Increasing Contrast in the LX200 OTA 

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Subject: Lining OTA with Black Flocked Paper   Top 

From: Rob Roy <>

>Could you explain how you lined your OTA w/black paper
>--and is it reversible? Did you notice any increase in contrast, etc.?

Easier to answer both at the same time. I thought there was a noticeable increase in contrast. Not only that, but there seemed to be fewer of the glares creeping in from the side that you would get on occasion near a very bright object (never counted- not 100% positive.) I love double stars and I'm sure those that I routinely look at show more black between them. I did do my finder first for practice, but use a Telrad exclusively now.

The black flocked paper came from Edmund Scientific. I used the sticky-one-side stuff. It seemed to have some glare or reflection when glancing light struck it, so I sprayed the whole sheet (minus a tiny piece for comparison) lightly with Krylon's "Ultra Flat Black." It was much better than the flock alone which in turn made the original interior surfaces look like semigloss by comparison. The flock paper was thoroughly vacuumed before and after spraying.

The outside of the secondary baffle was wrapped. The inside is curved and I just couldn't get the paper to fit, so I had to be satisfied with just spraying it with the much flatter Krylon paint. Mirror was removed first of course. The inside of the primary baffle tube was also covered--read further down for spray technique.

To do the main OTA, the primary mirror was removed for obvious reasons (see MAPUG-Astronomy Topical Archive listing) Just one edge of the shiny protective paper was lifted up, the flock paper started and as it progressed around the inside more of the shiny stuff was peeled back a little at a time. I didn't get it quite square to start, and the top and bottom edges are out by 1/4"- big deal- you're not going to get any reflections from the top and bottom 1/4" anyway!

Reversible?- I have no idea and don't intend to find out. I was concerned about peeling away from the inside of the OTA because of heat/cold- never happened. I was concerned about little hairs of flock getting all over the inside- never happened. So it's there to stay, I guess, and it HAS made a difference.

Editor's Note: I covered the inside of my OTA as well with the Edmund Scientific flocked paper (search on that phrase); however, my piece of adhesive backed was very absolute black and didn't require the dusting with Krylon. A sheet of non-adhesive back that came in the same order was not nearly as absolute black and does need the dusting?? I managed to do the inside of the OTA without removing the mirror by using pieces about 1/4 in circumference as I found that trying to do it with a full sheet was too frustrating. Don't overlap the paper because it won't stick long term. Instead, after overlapping, cut through both layers with a razor blade and peel out the cut-offs--will make a nice butt joint. Use the smaller pieces to line the diagonals, eyepiece barrels, etc.


Subject: Flocking & Black Painting -- Best & Easiest Methods to Increase Contrast? --part 1 of 3  Top Button

From: Alan Voetsch <>

Here is a post from Peter Erdman that talks about increasing contrast. I am specifically interested in one aspect of his post. Using spray paint to blacken the baffle knife edges. I'd like your thoughts on the following questions. Is the 'spray paint' method valid?

If so, what should the drying time be? The paint may be dry in 30 minutes but, should the tube be left open for a specified time to let fumes air out?

I am leaving the original post intact (even though it's long) because there is plenty of good material in it.

--- Peter Erdman <> wrote:
Subject: Increasing Contrast in an SCT --part 1 of 3   Top Button
One often sees statements that refractors are observed to have higher contrast than SCTs of equal aperture. A couple of reasons are frequently offered. One is usually ascribed to the lack of a secondary obstruction in the refractor, although computer simulations show minimal deterioration of images at an obstruction ratio of 25% or so. Commercial SCTs have an obstruction ratio a bit higher than this, yet they are available in much larger apertures (for finite dollars). One might think that the larger aperture should decrease the size of the diffraction pattern in favor of the SCT, thus mitigating the effects of their increased energy in the higher order diffraction rings.
Another rational for lower contrast is that "reflective optics have intrinsically higher scatter than refractive optics". I have seen this statement several times, but never with any reference to data which would support such a claim. In fact, one might easily imagine that refractive optics should scatter more light than a reflective surface since in the former the energy must propagate through the medium of the optics and any inhomogeneity would then result in scatter. The details determine the outcome here.
However, there is one place where it is very clear that contrast in an SCT (or any Cassegrain system) can suffer, badly--the baffle design. The baffle in a refractor can be almost trivial in comparison. We've all seen flashes of light while observing and moving the scope near a bright object--clearly a result of light scattering off some surface. The baffle system in an SCT is always a compromise between reducing the size of the central obstruction, the size of the field of view (FOV), and the quality of the off-axis energy rejection. The Meade systems have one particular compromise, maybe it could be better, maybe worse. There is no unique answer to the design problem since all factors cannot be optimized simultaneously.
However, we can improve the existing Meade baffle by the very simple means of reducing the reflectivity of the surfaces responsible. For example, the walls of the OTA are painted "flat" black, but try shining a light in through the corrector plate and you can easily observe that they are actually rather reflective. The paint surface is very nicely smooth, a fact which guarantees good reflectivity at grazing angles. Also see Baffling in Regards to Rear Aperture & Diagonals topic below.
Also, the interior of the central baffle tube is machined and anodized black. Remove the eyepiece, look directly up the baffle and you can see every one of the knife edges, some are very bright. We can see the same problem with smooth surfaces as for the OTA walls, maybe worse.
Now think about which surfaces could contribute the most to scattered light and therefore reduced contrast in the final image. The interior of the central baffle is certainly the worst potential offender. A single scatter of an off-axis ray is quite likely to strike the focal plane. The fact that these off-axis rays will be at nearly grazing angles to the baffle surface also means that they will have high reflectivity (just as we verify by looking up the tube). This is exactly why Meade went to the expense of the knife edges in the tube in an attempt to reduce this reflectivity.
There have been a couple of notes here from people who experimented with inserting black felt into the baffle tube. Exactly the right idea for reducing scatter, but I didn't want to do that because of the increased vignetting. I wished to preserve the FOV, which is already vignetted for 2" eyepieces. I needed to find a suitable paint for the interior of the baffle and a way to apply it.
The solution I attempted was Krylon "Ultra-Flat Black" spray paint, the "flattest" I have found available. As a comparison test of reflectivity with the Meade flat black paint, I painted a swath on the inside of a Meade dew shield. I held the spray can away from the surface and moved it rapidly to avoid "wetting" the surface, making more of a fog of paint, and letting it dry between applications. The result was a rather textured surface that was substantially blacker than the Meade paint. Side by side, the difference was very noticeable.
I used the same painting technique on the interior of the baffle tube. With the corrector plate removed, the primary protectively covered, and the OTA in a horizontal position, I sprayed directly down the center of the open baffle tube--deliberately trying to avoid directly hitting the side walls. I ended up spraying from both ends of the baffle tube to get both sides of the knife edges fogged with paint. It came out very "flat" looking as I had hoped.
But did it work? As a control, before the painting I had set up the scope in my garage pointing at a white wall illuminated by the interior garage lights, so I had a target that was sort of repeatable. Before painting I could look up the baffle tube and see every knife edge and some VERY bright places in between. After painting, I can't see any of them. I'm estimating that implies more than an order of magnitude reduction in scattered light from this particular source. If I increase the illumination I can finally see the baffle's knife edges, but I have to look directly at a florescent light fixture to do so.
Since the scope was all apart anyway, I also proceeded to install black felt <> with the adhesive backing on the OTA walls. I found it necessary to install it in two pieces, and can appreciate the dilemma of the person who had it fall on his primary. This is very aggressive adhesive, and the felt is very black. In fact the biggest problem I had was seeing what I was doing. Once the felt was inserted into the OTA, it was too dark in there to see the edges of the felt as I tried to align it.
However, I don't think that the OTA walls are the most important source of scattered light. An off-axis ray reflected from the OTA walls could easily be reflected from the primary and then the secondary, but it is still off-axis and therefore must survive another scatter off the interior of the central baffle tube before it could strike the focal plane. Hence my emphasis on darkening the central baffle interior. Since the net reflectivity would be the product of the reflectivity of the two surfaces involved, a blacker OTA still helps (and it's easy to do). I could have painted the OTA walls instead of covering them with felt, but I didn't want to pull out the primary mirror and didn't think I could protect it well enough without removing it.

Subject: Increasing Contrast in an SCT --part 2 of 3 Top Button

From: Doc G, Date: Apr 2004
----- Original Message -----
From: Bill Arnett
> > ..."20 years ago flocking was a very desirable and common practice, but
> > with a properly baffled SCT with such extensive computer-generated
> > baffling design for eliminating the introduction ofinternal glare, there
> > is absolutely no reason whatsoever to flock the inside of a SCT OTA...."
> That may be true for a "properly baffled SCT" but that does not include
> Meades! But there's no need to take any of this on authority.
> Just point your LX near the bright limb of the Moon, remove the
> eyepiece and look up the tube. You won't like what you see. Any of the
> flocking methods described on this thread will make a dramatic improvement.
I have followed this thread for a few days now. I have to agree totally with Bill Arnett. Flocking helps with Meade SCTs. Folded scopes are extremely hard to baffle without causing vignetting. There is a conflict in the design between good baffling and vignetting. The coating on the inside of the Meade SCTs does not absorb well at small incident angles. All telescopes benefit from excellent baffling, but with the SCT you really need to treat the surface as well.
Contrast is one of the very important properties of a telescope used for deep space objects. DSOs are of extremely low contrast and need al the help they can get. It is easy to do the baffling and you do not have to remove the primary mirror. All you have to remove is the corrector plate.
Another thing that helps a great deal to hold up the contrast is to use a very long dew shield since it also acts as a light shield or lens hood.
One final comment about scattered light and getting the highest possible contrast. Use a long hood and flock it as well. The hood should have a length at least twice the diameter of the objective. I line the hood with black felt. This not only cuts reflected light but absorbs moisture as well.

Subject: Increasing Contrast in an SCT --part 3 of 3

From: Alan Voetsch <>
When you're taking 2-4 hour exposures, any excess, scattered light is the last thing you want. If you're serious about observing, or astrophotography, spend the $15 bucks and the 30-90 minutes of time, and FLOCK IT. While you're at it, read Peter Erdman's post from a couple days ago, buy a can of Krylon Ultra Flat Black and spray the baffle tube also. Another $3 well spent.


Subject: Flocking & Black Painting -- Best and Easiest Methods to Increase Contrast? --part 2 Top Button

From: Roger Thompson <>

I flocked my OTA a few months back, and I had the same concerns, here is what I did. I went out to Staples (a local stationary/office supply outlet) and purchased a laminated calander, the one I got was about 24" x 36" and was only about $9.00

The lamenated sheet was cut to fit against the inside wall of the OTA, when I did a test fit, it actually snapped solidly into place, I then removed the lamenated sheet and flocked the sheet! I then put the flocked sheet back into the OTA, and it snapped in perfectly. The seam is invisable, I highly recommend this method since you do not get any of the sticky flocking paper anywhere near any optics. The baffle can be removed and flocked also. It really is not that difficult to do if care is taken. I had a large bench set-up with nothing nearby to trip over or hit. The primary mirror does not have to be removed. (your choice, depending on how much you want to flock. As the for the corrector, just be sure to have a mark on the corrector edge to align the corrector before you remove it so you can align it when you replace it. Note: See how to remove a stuck corrector here in the Archives.

I used any extra flocking material to flock diagonals, and eyepieces.


Subject: Flocking & Black Painting -- Best and Easiest Methods to Increase Contrast? --part 3 Top Button

From: Bruce Johnston

To help you with your fears, I just now uploaded a copy of an email I just sent a friend with the same concerns about removing the corrector. I think that if you follow the instructions *TO THE LETTER*, you'll have no problems at all with the corrector.

Take a peek at: <>


Subject: Flocking & Black Painting -- Best and Easiest Methods to Increase Contrast? --part 4 Top Button

From: Peter Erdman <>

Bruce Johnston already sent a message on the corrector plate removal that I would find impossible to improve upon. I would add one thing--the corrector plate may well be stuck to the OTA and take some effort to loosen. Note: See how to remove a stuck corrector here.

As for the flocking, I used this same Protostar material for my OTA. I applied it directly to the tube and to the interior of the secondary baffle. I can't imagine how it could come off and fall onto the primary--this is a very aggressive adhesive. Once on, it's on.

Run the primary all the way to the rear so it's out of the way. Don't bother with flocking all the way to the back, reflections there are very unlikely to enter the optical path. This operation doesn't need to be perfect, the improvement is proportional to the area you cover.

I quickly gave up on the idea of applying it in one piece. Too scary. Instead, I cut the necessary piece into quarters (along the axis of the OTA). The only reason to worry about nice seams is cosmetic, and since it will be so dark inside the OTA after you are done you won't even be able to see them. I just overlapped them a little and didn't bother doing any trimming.

The conical interior of the secondary baffle makes it imperative that you use small pieces of the flocking to cover it. This area is probably even more important than the interior of the OTA. Grazing incidence reflections here can easily enter the optical path.

Now, with the corrector still removed, point the OTA at an illuminated wall and look up the center baffle from the rear towards the wall. You will notice that the interior of the baffle is easy to see, and that some areas gleam like the mouth of hell. Bright stars slightly out of the field of view will also reflect from these surfaces, so I also painted the baffle interior (already had the corrector off, so why not?).

I covered the front of the primary, and sprayed "ultra flat black" paint in both directions down the center baffle. Never spraying directly at the walls, just letting a "fog" of paint slowly build up on the surfaces. This is deliberate bad spray painting technique since you don't want a smooth surface, rather the roughest you can create.

Afterwards, looking at the same wall, I couldn't see the baffle sides. I had to get another light to shine down the baffle to see where I needed further improvement.

I've yet to do the side by side comparison with an "unimproved" OTA, but this MUST improve the scattered light problem. The original machined surfaces of the baffle were especially problematic.


Subject: Flocking & Black Painting -- Best and Easiest Methods to Increase Contrast? --part 5 of 5 Top Button

From: Peter Erdman <> Date: Dec 2002

Alan Voetsch wrote:
You may have noticed that I forwarded a post you made over a year ago regarding spray painting the baffle tube. The reason for that was I wanted feedback on how long the tube should be left open to allow for curing. I've heard of weird things happening when paint cures in a closed environment. What I'd like to know is: should I spray the baffle before putting the flocking in, or does it matter? And how long should the paint cure before closing the tube back up?

Do the painting first (see primary shielding below). I probably let it dry for at least a week before reinstalling the corrector plate. I didn't want whatever volatile material the paint contained to condense on the optics, so I left it as open as possible. I even used a hair dryer (on cold) to blow up the baffle after I first painted it to force most of the "whatever" out of the OTA. I think I waited until I couldn't smell it any longer. Afterwards, I would take off the rear adapter when not using the scope and let it continue to vent any vapor build up.

I plan on doing these operations outside in the observatory. The OTA will be exposed to colder damper air. But I think that is a better option than bringing it inside where there will be dust and cat hair floating around.

Just keep the temperature high enough. Most paint isn't happy at temperatures below something like 55F. Keep all the optics covered (even the corrector across the room), spray has a way of getting everywhere. I used a piece of cardboard pushed down over the baffle to shield the primary. Duct tape between the cardboard and the OTA walls filled in the gaps to create a good seal. Same for sealing to the baffle exterior. Another tight fitting piece of cardboard at the rear of the scope to prevent over spray from getting all over the OTA's exterior.

Spray in bursts, don't let any areas get "wetted" with paint. If they do, let them dry and start over. Wetted areas smooth out too much, and that's the opposite of what you want. Take your time.

The worst thing about removing the corrector (other than the panic) it that you will have a serious collimation task to perform afterwards. Note: see collimation topic here.


Subject: Baffling in Regards to Rear Aperture & Diagonals --part 1 of 3  Top

From: Doc G, Date: Jan 2003

From: Peter Talmage
It seems to me that the rear aperture on SCTs is sized so the baffling all works properly. In effect it appears that the stock rear aperture is an important part of the baffling because a larger aperture allows direct unfocused sky light to enter the eyepiece and thus lower contrast. If you look through the focuser tube with your eye and you can see a ring of the great outdoors around the secondary then direct light is getting in. It seems preferable to have some edge vignetting and higher contrast. Am I missing something here?

As with so many things, it depends! (G) I understand what you are saying and I have studied this question for some years on and off. Here is my understanding and conclusion. The problem as I see it is not with the baffling of the telescope structure itself. Even there any SCT has baffling that will cause some vignetting for a 2" field stop at the eyepiece. There is not much one can do about that. The problem I believe is with the Schmidt threaded adapter on the back of the rear plate of the telescope. This adapter was originally designed for small SCT scopes (8 inch) and for 1 1/4 inch eyepieces. The inside diameter of the tube is about 33 to 36 mm depending on the wall thickness and design details.

This is certainly adequate for 1-1/4 inch eyepieces since the field stop for the small eyepieces cannot be larger than about 26 mm. The field stop will be as fully illuminated as possible, limited only by the telescope baffling. But for a low power 2 inch eyepiece the field stop can be as large as about 46 mm. Clearly then the diameter of the Schmidt threaded tube is too small and will cause additional vignetting of the illumination of the field stop in the larger eyepieces.

Putting a 2 inch eyepiece on an 8 inch telescope probably does not gain much in field illumination because of the restrictive central baffle tube in such a scope. However, larger telescopes such as the 12 or 14 inch scopes have a proportionally larger baffle system. In that case, the Schmidt size tube is a very definite restriction of the illumination of the field stop. Thus it is desirable to increase the size of the rear opening so that the field stop of the larger 2 inch eyepieces can be fully illuminated. Even with the larger telescopes, there will be some vignetting of the field by the normal telescope baffling.

You are certainly right about not allowing light from around the central stop (the secondary) to get into the eyepiece. And, that is certainly the purpose of the normal SCT baffles. It is a tight squeeze to get as much light as possible into the eyepiece and still not get too much stray light. Most users, I believe, feel that reducing vignetting is as important, possibly more important, than perfect baffling. I have made some measurements of unrestricted rear opening illumination and found that the larger telescopes will illuminate a field of well over 50 mm without allowing stray light to enter the field. Thus I have always felt it safe to open up the rear of the telescope at the back to a full 50 mm (2 inches). A larger field of full illumination may not be noticeable to many in visual observing, but it certainly is when using larger CCD chips and especially full 35 mm film format. In imaging a small reduction in edge illumination is greatly accentuated and must be removed with a flat field technique.

Thus, I have become a strong advocate of using a full 2 inch adapter at the back of the telescope and using 2 inch tubing everywhere possible. I personally use the 2 inch adapter made by Peterson Engineering.
See it at: <>


Subject: Baffling in Regards to Rear Aperture & Diagonals --part 2

From: Peter Erdman <>

These are all very good points. Another reason to switch to a 2" system is for improved mechanical rigidity (especially for imaging). I always use a 2" diagonal just so that I have the option of which eyepiece to use. While the smallish I.D. of the 8" SCT adapter will result in vignetting for the largest actual field of view (AFOV) eyepieces (such as the 40mm Pentax that I am fond of), there are other nice 2" eyepieces with AFOVs larger than the ~26mm limit of 1 1/4", but smaller than the 2" limit, that I also enjoy using.

In fact, a 2" diagonal itself will vignette an AFOV of 46mm. While the O.D. of the diagonal is 2", its I.D. is barely the 46mm of the AFOV, and making matters worse, this limiting aperture is well forward (by about 3") of the desired field stop location at the eyepiece. To fully illuminate an AFOV of 46mm would require an even larger diameter diagonal, but that is not a standard item.


Subject: Baffling in Regards to Rear Aperture & Diagonals --part 3 of 3  Top

From: John Hopper <>

Peter Erdman writes:
> In fact, a 2" diagonal itself will vignette an AFOV of 46mm. While the O.D. of
> the diagonal is 2", its I.D. is barely the 46mm of the AFOV, and making matters
> worse, this limiting aperture is well forward (by about 3") of the desired
> field stop location at the eyepiece. To fully illuminate an AFOV of 46mm would
> require an even larger diameter diagonal, but that is not a standard item.

I can picture people now shortening and boring out their 2" diagonal tubes to aluminum-foil thickness.

From selling the Aries Chromacor color corrector for achromatic refractors, I have some knowledge of 2" diagonal tubes, due to spacing issues particular to that device which screws onto a diagonal's 48mm threads. People with the A-P MaxBright need the most spacers added due to its shorter-than-normal tubes and hence optical path length.

So if you're looking for an off-the-shelf where the end of the diagonal creates the least-upstream obstruction in an "Eye-Opened" SCT back, I would recommend the A-P. But I'd still like to see photos of chopped and bored Meade and Tele Vues. Maybe a Meade diagonal with precise 48mm threads could also be created!

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