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PRACTICAL METHODS FOR FOCUSING FILM CAMERAS

1. Background
2. General Discussion of Focusing Techniques
3. Popular Focusing Methods in the U.S.
4. High Precision Focusing Methods - The B and K Astrofocuser
    4a. Focusing Devices Applicable to Cameras with Removable Pentaprisms
5. Very High Precision Focusing Methods - Knife Edge Methods
    5a. Takahashi FM60 Focusing Microscope


1. BACKGROUND

Achieving correct focus at the film plane can be arduous for even practiced astrophotographers with any camera. This is especially true under average to rather poor seeing experienced by most amateurs. Focusing techniques that are quite adequate under excellent seeing and imaging conditions may be virtually unusable for many under their sky conditions. One of the best (though undesirable) ways to turn reasonably good 3 arc second images into 4-5 arc second images is through poor focus and/or focal plane movement. Strong focal reduction and fast lenses (low f-ratio) require rather critical focus and should be checked often to assure focus is maintained when ambient varies by more than a bit. For example, in a compound moving mirror telescope, I have measured considerable focal plane movement during a 0.6 degrees C. temperature change. This focal plane movement may exceed the limit for the maximum depth of focus desired for a selected focal ratio. At first this may sound surprising, but not so when one considers the optical leveraging of these telescopes.

Temperature changes, optical changes, and accidental focus changes all call for focus checks. Frequent focus checks are easier and encouraged if they are fast, convenient, don't require camera or film removal and don't cause horrible head and body positions. In this article, I mention a few companies by name and some fine companies are not mentioned at all. This should not be construed as a specific recommendation, but rather, certain companies are mentioned for purposes of clarity, example and reference.

This article is broken down into four general categories of practical methods for focusing film cameras: General Focusing Techniques, Popular methods, High Precision Methods, and Very High Precision Methods. The material presented is not intended to be all-inclusive. Devices that are not commercially available which require rather significant skills to fabricate are not included. Rather, only methods that are known to be reliable under actual imaging conditions using typical amateur equipment and experience are discussed. It is not enough for a device to have the potential to perform a particular function; it must work reasonably well in the field at the telescope. I have provided rather detailed first-hand experience with each of the devices that I have personally used under typical imaging conditions. There is one exception- the Doc G Focusing Devices. I included these because they work in a similar fashion to one that I have used, while providing superior implementation. The reader must decide which method or combination of methods is adequate for their imaging needs.


2. GENERAL DISCUSSION OF FOCUSING TECHNIQUES

If we have enough contrast, one can focus quite well with rather low power (5X) magnified camera viewfinders. Unfortunately when contrast is low, the eye accommodates rather significant focus errors quite well. Here, point source focusing is particular useful for those imaging in light polluted areas where contrast is poor do to the bright sky background. Point source focusing may be improved with higher magnification. In many cases, perhaps most, the standard 5-6X viewfinder magnification is not adequate.

Even simple framing of an object can be quite difficult through a standard viewfinder due to limited eye relief. Worse, focusing and framing is often especially difficult for objects at their ideal photographic location near the zenith. In addition, some telescope designs such as compound telescopes and many Newtonians may not maintain precise focus if moved too far from the object of interest. Moreover, rather than use a finder scope for object framing, many prefer to use the camera viewfinder for framing the object of interest.

Many owners of cameras used for astrophotography find standard focus screens are too dim for focussing and framing. They can replace the standard focus screen with brighter version. Exactly which versions are best for a given camera is debatable. Then, if one is using one of the discontinued cameras that are popular for astrophotography, the focus screen of choice may not be readily available. However, the Beattie IntenScreen Plus is readily available for popular astrophotography cameras such as the Olympus OM-1, OM-1n, and Nikon F. While it may not be the their first choice, I believe most amateurs can use this focus screen for astrophotography work.

A brighter focus screen helps with framing, increases contrast and aides rough focus, but does little for critical point source focus by itself and nothing for comfortable object framing. A typical viewfinder lens and pentaprism such as found in the Olympus provides about 5X magnification of the focus screen. Others provide around 6X, which are just adequate for high contrast objects but not adequate for the critical focus of point sources.

It is a good idea to focus on a star 5-6 mm from the center of a 35 mm film frame to average out any field curvature. If one is within the maximum depth of focus for a particular focal ratio, the use of this technique can produce what appears as a flatter image with little to no loss of focus in the center. This is so because the smallest star size is determined by the small circles produced by seeing and the film structure. As long as the stars Airy disc is smaller than the small circles formed, the image recorded will appear to be in focus, when in fact, it is a bit out of focus in the center.


3. POPULAR FOCUSING METHODS IN THE U.S.
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The Hartmann Mask

Some amateurs use a Kwik Focus or similar Hartmann Mask style focus tool based on the principle of using two or more holes in a cover or mask. The mask is placed over the scope aperture. The result is that an out of focus condition produces multiple images or multiple stars, which converge as focus is reached. In practice this seems to work better for CCD than for visual use. Under average seeing, it is a bit difficult to determine when the images have fully merged, as the edges are rather soft under these conditions. One can achieve good focus with this method, but not precise focus.

Review of Low Power Loupe Methods

The method of preference for many U.S. amateurs is focusing directly on the camera screen with a low to medium power loupe or variation of this. The technique is as follows - one focuses directly through the camera for maximum contrast and/or minimum point source size, much as focusing a low power eyepiece. Accurate position of the focus screen and  camera mirror is important. I typically prefer and use a knife-edge film plane method for critical focusing which is also useful to verify a particular camera or focus accessory will accurately focus. It is wise not to assume that the focusing device is working properly. Using an in-camera knife-edge test will verify other schemes for focusing. It may seem pretty risky to depend on the camera mirror position, mirror stops, mirror pivots, and camera focus screen position to accurately represent the true focal plane. However in practice, it is possible to do so, especially with high quality cameras and at larger focal ratios where the depth of focus tolerance is more relaxed.

The Nikon DW-2 Chimney Style Viewfinder

Users of the Nikon F2 camera can remove the pentaprism and replace it with the Nikon DW-2. The DW-2 provides direct viewing of the focus screen at about 6X magnification; this is not as high as I would like for point source focussing. However the comfortable right angle use helps to make the most of the low magnification.
The DW-2 will fit the Nikon F by removing a rubber skirt on its bottom side. The Nikon DW-2 is no longer available for the popular older astrophotography cameras. Those that want the DW-2 must look to the used marketplace. Likely used costs are in the $ 200 range. These types of chimney style finders are available for many other cameras with removable pentaprisms. However, they all have the disadvantage that they provide less than optimum magnification.

Fabricating a Focus Screen Magnifier

Others (including myself) have fabricated their own magnifier for the focus screen of the Nikon F for even higher magnification of 8-15X. The general procedure is to remove the pentaprism and fit a relatively high quality loupe to focus directly on the view screen. The rectangular type designed for 35 mm film work have the correct aspect ratio, though a bit of sanding or cutting may be required to allow it to rest fully on the focus screen. At these magnifications, one can focus by object contrast as well as focus with a point source. Those in suburban locations may find focusing by contrast very difficult simply because of the lack of contrast due to the brighter sky background. Here, point source focusing is useful. With the OM-1, the only real choice for higher magnification has been to physically cut out the pentaprism and fabricate your own magnifier (I've done this), use the Olympus Varimagni Finder, or purchase the Astrofocuser (described below).

Using the Olympus Varimagni Finder

The Olympus Varimagni Finder is just adequate for my 20-10 (corrected) vision with point source focusing. It produces a magnification through the viewfinder of 6X in the 1.2X position and 12.5X in the 2.5X position. Others find the magnification of the Varimagni Finder inadequate. The Olympus Varimagni Finder is field stopped at 3 & 5 mm at the 1.2X a 2.5X positions respectively which results in a sharper but dimmer image while making this device much less suitable for contrast based focusing. The Varimagni also readily falls off the camera, requiring its helical focuser to be re-focused to the camera focus screen. I use a bright star slightly defocused as a view screen focusing light source or monochromatic light introduced through a guiding port. Like the Nikon DW-2, you will have to find a used one, as these are no longer in production.

We have described many of the popular methods of focusing film cameras by many U.S. amateurs. However, many amateurs still lack confidence in the results produced with these devices under their imaging conditions. Let's examine a few methods that promise greater precision and ease of use:


4. HIGH PRECISION FOCUSING METHODS
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The B&K Astrofocuser-

I have tested the B&K Astrofocuser on a standard SCT with a modified JMI NGF-S focuser to provide fine focus adjustments needed to verify the Astrofocuser accuracy against one of the most, if not the most precise focus method of all - the inside the camera knife edge test. The unit is shown below attached to an Olympus camera.  (For availability and further details on this unit (cost approx. $240) go to the web site:  www.starfinder.com/maxx/images/Web-om-1a.jpg)
 
 

B&K Astrofocuser

Accessories Needed to Use the B&K Astrofocuser

The B&K Astrofocuser was shipped with a black anodized Astrofocuser body which contains the lens, camera bracket and heavy-duty 1/4" thumbscrew. The Astrofocuser bracket is attached to the camera body with the supplied 1/4" thumbscrew. A 1-1/4" star diagonal is inserted into the bracket holder. The Astrofocuser body assembly (with lens) is inserted into the star diagonal. Finally, the eyepiece is inserted into the body assembly and focused.

The B&K Astrofocuser requires a user supplied diagonal and eyepiece. Many can use what they already own, so this is not as bad as it first seems. I chose a chrome plated brass barrel star diagonal without a grooved barrel rather than a machined aluminum barrel. Barrels with machined undercuts (grooves) sometimes induce optical axis misalignment upon tightening. The smooth chrome was sufficiently hard enough to resist scratching by the bracket while allowing easy diagonal rotation. Owners of newer 1-1/4 prism diagonals from Orion and Celestron may want to check collimation. Two Orion and one Celestron sample that I compared with an older Celestron prism diagonal were badly out of collimation- beyond simple user adjustments. This could unjustly affect first impressions of the Astrofocuser, which is why I mention this cautionary note.

The complete focuser assembly weighs roughly 12 ounces with a user supplied diagonal, user supplied 15 mm eyepiece, Astrofocuser bracket, Astrofocuser body, and thumbscrew. This could be enough weight to throw off declination drift alignment on light duty mounts with the effect of increased field rotation. During declination drift alignment, those with concerns can minimize flexure induced polar axis misalignment, (which may lead to field rotation) by using weights to simulate the expected Astrofocuser/camera weight combination.

The Olympus Varimagni weighs roughly 4 ounces. Still, the complete Astrofocuser assembly weighs only 4 ounces more than my Nikon F with modified loupe for right angle viewing and focusing. The Astrofocuser is mounted by a large 1/4" thumbscrew to the camera body. A piece of 0.2" X 1.25" key stock securely positions the user supplied 1-1/4" diagonal against the camera viewfinder. The single 1/4" thumbscrew is more than adequate to secure the entire assembly and makes removal very fast. Those that purge their cameras with nitrogen and intend to leave their Astrofocuser in place on the camera can drill the thumbscrew to accept a short piece of brass tubing for attaching a hose.

Focusing with the B&K Astrofocuser

If you are using the Beattie IntenScreen Plus, focus the eyepiece on the side of the screen that faces away from your eye. A red light shown through an affixes guiding port will reflect enough light to quite easily fine tune focus of the eyepiece to the screen. The red light will reflect off the focal reducer and back through the camera just like that over bright illuminated reticle guiding eyepiece that spoils images. The red light will illuminate the concentric Fresnel rings found on many focus screens that are practically invisible to the unaided eye. If you have difficulty seeing the Fresnel rings, try a 15 mm eyepiece, though focusing on the focusing screen is pretty close. The light does not need to be red, but indirect lighting from a window or diffused light doesn't illuminate the Fresnel rings as well as a monochromatic light, I believe.

The ability to focus at the Astrofocuser at the telescope is useful when changing eyepieces from a 15 mm to a 32 mm framing eyepiece. Done correctly, you will see concentric Fresnel rings on the correct side of the Beattie screen. You can also use a bright star slightly defocused for the light source as well. I use a similar technique with the Olympus Varimagni Finder, though the Varimagni requires a brighter star. As the instructions recommend, move the focal point between inside and outside focus several times to learn where the best focus is. At 20X or so, this is quite easy as the star will flare a little as one just approaches focus, then become very small (size varies with seeing) and flare again outside focus.

As compared to the Varimagni Finder at 1.2 X setting (about 6X through the viewfinder lens and pentaprism), the Astrofocuser is much brighter, in part because the Varimagni has a 3 mm field stop. At the 2.5 X position (12.5X), the Varimagni has a 5 mm field stop. With the Astrofocuser, there are no field stops until the eyepiece, except the 11 X 16 mm camera viewfinder. With the B&K Astrofocuser, magnifications of the focus screen of 20X and more are quite possible. This higher magnification can considerably improve results for those that prefer through the camera focusing.

Framing with the B&K Astrofocuser

One of the most important uses of the Astrofocuser is not focusing at all, but for framing the object at various scope positions including objects near the zenith. Here, a 32 mm Plossl provides almost the entire field of view through the viewfinder and pentaprism while increasing eye relief. For framing near the zenith, the diagonal is extremely useful. In this case, any spherical aberrations become more important if you use the Astrofocuser for framing. The visual result is that the center of the field is in focus while the outside edges are blurred. Spherical aberration was evident with a 32 mm Plossl in my early Astrofocuser (one of the first made), which did not affect focus results at the center of the field. Models currently shipping have good correction for spherical aberration.

While B&K promotes the Astrofocuser as a finding and focusing device and not a framing device, reduced spherical aberrations of current models allow the Astrofocuser to be used for framing as well. I found I could not resist using the Astrofocuser along with a 32 mm Plossl for framing as the bright view and diagonal made precision framing very easy. I consistently switched and focused my eyepiece to the focus screen in 30-45 seconds at the scope in 20 degree weather. Still, others may find they prefer avoiding eyepiece changes. The Astrofocuser was tested with several lenses (Plossls, Ultimas, Naglers, Kellners, Orthoscopics, and Ultrascopics from 32 mm to 4.8 mm). All worked well and reached focus properly including a no name Kellner.

Comparing the Inside Camera Knife-edge to the B&K Astrofocuser

I checked the Astrofocuser with a knife edge tool of my own design precisely calibrated for my OM-1n. I couldn't improve focus with my camera back knife edge tool. However, others may not obtain similar results if their camera mirrors and/or focus screens are off spec. The Astrofocuser generally provided excellent focus results with a 15 mm Ultrascopic eyepiece for me, but some may need eyepieces from 12.5 to 10 mm to achieve my results as I have superb corrected vision (20-10).

4a. The Doc G Focusing Device for Cameras with a Removable Pentaprism

Just recently, Doc G has designed two focusing devices for cameras with removable pentaprisms. They consists of a small high quality telescope and transfer lens designed to enable focusing directly on the camera view screen at a moderate to high power of 26X and 48X. A nice feature of  his devices is that they are positioned at right angles to the camera thus enabling comfortable high altitude focusing for SCT's and refractors. They are secured to the camera which prevents jiggling of the mount during focusing and provides hands free use. The following link has a description of  two versions of the adapters for attachment  to the Canon camera:  The same devices could be designed to work with other cameras with removable pentaprisms. ragreiner/focusdevice.html


5. VERY HIGH PRECISION FOCUSING METHODS
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Description of the Knife-edge Focus Method

The knife edge method for focusing film cameras is the method against which most others are compared. It is quite simple in principle, but not particularly intuitive because unlike other methods, it does not bring an image to focus through an eyepiece or similar lens. If one places a knife-edge at the desired focal plane, the knife-edge will cut the light from a point source (star) when a focus solution is reached causing an instantaneous flooding or darkening of light. This is easy to say, however, I have found a great many amateurs are reluctant to admit they really don't know how to do it or are very are uncomfortable with this method. This is unfortunate, because once mastered, it is powerful, effective and almost never fails. A bit of tutoring (less than 10 minutes) at the telescope reinforced with personal practice over a few months is usually enough to master this technique, I think.

Below, left, is shown the open back of a camera where the rails are located and where the knife edge is placed. There are two sets of rails used to guide the film. It is important to set the knife edge in the plane that defines the film emulsion as described in the text.

To the right is shown a Ronchi based focusing tool. The tube with a length of 55 mm is screwed onto the T-thread adapter in place of the camera for focusing. Care needs to be taken when using  this device as described in the text.
 

Camera back Ronchi focuser

For many, it is a good idea to rough focus through the camera viewfinder or perhaps a parfocal eyepiece. The knife-edge tool is inserted on the inside camera rails- a surrogate open back camera can be used in place of the imaging camera. A real knife-edge or Ronchi screen of 50-200 lines per inch on thin Plexiglas or glass is taped to the camera rails. The Ronchi screen works well on an inexpensive amateur mount that might tend to exhibit flexure as well as high end mounts.

One should move the knife-edge fixture away from the inside rails (towards the camera back) a bit. About 0.005" backset is required for most films & 0.006" for 2415 film to accommodate the difference between the film thickness (about 0.005" for most films and about 0.004" for 2415 film) and the 0.010" difference (average) between the inner and outer rail heights. This is because the film curl pushes the film away from the inside rails into the spring loaded pressure plate residing on the inside of the camera back which helps keep the film flat without touching the image side. The outside rails are used to set the distance between the pressure plate and the film, thus allowing the emulsion to move freely. I use a cut out piece of discarded film for the 0.005" backset needed for standard film and voice coil shims for the 0.006" 2415 film backset. As a precaution for absolute precision, one should obtain precision measuring tools to verify the difference between the inside and outside rail heights.

The Ronchi screen faces towards the front of the camera, away from the eye. One can disassemble the Celestron MFFT 55 and use it's reticle inside many cameras such as the Olympus OM-1 on the inside film rails (with the addition of 0.005" added to the inside rails) and between the outside rails. As another alternate, one can use a commercial knife edge focus tool which assume a 55 mm film plane to T-thread distance. Unfortunately, I have found this distance to vary. My Nikon F camera is around 56 mm. This is of no surprise to me as the 55 mm "standard" would not be important to a given camera manufacturer using their lenses mated to their mounts.

How to Focus with the Knife-edge Method

With a discussion of knife-edge methods above, it is appropriate to discuss how to use the knife-edge method because it is arguably not as intuitive as other methods. However, I believe it is useful to know how to do it to evaluate the suitability of alternative focusing methods and verify the correct positioning of a camera mirror and focus screen.

Choose a bright star located well above the horizon to avoid seeing and scintillation problems. As seeing worsens, finding exact focus worsens as well. This is good, because this is an excellent indicator that imaging should be postponed. Center the star about 5-6 mm from the center of the 35 mm film to average out any field curvature. If one is within the maximum depth of focus for a particular focal ratio, the use of this technique can produce what appears as a flatter image with little to no loss of  focus in the center. Get rather close to the knife-edge or Ronchi screen (5-10 mm) and view the image. With the Ronchi screen, one typically moves the telescope. With a real knife-edge, one often moves the knife-edge, which works well on high end amateur mounts with good stability but may not work as well with some entry level amateur mounts.

When viewed from behind, moving the telescope (or knife-edge) slowly moves a point source into the knife-edge. As the star moves across the knife-edge, a darkening or lightening from one side will be noticed. It may help speed things up a bit if one notes the direction of the darkening. If the darkening moves in same direction as the knife-edge, the focus point is too far inside. If the darkening moves in the opposite direction as the knife-edge, the focus point is too far outside. However, if one starts bit inside of focus, one can forget about the direction of the curtain and concentrate on continuing in the same direction until the focus solution is reached. For an SCT where one wants to finish focusing the moving mirror in a given direction to pre-load the focus knob threads, this method is unequalled. By the way, for those that are wondering, an SCT is inside of focus when turning the focus knob CCW sharpens the blurred image.

The darkening (or shadow) appearing from one side of a slightly out of focus star is often referred to as a "curtain". As one gets quite close to focus, the light or darkness engulfs more and more of the entire image, rather than just on one side. When getting even closer to focus, the lightening or darkening dims or brightens evenly throughout the image. Finally, perfect focus will show almost instantaneous lightening or darkening of the image everywhere. The knife edge method of focus (based on the Foucault Test) is arguably the most precise method of all, especially when used at the film plane. It is my personal favorite because of speed and ease of use. However, there is one drawback- the camera or film must be removed to use it.
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5a. The Takahashi FM-60 Focusing Microscope
Gene Horr has been kind enough to loan me his personal Takahashi FM-60 focus tool for testing, review, comparisons, and comments. This tool is shown below being applied to the film rails of a camera. The Allen wrench is used to tighten the position of the microscope objective after adjustment.
 

Takahashi microscope

The Takahashi FM-60 is a film plane focus tool, which utilizes a microscope type objective and 60X magnification to focus a film camera not unlike focusing a high power eyepiece. This makes it rather intuitive to use. As compared to loupes and other lower power magnifiers used away from the film plane via internal mirrors and/or prisms, this device clearly produced equal or better results than all other methods- as one would expect with such high power. While the use of a microscope objective at high power does limit the field of view, it also improves the image considerably. At around $ 200, it is difficult to duplicate similar performance for less. One of the best ways to improve image results is with accurate and dependable focus techniques. The FM-60 appears to be quite adequate for this task.

How to Use the Takahashi FM-60 Focusing Microscope

The FM-60 is designed for use without film or in a surrogate camera. Functionally, it works exactly as my custom ground glass and microscope focus tool, so I am familiar with its use. This is fortunate because Gene reports the instructions are in Japanese. While I believe the use of this tool is rather straightforward, some new to these devices will benefit from the overview that follows. (For availability and  further information contact Texas Nautical at  http://www.lsstnr.com/)

The objective's distance to the ground glass screen is adjustable by a small set screw on the side of the threaded end piece opposite the rubber covered eyepiece end. Loosen the set screw and adjust the distance needed to reach focus with the eyepiece draw tube. This is useful for eyeglass users preferring not use their corrective lenses, though the FM-60 works quite well with prescription glasses. The eyepiece is contained inside an adjustable slider tube, which is secured with a hand nut and split tapered nylon bushing.

A rubber eyecup prevents damage to eyeglass lenses and a nylon ring around the recessed objective prevents scratching of the screen and a bearing surface allows moving the front of the microscope rather freely. The glass screen should be secured to the camera over the inside rails (etched side toward the telescope), but between the outside rails. With typical 35 mm cameras, it may be prudent to add about 0.005" under the glass screen to move the front of the screen toward the camera back where the film usually resides. This is described in greater detail above. To improve ease of use, it is a good idea to remove the camera back. To do this, look for a small, spring loaded sliding bolt on the hinge side of the back. Apply pressure toward the opposite side to release the hinge pin.

The microscope objective must be pre-focused to front side of the ground glass (frosted side). This can be done earlier by simply providing illumination to the front of the glass. Gene Horr prefers to dot the glass front with a permanent marker, hold the glass up to sky light or on a light box, then focus on the dot. I prefer to focus similar devices to the front of the glass screen using a monochromatic light source shown through an affixed port. This readily illuminates the irregular front surface allowing one to focus the microscope to the front surface of the glass at the film plane quite precisely.

You will likely find pre-focusing the scope or camera lens helps. Next, point the scope to a rich star field. Place the microscope directly on the ground glass, moving it about looking for a rather dim star for best results. Focus the scope as you would a high power eyepiece. If the seeing is good, the point of optimal focus will be between the points at which the star just starts to flare. However, at 60X, optimal focus is easier to see than at 6 to 10X.

Using the Takahashi FM-60 Focusing Microscope on SCT's, Newts and Refractors

I used the focus tool on an SCT, Refractor and Newtonian held in rotating rings by a GE mount. I asked my colleague to use the FM-60 focus tool as well. We used it at several focal ratios from f/10 to f/4. We were able to get quite close or dead-on as compared to my custom knife-edge inside camera tool.

The scopes that were more difficult to focus with the FM-60 were the SCT and the refractor. This was in part because the high 60X magnification and small field of view (typical of a microscope objective) required a bit of a steady hand to keep the selected star in the objective field. We needed to hold the tool, find the star on the glass screen at high magnification (yes, a dim star is best), focus while holding the tool and not shake the mount too badly while our head was twisted to the best viewing and focusing position. The SCT proved the most difficult as we opted to use the moving mirror focus knob which is best focused in one direction (CCW for most except the 12" & 16" Meade SCT's). Focusing near the zenith would be all but impossible for the SCT. With the refractor, we had to lie on the ground for zenith focusing. The Newtonian was the easiest to focus. We positioned the tube to allow a comfortable head position. We were able to achieve consistent focus every time.

I believe the less than perfect focus on the SCT and refractor were primarily due to the difficulty in holding the tool while turning the focus knob and viewing simultaneously. For example, when focusing much closer to the horizon, focusing confidence and subsequent results improved. However, there are many situations and reasons why it is prudent to focus at much higher altitudes. In addition, the desire to finish focus the SCT in one direction left us with the uncertainty whether moving the knob a bit further would improve focus. Of course an auxiliary focuser such as the JMI NGF-S, Feather Touch, or several Van Slyke models would alleviate this problem.

Comparing the Takahashi FM-60 Focusing Microscope to the Inside Camera Knife-edge

We compared the FM-60 to my custom knife edge tool, which by its nature is less intuitive, but doesn't require holding the tool while focusing and allows a more liberal head position. The knife-edge method allows one to reach a focus solution from one direction every time. There is no need to move between inside and outside of focus to assure the best possible focus is attained because the end-point is very clear. Both of us were able to achieve focus in less than one minute on all three scopes with a high degree of confidence. We were able to approach similar speeds using the FM-60, but not consistently. However, we have many years of prior knife-edge focusing experience.

Final Thoughts about the Takahashi FM-60 Focusing Microscope

One would like to start imaging when the air is steady and temperature nearly stable. However, in summer, it is quite possible the rather short period of astronomical darkness will require at least one focus check to allow a bit earlier imaging. This will require removal of the film to use the FM-60, a surrogate body, or a machined adapter.

I believe those that prefer to focus their camera as they would focus an eyepiece will not be disappointed with the Takahashi FM-60 microscope focus tool. Currently, I believe there is no better commercial focus tool using standard focus methods that will produce better results for good reasons- it is placed at the film plane and uses high magnification. However, those that are comfortable and experienced with knife-edge methods will not likely see any improvement in ease of use, speed, or results when using the FM-60 focus tool.

I believe most can obtain improved consistency and results providing they learn to accommodate the manual dexterity and head position often needed to use this tool with SCT's and refractors. If there was an accessory holder to hold this tool in place, I believe ease of use with SCT's and refractors would improve.
--

Michael Hart
Husen Observatory

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