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I read some things here which I think are not correct but I will give you my point of view.
The mount:
- GEM versus Fork
The big advantage of a fork mount is that it does not need counter weights. The big disadvantage is its instability. The rule of thumb is a fork mount with the same stability of GEM mount will weight twice as much!!! ...there goes the advantage of not needing counter weights. The next thing is that fork mounts are usually Alt/Az mounts. What does it mean for observation: the image will rotate! To overcome that you can either rotate the camera (Meade offered those systems for their SC-telescopes a few years ago) or you can have a wedge to make the mount equatorial. Now you brought a lot more instability in the system.
Back to the GEM mount... the downside as mentioned before is the need of counter weights. Otherwise they are more compact compared to the fork mounts.
That is why all professional deep sky photographer use GEM mounts for their portable systems.
Another important rule of thumb for photography is you spend 2/3 of your money on the mount and 1/3 on the telescope.
-Tracking accuracy
First you start of with a mount which tracks accurate without any correction electronics (namely: all Losmandy mounts, all Takahashi mounts, Vixen: GP-DX and New Atlux and perhaps the Sphinx).
Now it is a big advantage if you have PEC system. People might say a PEC system makes every mount accurate but that is not the case. PEC systems are limited by magnitude of the worm error and its gradient. PEC systems don't work if the worm error is too large or occurs suddenly. This is the case with 80% of Chinese mounts like EQ-1to5, HEQ-5, EQ-6. Some dealer (very few) actually test the worm error of those Chinese mounts and sort them for visual use and photographic use. The once for photographic use are more expensive but still cheaper than equivalent mounts from Losmandy or Takahashi). So if you know a dealer who does that test buy your mount there even if you pay more money!!!
Once you have a stabile, accurate mount with a PEC system you can do auto guiding and you do very long exposures without any trouble. For auto guiding you need a port on your mount drive controller. Most are standardized to process signals from an ST-4. The ST-4 auto guiders are nice because you do not need a pc or notebook. Losmandy mounts have this port already. The Vixen Sphinx mount has this port too and is very compact. For the Chinese EQ mounts you need to alter your controller or get a different one. You will get this from the US. There are many other ways of auto guiding but most of them will require a computer.
The telescope:
- F ratio
Here an important rule, as smaller the F ratio as shorter the exposure time and as larger the field of view. A raw rule of thumb is one F number down shortens the exposure time by half. Also the light gathering increases with aperture. If you want to do deep sky photography you need a telescope with a small F number.
- Telescope choice
Newton telescopes are a good choice because they are cheap for the aperture you get. From F7 or F6 down you will need a coma corrector and they are not cheap.
Refractor telescopes are suitable from F7 and down but you definitely need a field flattener. The field flattener come often as flattener/reducer what brings the F number down again. If you want to do one shot colour imaging you need an apochromatic refractor but if you work with colour filters and do a composite (RGB) a simple achromatic telescope will do the job. In this case focus will differ for each colour.
SC telescopes need a reducer or some of them offer the possibility to attach the camera where normally the secondary mirror is. In this case the SC is actually a Schmidt camera. But for this the camera should not be too big (and the D20 is big).
For SC’s and Newton’s you will find that bright stars on long exposure times appear as discs rather than points (due to the central obstruction of the secondary mirror). Also with the Newton you will have spices around bright stars in the image.
For planetary photography you should have a long focal length and at least 8” aperture.
The guiding
Guiding with a second telescope is much easier than off axis guiding. For the guiding scope you will not need much aperture but you need focal length. At least the same as the telescope you use for the camera but preferably more (twice as much is a good number)
Equipment wise I suggest a Losmandy G8 or a Vixen GP-DX with Skysensor 2000 or a Vixen Sphinx for a mount. For DS photography an Orion 80ED (or Sky Watcher or Saxon) with a Televue 0.8 Reducer/Flattener should do for a start. For planetary photography a second hand Celestron C8. As guiding scope a second hand 60 F15 achromatic scope will do the job perfectly. If you shop around you might even have some spare money towards a ST-4 auto guider…
So now you can think about what you want to do…
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