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Here is a list of over 300 interesting extended objects. They are listed by size in order to emphasize the idea that when imaging extended objects is necessary to try to get a significant sized image on the film or chip so as to make good use of the resolving power of the film or imager chip. The only way to essentially "fill the chip" with the image is to use the proper focal length telescope. The proper focal length can be obtained by choosing the correct telescope focal length. The size of the real field in the sky is generally measured in arc seconds or minutes. This value is dependent only on the size of the imaging chip and the focal length of the telescope. Thus if the imager wants to encompass a certain range of object sized effectively, it is necessary to choose a telescope of a given focal length to match the chip. It can be a problem to get focal lengths that are short enough to image larger objects and also to get enough image size to image planets and some very tiny objects. Often a telescope of excessive ly long focal length needs to be augmented with a focal reducer to allow getting larger objects on the ship. This can be done with focal reducers. Unfortunately focal reducers not only reduce the effective focal length of the telescope, but they reduce the circle of illumination of the telescope. The reduced circle of illumination causes vignetting and often serious degradation of the image quality near the outer edges of the image. The full list is divided into groups with comments about a suitable focal length to go with various telescope focal lengths and cameras. I generally like to suggest “filling the chip” so as to make good use of the pixel array and get the best possible resolution. (At the very end of the list is a short discussion of what I have used as a criterion for "filling the chip.") Planets are a special case, being significantly less than 0.5 minutes of arc and are discussed separately at the end of this list.
The first set of objects is very small, up to about 1 by 1.5 arc minutes. With a 3000 mm telescope (12" f10) this yields a largest image size at the chip of only 0.9 mm. That is, about 90 pixels with typical 10 micron pixels, but a few more with most digital cameras. This is not a very high resolution situation but probably all that can be expected. The atmospheric resolution, under good seeing conditions, is only about 1 arc second which is about one pixel with this focal length. There are 28 objects in this category including 4 M objects. While it is difficult to get great detail in these objects with a telescope of 3000 mm focal length, it would probably be wise to use a 2X focal extender (Barlow lens) to get a bigger image on the chip. There is another imaging factor that must be considered. When using a telescope of 3000 mm with a 2X extender the effective focal length of the telescope is 6000 mm. It requires an exceptional mount to stabilize such a long focal length for imaging. These objects are tiny and quite faint and thus very difficult to image. It seems amazing that Messier was able to see the four he designated on this list. Note that all of the planets are in this range or much smaller than the smallest of these objects. Jupiter, the largest is about 40 arc seconds (0.7 arc minutes), Saturn about 20 to 30, Venus 10 to 60 and the rest very small. To image such small objects, projection imaging or a 3X to 5X focal extender is usually used to get a larger image. However the planets are quite bright and different cameras and techniques are usually used to image planets. With very dim objects, such as those listed here, there is not enough light to use projection imaging. There is no substitute for a large aperture, fast focal ratio and long focal length telescope for imaging these tiny, dim objects.
NUMBER Messier RA DEC SIZE MAG TYPE
UGC 7772 M 040 12 22.4 58
05 9.0 Double
Star
NGC
1535 04
14.4 -12 44 0.3x0.3 9.0 Planetary Nebula
NGC
6210 16
44.6 23 47 0.3x0.3 9.7 Planetary Nebula
NGC
7027 21
07.0 42 14 0.3x0.2 9.0 Planetary Nebula
NGC
6826 19
44.7 50 31 0.4x0.4 8.8 Planetary Nebula
NGC 7009 21 04.1 -11 22 0.4x0.4 8.0 Planetary
Nebula
NGC
6543 17
58.8 66 38 0.4x0.3 8.6 Planetary Nebula
NGC
7662 23
25.9 42 33 0.5x0.5 8.5 Planetary Nebula
NGC
4782 12
54.6 -12 35 0.5x0.5 12.9 Elliptical Galaxy 0
NGC
4783 12
54.6 -12 34 0.5x0.5 12.9 Elliptical Galaxy 0
NGC
2392 07
29.2 20 55 0.7x0.7 8.3 Planetary Nebula
NGC
3242 10
24.8 -18 38 0.7x0.7 8.9 Planetary Nebula
NGC
4476 12
30.0 12 20 0.7x0.4 13.3 Elliptical Galaxy 4
NGC
0040 00
12.9 72 32 1.0x0.7 10.5 Planetary Nebula
NGC
4889 13
00.1 27 59 1.0x0.6 13.2 Elliptical Galaxy 4
NGC
6729 19
01.8 -36 58 1.0x1.0 ? Nebula
NGC 6994 M 073 20 58.9 -12
39 1.0x1.0 10.0 Open Cluster
NGC
7006 21
01.4 16 12 1.0x1.0 11.5 Globular Cluster
NGC
0604 01 34.5 30 46 1.0x1.0 ? Nebula
NGC
2346 07
09.3 00 49 1.0x0.9 10.0 Planetary Nebula
NGC
4413 12
26.5 12 36 1.1x0.7 13.2 Barred Spiral a
NGC
2438 07
41.9 -14 43 1.1x1.1 11.0 Planetary Nebula
NGC
0278 00
52.0 47 34 1.2x1.2 11.6 Spiral Galaxy c
NGC 6720 M 057 18 53.6 33 02 1.3x1.0 9.0 Planetary
Nebula
NGC
5367 13
57.7 -39 59 1.3x1.0 10.0 Nebula
NGC 4435 12 27.7 13 04 1.4x0.9 11.8 Elliptical
Galaxy 4
NGC 0650-1 M 076 01
41.9 51 34 1.5x0.7 10.0 Planetary Nebula
NGC
4459 12
29.0 13 58 1.5x1.0 11.7 Spiral Galaxy 0
The next batch of objects goes up to about 3 arc minutes in extent. Even these objects create quite a small image with the 3000 mm telescope. But these images are up to 200 pixels in size. This is a good sized image on an ST-7 size chip which is about 500 by 750 pixels. With modern larger chips especially those with small pixels in the 6 micron range it is possible to image these objects even though the resolution is not ideal. Pictures taken with this resolution are nice to look at but still do not use up the chip area available and are nowhere near photographic quality. There are 39 objects in this group. Only 5 are M objects.
NGC
4479 12
30.3 13 35 1.5x1.5 12.5 Spiral Galaxy b
NGC
4105 12
06.7 -29 47 1.5x1.5 12.0 Elliptical Galaxy 3
NGC
4478 12
30.3 12 19 1.0x1.8 12.4 Elliptical Galaxy 1
NGC
2782 09
14.1 40 07 1.8x1.6 12.4 Spiral Galaxy b
NGC
4106 12
06.8 -29 48 1.0x1.8 12.0 Elliptical Galaxy 2
NGC 2732 09 13.4 79 11 1.8x0.8 11.9 Spiral
Galaxy c
NGC
4387 12
25.7 12 49 1.9x1.1 12.0 Elliptical Galaxy 5
NGC
4458 12
29.0 13 15 1.9x1.8 12.0 Elliptical Galaxy 0
NGC
2261 06
39.1 08 43 2.0x2.0 10.0 Nebula
NGC
2419 07
38.2 38 53 2.0x2.0 11.0 Globular Cluster
NGC 4374 M 084 12 25.1 12 53 2.0x1.8 10.5 Elliptical
Galaxy 1
NGC
4402 12
26.1 13 07 2.0x0.8 13.0 Spiral Galaxy b
NGC
4425 12
27.2 12 44 2.0x0.5 12.9 Spiral Galaxy 0
NGC
4461 12
29.1 13 11 2.0x1.0 12.2 Spiral Galaxy 0
NGC
4473 12
29.8 13 25 2.0x1.0 11.3 Elliptical Galaxy 4
NGC 4621 M 059 12 42.0 11 39 2.0x1.5 11.0 Elliptical
Galaxy 3
NGC
6302 17
13.9 -37 07 2.0x1.0 ? Nebula
NGC
5195 13
30.0 47 16 2.0x1.5 11.0 Peculiar Galaxy
NGC
1999 05
36.5 -06 43 2.0x2.0 9.0 Nebula
NGC
6638 18
31.0 -25 30 2.0x2.0 9.5 Globular Cluster
NGC
6642 18
31.8 -23 29 2.0x2.0 8.0 Globular Cluster
NGC
6652 18
35.8 -33 00 2.0x2.0 8.5 Globular Cluster
NGC
6934 20
34.1 07 24 2.0x2.0 9.0 Globular Cluster
NGC
6569 18
13.7 -31 49 2.0x2.0 10.0 Globular Cluster
NGC
7335 22
37.3 34 27 1.7x0.8 14.7 Galaxy so
NGC 3379 M 105 10 47.8 12 35 2.1x2.0 10.6 Elliptical
Galaxy 1
NGC
3389 10
48.4 12 32 2.2x1.0 12.2 Spiral Galaxy c
NGC
4567 12
36.5 11 15 2.4x1.6 12.0 Elliptical Galaxy s1
NGC
5908 15
16.7 55 25 2.4x0.4 13.0 Spiral Galaxy b
NGC 1068 M 077 02 42.7 -00 01 2.5x1.7 10.0 Spiral
Galaxy b
NGC 3587 M 097 11 14.9 55 02 2.5x2.5 11.0 Planetary
Nebula
NGC
4038 12
01.9 -18 52 2.5x2.5 11.0 Peculiar
NGC
4039 12
01.9 -18 53 2.5x2.0 12.0 Peculiar
NGC
4697 12
48.6 -05 48 2.5x1.3 10.5 Elliptical Galaxy 3
NGC
2207 06
16.4 -21 22 2.5x1.5 12.3 Spiral Galaxy c
NGC 3077 10 03.4 68 44 2.6x1.9 11.0 Elliptical
Galaxy 2
NGC
1275 03
19.8 41 31 2.6x1.0 11.6 Syfert Galaxy
NGC
2748 09
13.7 76 29 2.8x1.1 11.7 Spiral Galaxy c
NGC
3513 11
03.8 -23 15 2.9x2.3 11.5 Spiral Galaxy c
The next group of objects is up to 3.6 arc minutes in size. This corresponds to an image of almost 400 pixels in size. These objects really fill the ST-7 chip using the full focal length of the 12" f10 which is again 3000 mm. Even the 10" f10 with a focal length of 2400 mm gives a reasonable image size and might be considered for imaging these objects. There are 21 objects in the group but only 8 of them M objects. Again, the digital camera chips of about 15 mm by 23mm will show a small inage, but of reasonable resolution. A 2400 mm focal length telescope still requires a very good mount to hold it steady.
NGC 4382 M
085 12 25.3 18 11 3.0X2.0 10.5 Spiral
Galaxy 0
NGC 4406 M 086 12 26.2 12 56 3.0X2.8 10.5 Elliptical
Galaxy 3
NGC 4486 M 087 12 30.8 12 23 3.0x3.0 8.6 Elliptical
Galaxy 1
NGC 4649 M 060 12 43.6 11 33 3.0x2.5 10.0 Elliptical
Galaxy 1
NGC 6171 M 107 16 32.5 -13 03 3.0x3.0 8.1 Globular
Cluster
NGC 6864 M 075 20 06.1 -21 55 3.0x3.0 8.0 Globular
Cluster
NGC 6981 M 072 20 53.5 -12 33 3.0x3.0 8.6 Globular
Cluster
NGC
3626 11
20.1 18 21 3.0x2.0 10.9 Spiral Galaxy b
NGC
5824 15
04.0 -33 05 3.0x3.0 9.5 Globular Cluster
NGC
6441 17
50.2 -37 03 3.0x3.0 8.0 Globular Cluster
NGC
6624 18
23.7 -30 22 3.0x3.0 8.5 Open Cluster
NGC 6712 18 53.0 -08 43 3.0x3.0 9.0 Globular
Cluster
NGC
7339 22
37.8 23 47 3.0x0.9 12.1 Spiral Galaxy c
NGC
5248 13
37.6 08 53 3.2x1.4 11.0 Spiral Galaxy c
NGC
4298 12
21.5 14 36 3.2x1.9 11.4 Spiral Galaxy c
NGC
4710 12
49.6 15 10 3.4x0.5 12.0 Spiral Galaxy 0
NGC
0185 00
38.5 48 21 3.5x2.8 11.8 Elliptical galaxy 1
NGC
7479 23
04.9 12 19 3.2x3.5 11.8 Barred Galaxy b
NGC
6229 16
47.0 47 32 3.5x3.5 8.9 Globular Cluster
NGC 0221 M 032 00 42.7 40 52 3.6x3.1 9.5 Elliptical
galaxy 2
NGC
5694 14
39.6 -26 32 3.6x3.6 10.2 Globular Cluster
The next group of objects goes up in size to 5.5 arc minutes. Now the largest of these images is a full 480 pixels in size and will require a slightly larger imaging chip like that in the ST-8. These images will of course fit well onto the typical digital camera chip. With a 12” f 10 telescope, a focal reducer of about 0.63 strength migh be a good attachment to shorten the effective focal length a bit. The objects also give a nice sized image with the 10" f10. (2400 mm focal length) There are 37 objects in this group with 13 of them being M objects.
NGC
0246 00
47.1 -11 53 4.0x2.5 8.5 Planetary Nebula
NGC
3115 10
05.3 -07 43 4.0x1.0 10.0 Elliptical Galaxy 7
NGC 3351 M 095 10 43.9 11 42 4.0x3.0 11.0 Barred
Spiral c
NGC
3384 10
48.3 12 38 4.0x2.0 11.0 Elliptical Galaxy 7
NGC
4438 12
27.8 13 00 4.0x1.5 11.0 Spiral Galaxy a
NGC 4472 M 049 12 29.8 07 59 4.0x3.4 10.1 Elliptical
Galaxy 3
NGC
4477 12
30.0 13 38 4.0x3.5 10.4 Barred Spiral a
NGC 4579 M 058 12 37.6 11 48 4.0x3.5 10.5 Spiral
Galaxy b
NGC 6333 M 009 17 19.1 -18 31 4.0x4.0 8.0 Globular
Cluster
NGC 6637 M 069 18 31.4 -32 21 4.0x4.0 7.5 Globular
Cluster
NGC 6681 M 070 18 43.3 -32 18 4.0x4.0 8.0 Globular
Cluster
NGC
0559 01
29.5 63 18 4.0x4.0 9.5 Open Cluster
NGC
7235 22
12.5 57 15 4.0x4.0 9.0 Open Cluster
NGC
6603 18
18.4 -18 26 4.0x4.0 11.4 Open Cluster
NGC
4449 12
28.2 44 05 4.2x3.0 10.5 Irregular Galaxy
NGC
7332 22
37.4 23 47 4.2x1.3 11.0 Spiral Galaxy c
NGC
4111 12
07.1 43 04 4.4x0.9 10.7 Spiral Galaxy c
NGC 4254 M 099 12 18.8 14 25 4.5X4.0 10.4 Spiral
Galaxy c
NGC
2775 09
10.3 07 02 4.5x3.0 10.3 Spiral Galaxy b
NGC 4302 12 21.7 14 36 4.5x0.5 12.9 Spiral
Galaxy c
NGC
5005 13
10.8 37 03 4.7x1.6 10.8 Spiral Galaxy b
IC
2233 08
14.2 45 44 4.7x0.6 13.0 Spiral Galaxy c
NGC 1952 M 001 05 34.5 22 01 5.0x3.0 9.0 Nebula
NGC
4388 12
25.8 12 39 5.0x1.0 12.0 Barred Spiral b
NGC 4736 M 094 12 51.0 41 07 5.0x3.5 8.9 Spiral
Galaxy b
NGC 6779 M 056 19 16.5 30 10 5.0x5.0 8.0 Globular
Cluster
NGC
2129 06
01.1 23 18 5.0x5.0 7.0 Open Cluster
NGC
5466 14
05.4 28 32 5.0x5.0 9.0 Globular Cluster
NGC
5986 15
46.1 -37 46 5.0x5.0 8.0 Globular Cluster
NGC 6520 18 03.5 -27 54 5.0x5.0 9.0 Open
Cluster
NGC
0779 01
59.7 -05 57 3.0x5.0 11.8 Spiral Galaxy b
NGC
4490 12
30.7 41 38 5.0x2.0 10.1 Spiral Galaxy c
NGC 4321 M 100 12 22.9 15 49 5.2x5.0 10.4 Spiral
Galaxy c
NGC 5866/79 M 102? 15
06.5 55 45 5.2x2.3 10.0 Elliptical Galaxy
NGC 4548 M 091 12 35.5 43 09 5.4x4.4 10.2 Spiral
Galaxy c
NGC
7814 00
03.3 16 09 1.5x5.5 11 Spiral Galaxy
b
NGC
3511 11
03.4 -23 05 5.5x1.0 11.0 Spiral Galaxy c
The next group of objects goes up to 9.9 arc minutes in size. This is 870 pixels with the 12" f10. Clearly these objects give images that no longer fit on the ST-7 chip at all and a focal reducer is required. At this point it is good to consider the ST-8 which has a chip of twice the size and four times the area. The 10" f10 (2400 mm) now gives the excellent sized images up to 580 pixels. But the 10" f6.3 looks like a good choice as well, giving images up to about 360 pixels. With the 10" f 6.3 scope and the ST-8 a very comfortable match is had. Again, the digital camera will handle this image size easily and is a good choice for this group with a 10“ f 6.3 telescope of 1600 mm focal length. This is a very large group of 67 objects with no less than 35 M objects. This group fives image sizes that start to look very good for a digital camera with a 15 mm by 23 mm chip or one of the larger SBIG cameras like the 8, 10 or so. There are a wonderful set of objects in this group.
NGC 4303 M
061 12 22.0 04 28 5.7x5.5 10.2 Spiral
Galaxy c
NGC 4501 M 088 12 32.0 14 25 5.7x2.5 10.5 Spiral
Galaxy b
NGC
6934 20
34.2 07 24 5.9x5.9 8.9 Globular Cluster
NGC
5377 13
56.3 47 12 3.0x6.0 12.0 Spiral Nebula a
NGC
2362 07
18.7 -24 57 6.0x6.0 4.0 Open Cluster
NGC 3368 M 096 10 46.8 11 49 6.0x4.0 10.2 Spiral
Galaxy b
NGC 6266 M 062 17 01.3 -30 07 6.0x6.0 6.5 Globular
Cluster
NGC 6273 M 019 17 02.6 -26 15 6.0x6.0 7.0 Globular
Cluster
NGC 6402 M 014 17 37.6 -03
15 6.0x6.0 9.0 Globular Cluster
NGC 6626 M 028 18 24.6 -24 52 6.0x6.0 8.0 Globular
Cluster
NGC 6715 M 054 18 55.2 -30 28 6.0x6.0 9.0 Globular
Cluster
NGC 6838 M 071 19 53.7 18 47 6.0x6.0 9.0 Globular
Cluster
NGC 7099 M 030 21 40.3 -23 11 6.0x6.0 8.0 Globular
Cluster
NGC
7793 23
57.9 -32 34 6.0x4.0 9.7 Spiral Galaxy
NGC
1300 03
19.8 -19 24 6.0x3.2 11.3 Barred Spiral b
NGC
6819 19
41.3 40 13 6.0x6.0 10.0 Open Cluster
NGC
3432 10
52.5 36 37 6.2x1.7 10.6 Spiral Galaxy c
NGC 3992 M 109 11 57.6 53 22 6.4x3.5 10.9 Barred
Spiral b
NGC
0147 00
33.1 48 31 6.5x3.8 12.1 Elliptical galaxy 4
NGC 1982 M 043 05 35.6 05 16 7.0x5.0 8.0 Nebula
NGC 4569 M 090 12 36.8 13 09 7.0x2.5 9.0 Spiral
Galaxy b
NGC 4594 M 104 12 39.9 -11 37 7.0x1.5 8.2 Spiral
Galaxy a
NGC 6093 M 080 16 17.1 -22 59 7.0x7.0 8.0 Globular
Cluster
NGC 6613 M 018 18 19.9 -17 08 7.0x7.0 8.0 Open
Cluster
NGC 6913 M 029 20 24.0 38 31 7.0x7.0 7.0 Open
Cluster
NGC 7089 M 002 21 33.5 -00 50 7.0x7.0 6.0 Globular
Cluster
NGC
6885 20
12.0 26 29 7.0x7.0 5.7 Open Cluster
NGC
2506 08
00.2 -10 47 7.0x7.0 7.6 Open Cluster
NGC
6723 18
59.6 -36 38 7.0x7.0 6.0 Globular Cluster
NGC
6997 20
56.5 44 39 7.0x7.0 10.0 Open Cluster
NGC 1904 M 079 05 24.3 -24 31 7.5x7.5 8.4 Globular
Cluster
NGC 4826 M 064 12 56.8 21 41 7.5x3.5 8.6 Spiral
Galaxy a
NGC
1055 02
41.8 42 21 7.6x3.0 10.6 Spiral Galaxy c
NGC
3079 10
02.0 55 41 7.6X1.7 10.6 Spiral Galaxy c
NGC 3556 M 108 11 11.6 55 41 7.8x1.4 10.8 Spiral
Galaxy c
NGC 3623 M 065 11 18.9 13 07 7.8x1.6 10.3 Spiral
Galaxy c
NGC
5746 14
44.9 01 57 7.9x1.7 10.6 Spiral Galaxy c
NGC 0205 M 110 00 40.3 41 41 8.0x3.0 10.8 Elliptical
galaxy 6
NGC 0581 M 103 01 33.2 60 42 8.0x8.0 8.0 Open
Cluster
NGC 2068 M 078 05 46.8 00 03 8.0x6.0 8.0 Nebula
NGC 3034 M 082 09 56.1 69 42 8.0x3.0 9.2 Irregular
Galaxy
NGC 3627 M 066 11 20.2 13 01 8.0x2.5 9.7 Spiral
Galaxy b
NGC 6254 M 010 16 57.1 -04 07 8.0x8.0 7.0 Globular
Cluster
NGC 6341 M 092 17 17.1 43 09 8.0x8.0 6.5 Globular
Cluster
NGC 6853 M 027 19 59.6 22 43 8.0x6.0 8.0 Planetary
Nebula
NGC
6946 20
35.0 60 08 8.0x8.0 11.1 Spiral Galaxy c
NGC
1365 03
33.7 -36 08 8.0x3.5 11.2 Spiral Galaxy b
NGC
1502 04
07.4 62 19 8.0x8.0 7.0 Open Cluster
NGC
2627 08
37.3 -29 56 8.0x8.0 9.0 Open Cluster
NGC
6866 20 03.7 44 10 8.0x8.0 8.0 Open
Cluster
NGC
6939 20
31.4 60 38 8.0x8.0 10.0 Open Cluster
NGC 4192 M 098 12 13.8 14 54 8.2x2.0 11.0 Spiral
Galaxy b
NGC
4216 12
15.9 13 09 8.3x2.2 10.0 Spiral Galaxy c
NGC
2683 08
52.7 33 25 8.4x2.4 9.7 Spiral Galaxy c
NGC
5897 15
17.4 02 05 8.5x8.5 9.0 Globular Cluster
NGC 0628 M 074 01 36.7 15 47 9.0x9.0 9.0 Spiral
Galaxy c
NGC
1097 02
46.4 -30 16 9.0x5.5 10.6 Barred Spiral b
NGC 4590 M 068 12 39.5 -26 45 9.0x9.0 8.0 Globular
Cluster
NGC 5055 M 063 13 15.7 42 01 9.0x4.0 9.8 Spiral
Galaxy b
NGC 6694 M 026 18 45.2 -09 24 9.0x9.0 9.5 Open
Cluster
NGC
1857 05
20.1 39 21 9.0x9.0 7.0 Open Cluster
NGC
6649 18
33.5 -10 24 9.0x9.0 9.0 Open Cluster
NGC
2818 09
16.0 -36 37 9.0x9.0 11.0 Open Cluster
NGC
7788 23
56.7 61 24 9.0x9.0 9.4 Open Cluster
NGC
4762 12
52.9 11 14 9.1x2.2 10.3 Spiral Galaxy c
NGC
1560 04
32.8 71 53 9.2x1.7 11.5 Spiral Galaxy c
NGC 4517 12 32.8 00 07 9.9x1.4 10.4 Spiral
Galaxy c
The next group has sizes up to 14 arc minutes. With this group shorter focal lengths are required. A 10” f 6.3 scope of 1600 mm or even a scope in the 1000 mm range is a good choice. Here is where the ST-8 chip size is a clear advantage. Objects at the end of this part of the list can now be imaged nicely on the larger SBIG chips or with a typical digital camera chip of 15 mm by 23 mm. There are 35 objects in this group with 11 of them being M objects.
NGC
0457 01
19.0 58 20 10x10 7.0 Open Cluster
NGC
2023 05
41.7 -02 13 10x10 ? Nebula
NGC 2323 M 050 07 02.9 -08 20 10x10 6.0 Open
Cluster
NGC
2360 07
17.7 -15 38 10x10 9.0 Open Cluster
NGC
4559 12
36.0 27 57 10x3 10.5 Spiral Galaxy
c
NGC 5024 M 053 13 12.9 18 10 10x10 8.0 Globular
Cluster
NGC 5194/5 M 051 13
29.9 47 12 10x5 8.1 Spiral
Galaxy c
NGC 5236 M 083 13 37.1 -29 52 10x8 8.0 Spiral
Galaxy c
NGC 6218 M 012 16 47.2 -01 57 10x10 8.0 Globular
Cluster
NGC 6531 M 021 18 04.8 -22 30 10x10 7.0 Open
Cluster
NGC 7078 M 015 21 30.0 12 10 10x10 6.5 Globular
Cluster
NGC
7331 22
37.1 34 26 10x3 10.4 Spiral Galaxy
b
NGC
7635 23
20.7 61 10 10x5 11.0 Nebula
NGC
0288 00 52.6 -26 36 10x10 7.2 Globular
Cluster
NGC
1807 05
10.7 16 32 10x10 7.5 Open Cluster
NGC
6530 18
04.7 -24 20 10x10 6.0 Open Cluster
NGC
6645 18
32.7 -16 54 10x10 9.0 Open Cluster
NGC
4395 12
25.9 33 32 10x8 11.0 Spiral Galaxy
NGC
0663 01
46.0 61 16 11x11 7.0 Open Cluster
NGC
2903 09
32.1 21 31 11x5 9.7 Spiral
Galaxy b
NGC 0891 02 22.4 42 41 12x1 12.2 Spiral
Galaxy b
NGC 1960 M 036 05 36.2 34 09 12x12 6.3 Open
Cluster
NGC
3628 11
20.3 13 37 12x2 10.3 Spiral Galaxy
b
NGC 6705 M 011 18 51.1 -06 16 12x12 6.0 Open
Cluster
NGC 7654 M 052 23 24.2 61 36 12x12 7.0 Open
Cluster
IC
5146 21
53.5 47 16 12x12 10.0 Nebula
IC
1613 01
04.8 02 07 12x11 9.0 Irregular
Galaxy
NGC
1513 04
09.9 49 31 12x12 9.0 Open Cluster
NGC
1893 05
22.6 33 24 12x12 8.0 Open Cluster
NGC
6709 18
51.5 10 21 12x12 8.0 Open Cluster
NGC
5907 15
15.9 56 20 12x2 10.4 Spiral Galaxy
c
NGC
4631 12
42.2 32 33 12x1 9.7 Spiral
Galaxy c
NGC
4244 12
17.5 37 48 13x1 10.7 Spiral Galaxy
b
NGC 5904 M 005 15 18.5 02 05 13x13 6.2 Globular
Cluster
NGC
0225 00
43.8 61 46 14x14 8.0 Open Cluster
For the next set of objects which goes up to 29 arc minutes, the images are 15 mm with the 12" f10, 10 mm with the 10" f10 and 6.6 mm with the 10" f6.3. These are all good candidates for the ST-8 and larger chips and definitely for the digital cameras. A 1200 mm focal length gives an image for the largest of the objects of 630 pixels. . For these larger objects it is time to consider one of the excellent refractors in the 800 to 1000 mm range. A Takahashi FSQ 106 is a good choice. Even a good telephoto lens mounted directly on the CCD imager might be a suitable choice. It becomes more and more necessary to consider but 1000 mm optics as we move toward the larger objects. There are an astonishing 50 objects in this group with 19 of them being M objects.
NGC
0188 00
44.2 85 19 15x15 10.0 Open Cluster
NGC 2682 M 067 08 51.0 11 49 15x15 7.0 Open
Cluster
NGC 4565 12 36.4 25 59 15x1 10.5 Spiral
Galaxy b
NGC
1817 05
12.1 16 42 15x15 8.0 Open Cluster
NGC
2281 06
49.3 41 04 15x15 6.0 Open Cluster
NGC
6811 19
38.2 46 34 15x15 9.0 Open Cluster
NGC
6809 19
40.1 -30 56 15x15 7.0 Globular
Cluster
NGC
2403 07
36.8 65 36 16x10 8.8 Spiral Galaxy
c
NGC
7293 22
29.7 -20 51 12x16 6.5 Planetary
Nebula
NGC
7790 23
58.4 61 13 17x17 13.0 Open Cluster
NGC
0247 00
47.1 -20 45 18x5 10.7 Spiral Galaxy
c
NGC 2447 M 093 07 44.5 -23 52 18x18 7.0 Open
Cluster
NGC 3031 M 081 09 55.6 69 04 18x10 8.0 Spiral
Galaxy a
NGC 5272 M 003 13 42.2 28 23 18x18 6.0 Globular
Cluster
NGC 6656 M 022 18 36.4 -23 56 18x18 6.0 Globular
Cluster
NGC
6888 20
12.5 38 25 18x12 ? Nebula
NGC
7023 21
02.0 68 10 18x18 ? Nebula
IC
0342 03
46 68 06 18x17 9.2 Barred
Spiral c
NGC
6664 18
36.7 -08 14 18x18 6.0 Open Cluster
NGC 4258 M 106 12 19.0 47 18 19x6 9.0 Spiral
Galaxy b
NGC
4656 12
44.0 32 10 19x2 11.0 Irregular
Galaxy
NGC 1039 M 034 02 42.0 42 47 20x20 6.0 Open
Cluster
NGC 1912 M 038 05 28.7 35 51 20x20 6.2 Open
Cluster
NGC
2024 05
41.9 -01 50 20x20 ? Nebula
NGC 2099 M 037 05 52.3 32 34 20x20 6.2 Open
Cluster
NGC 2422 M 047 07 36.6 -14 30 20x20 5.0 Open
Cluster
NGC 6121 M 004 16 23.7 -26 31 20x20 7.4 Globular
Cluster
NGC
6822 19
44.9 -14 46 20x10 10.0 Irregular Galaxy
NGC
7243 22
15.2 49 53 20x20 8.0 Open Cluster
NGC 6809 M 055 19 40.1 -30 56 20x20 7.0 Globular
Cluster
NGC
1435 03
46.2 23 45 15x20 6.8 Nebula
NGC
1245 03
14.6 47 14 20x20 9.0 Open Cluster
NGC
6940 20
34.6 28 18 20x20 8.0 Open Cluster
NGC
6791 19
20.8 37 46 20x20 11.0 Open Cluster
NGC
7789 23
57.0 56 43 20x20 10.0 Open Cluster
NGC
0300 00
55.0 -37 42 21x14 11.3 Spiral Galaxy c
NGC
0253 00
47.6 -25 18 22x6 7.0 Spiral Galaxy
c
NGC
4236 12
16.7 69 28 22x5 10.7 Barred Spiral
NGC 5457 M 101 14 03.2 54 21 22x20 9.0 Spiral
Galaxy c
NGC 6205 M 013 16 41.7 36 27 23x23 5.7 Globular
Cluster
NGC 2437 M 046 07 41.9 -14 49 25x25 8.0 Open
Cluster
NGC
2477 07
52.3 -38 33 25x25 7.0 Open Cluster
NGC 6405 M 006 17 40.1 -32 13 25x25 6.0 Open
Cluster
NGC 6494 M 023 17 56.9 -19 01 25x25 7.0 Open
Cluster
NGC 6611 M 016 18 18.8 -13 47 25x25 6.5 Nebula
NGC
1528 04
15.2 51 15 25x25 6.0 Open Cluster
NGC
2546 08
12.4 -37 38 25x25 8.0 Open Cluster
NGC
2264 06
41.2 09 53 15x26 5.0 Open Cluster
NGC
0281 00
53.3 56 35 23x27 8.0 Nebula
NGC 6514 M 020 18 01.9 -23 02 29x27 8.5 Nebula
For the next group of objects, up to 60 arc minutes in size, one must use telescopes with shorter focal lengths. There are only 19 objects in this group with 8 being M objects. It is an important group of bright objects of great interest. These objects, because of their size, have great detail and are the focal point of many photographic and imaging techniques. We might note for reference that the moon and sun are about 30 arc minutes, right between this group and the one above. Telescopes like the 500 to 800 mm high quality refractors are the best choice for these objects. Typical SCT scope are too long in focal length and when reduced have too small a circle of illumination and poor edge of image quality.
NGC 2168 M
035 06 08.8 24 20 30x30 5.5 Open
Cluster
NGC 2287 M 041 06 47.0 -20 45 30x30 6.0 Open
Cluster
NGC 7092 M 039 21 32.2 48 26 30x30 5.0 Open
Cluster
IC
0405 05
16.2 34 16 30x19 6.0 Nebula
NGC 0869 02 19.0 57 09 35x35 4.4 Open
Cluster
NGC
0884 02
22.4 57 07 35x35 4.7 Open Cluster
NGC 6634 M 025 18 31.7 -19 15 35x35 6.0 Open Cluster
NGC
0055 00
15 -39 13 25x40 7.8 Irregular
galaxy
NGC
2244 06
32.4 04 52 40x40 5.5 Open Cluster
NGC 2548 M 048 08 13.7 -05 47 40x40 5.5 Open
Cluster
NGC
1647 04
46.1 19 05 40x40 6.5 Open Cluster
NGC
1977 05
35.5 -04 52 40x25 7.0 Nebula
NGC
2174 06
09.7 20 30 40x30 ? Nebula
NGC
0752 01
57.8 37 41 45x45 7.5 Open Cluster
NGC 6618 M 017 18 20.9 -16 11 45x35 6.0 Nebula
NGC
1746 05
03.6 23 48 45x45 6.0 Open Cluster
Sh2-155 22 56.8 62 37 50x10 7.7 Nebula
NGC 0598 M 033 01 33.9 30 39 60x60 5.3 Spiral
Galaxy c
NGC 6475 M 007 17 54.0 -34 49 60x60 5.0 Open
Cluster
The final group of objects is definitely candidates for the largest CCD chips and digital cameras. Even then short focal length telescopes and good telephoto lenses are a best choice. This group holds the really giant objects, those well over 1 degree in size. To fit M 31 on an ST-10 chip requires a focal length of only 180 to 200 mm. These objects are good size for a digital camera with a 200 to 400 mm lens. There are only 12 objects in the giant object group but 6 are well known M objects.
NGC 1976 M
042 05 35.4 -05 23 65x65 4.0 Nebula
NGC
6960 20
45.7 30 43 70x6 8.0 Nebula
NGC
6992-5 20
56.4 31 42 78x8 8.0 Nebula
NGC
2237-9 06
32.3 05 03 60x80 9.0 Nebula
NGC 2632 M 044 08 40.4 19 41 80x80 4.5 Open
Cluster
NGC 6523 M 008 18 04.7 -24 20 80x40 5.0 Nebula
NGC 6603 M 024 18 18.4 -18 26 60x90 4.5 Open
Cluster
M 045 03 46.9 24 07 100x100 1.4 Open
Cluster
NGC
7000 20
58.8 44 20 100x100 5.0 Nebula
NGC
1499 04
3.4 36 25 145x40 6.0 Nebula
NGC 0224 M 031 00 42.7 41 16 150x50 4.0 Spiral
galaxy b
IC
1396 21
39.1 57 30 140x170 Nebula
As I review this list, I wonder at the great variety and size of the non-stellar objects in the sky. Looking at the distribution of the Messier objects among them I wonder at how Messier saw some of them at all and how he missed others. It is a wondrous list. The list also demonstrates the difficulty of imaging all of them with a single telescope or camera. Just on the basis of size, it becomes clear that one selection of optics or one imager will not capture all of them equally well. In fact the range is so great that if imaging is a goal, the selection of the base telescope will have to be adjusted to cover with facility the range of objects of greatest interest.
This review of object sizes should now have shown that any one telescope and any one chip size will not do an optimum job with all of the interesting objects on even this small list. It would require a telescope of 1600 to 3000 mm to do the smaller group. In order to extend the range of objects covered by a single telescope, imagers often choose the scope to be rather too long in focal length and then to shorten it with focal reducers. Unfortunately the focal reducer also constricts the circle of illumination and introduces severe optical aberrations at the edges of the image. Thus I recommend it best to choose a telescope of shorter focal length in the first place. It seems that amateurs have aperture fever and think the best scope is the biggest one. This is often not the case. The best scope is the one that covers you range of needs. If you are interested in wider field images or images of the groups of larger objects, it may well be a medium to short focal length refractor that is the best choice.
What I am trying to encourage is for imagers to think about the options. The only things that determine you actual field of view are the focal length of the telescope and the size of your chip. You take the size of the chip in mm and divide it by the focal length of your telescope in mm. Then multiply the result by 57 to change radians into degrees. You can multiply again by 60 to get the result in arc minutes. Then you can window your actual field of view, in arc minutes, on the above list and see which objects you can get on the chip and which will give images that are too tiny and thus have poor pixel resolution.
In an effort to give some examples it turns out that there are just too many cameras and telescopes to exemplify many of them.
Thus cameras will be put roughly into four chip sizes Small chips like the ST-7 and some webcams and guider chips which are about
4 mm by 7 mm. Medium chips such as those in the ST-8, ST-10 and ST-2000 which are about 10 mm by 15 mm in size. Large chips which
are 15 mm by 22 mm such as are in some of the large format SBIG cameras and in the common Canon digital cameras. The final category
are Giant chips such as are in the largest SBIG cameras and the full format Canon digital cameras where the chips are about 24
mm by 36 mm.
The number of pixels run from 400,000 in the Small chips to several million in the Medium and Large chips. The Giant chips
are often 6 to 8 or even 16 million pixels. A few of the SBIG cameras have color chips; but all of the Canon digital cameras
have color chips. The costs of these cameras has some correlation with the chips size. The CCD cameras run from $1500 to about
$10,000. The Canon digital cameras run from $800 to $8000. Thus there is a vast range of cameras, chip sized, pixel densities
and costs. The examples here take into account only the issue of what chip sizes go well with what focal length telescopes so
that certain groups of object sizes can be imaged on the chip. More detailed calculations for specific chip sizes and telescope
focal lengths can easily be done using the formula given
above.
In the examples the chip is filled but with some room around the main object say for artistic reasons. This is my definition
of “filling the chip.” For an example, take the large group of objects in the list above that are in the 5 to 10
arc second size range. This is an interesting group which contains about 67 objects, 35 of which are the popular M objects. What
would be a good choice of telescope focal length and chip size for this set of objects. Even with a limited group of object sizes
it is apparent that the ideal focal length depends on the chip size and vice versa. Let us take a CCD camera of the Medium size
like the ST-8 or ST-10. These are quite popular sizes these days. Using a circle of interception for the chip of about 8 mm and
a size for the largest objects of 12 arc minutes, we find that a telescope of focal length of about 2200 mm is about right. Note
the example allows for some clearance for getting the object on the chip. But if we move down to another very nice group of objects
which are 15 mm to 30 mm in size, we need a telescope of about 800 mm. Now the problem of filling the chip with a great variety
of object sizes becomes obvious. It will be necessary to choose the shorter focal length telescope to get the larger objects
and
accept the smaller objects to simply have smaller images on the chip.
Digital SLR cameras which have chips of about 15 mm by 23 mm when used for the same two groups of objects as used in the above
example would require a telescope focal length of 4000 mm for the smaller sized objects and 1400 mm for the larger sized objects.
One can see that some of the larger telescopes which might be in the range of a 14 inch f 10 (3500 mm) or a 16 inch f 10 (4000
mm) require the larger chips that the digital SLR or the large format CCD cameras have in them. Those who use something as large
as a 20 inch f 8 telescope
also find that they are working at 4000 mm focal length and can well use the
larger chipped cameras.
It becomes apparent that those wishing to image the larger sets of objects in the final two groups with even Large CCD cameras
need to go to shorter focal length telescopes. When imaging the last group with a large chip it is possible to use 300 mm or
even 150 mm photographic lenses. The object of these exercises is to point out that a first and important issue to consider is
the focal length of the telescope you choose for imaging and its relationship to the chip size of your camera. This may not,
usually does not, dictate getting the largest aperture telescope. In fact for imaging a shorter faster telescope is usually a
better choice.
A brief not on very tiny objects is required as well. Planets are very tiny. They subtend under 40 arc seconds but are fortunately
very bright. In this case it is useful to have a focal length of as much as 15,000 mm to 40,000 mm. Wow, that is at the far opposite
end of the focal length spectrum. In the case of planets then a 2000 mm to 3000 mm can be used with a 5 power tele-extender like
the TeleVue power mate. It is almost impossible to get a mount steady enough for such a long focal length. Also the focal ratio
of the telescope goes to numbers like f 50. But since the planets are bright, it is still possible to image planets effectively
with
rapid image capture cameras. Much
excellent planetary work has been done with web cams.
Again it can be seen that there is no one telescope that is suitable for all imaging needs. To cover all situations, two or possibly three telescopes are required. A summary of the situation is to pay special attention to the size of the imaging chip and the focal length of the telescope chosen for your imaging needs.
Books which discuss many aspects of astrophotography are listed in the Bibliography/Applications section on this web site.