Buying a telescope for Astrophotography

[benklerk]

HI

I would like some comments on getting a Celestron 9.25" or 11".

1) What is the difference in between the two, I would like to get into taking deep sky objects, comets and planets. I know I need a guide scope. I do know you need to spend a few hrs doing stacking on the photos, does size make a difference in the end result?

2)What is the difference in between the CGEM and CGEM DX mount for astrophotography. Their is a few hundred dollar difference.

3) I do have a few barlow lens from my old 6" scope, a 2x and 3x, do people use them on their CCD cameras?

4) I live in Katoomba in the blue mountains, west of Sydney, so I do have ok dark skies.

5) If their is no difference between the two, is it better for me to get the 9.25" EdgeHD? As almost the same price as a normal 11".

I hope a have covered the basics.

Ben

[Shiraz]

Hi Ben
1. from images published here and elsewhere, both are good scopes. C11 has better resolution than C925 for planetary work, but most of the time the atmospheric seeing will ensure that there will not be whole lot of difference. Deep sky capabilities of either will be a bit limited by the long focal lengths - a focal reducer would probably be a good idea with most astro cameras.
2. get the heaviest/most accurate mount you can afford.
3. most of the time you will not need a Barlow, but for planetary imaging, a good quality ~3x Barlow is needed - get a good one.
5. edge seems to be a successful technology, but pricy. if you go that way, check that you can get a compatible focal reducer.

If you are set on Celestron, maybe a reasonable starting point would be a DX mount with a 925 (edge if affordable and if a focal reducer is available). that should give you a versatile system. Regards ray

[Gem]

C9.25 is a great scope for planetary. Despite the C11 being obviously larger, many people comment that the C9.25 has a slightly better design for planetary ( but I forget the details why).

The only problems with both are the their long focal lengths. It is easier imaging with a shorter focal length.

I understand that the only difference between the CGEM and CGEM DX is the DX has thicker, heavier legs. The C9.25 sits well on a CGEM even with two cameras and a guide scope. Personally, I wouldn't put a C11 on a CGEM if I was going to add a guide scope on top.

If you are going to have a large CCD chip, the EdgeHD would be great. Also, it cools faster than the standard C9.25.

You are always best to work well within your weight limits (and budgets). Better to have a 9.25 that is easy for mount than a 11" that is pushing the mount. For visual observing - have aperture fever. For imaging - have mount fever!

[lepton3]

Hi Ben,

As someone who has owned both a C9.25 and a C11, I would say for planets and comets, it would be the C11. The extra light grasp gives it the edge over the C9.25.

With planets, you want to keep the exposures down to less than 1/30s, preferably less than 1/60 s, which means high noise unless you have large aperture. A C14 is better, but I think a C11 is a reasonably manageable size, and works well on a CGEM for planets.

Comets are often moving reasonably fast, so you often want to keep the exposure time down to below 90s or so to avoid trailing. Again, the C11 has the edge over the C9.25 here, and 90s exposure with a small 50mm guide scope is no problem.

For portability, the C9.25 has it made. It is nice and light. The CGEM mount easily carries it along with an 80mm guide scope.

For deep space astrophotos, you probably don't want a f/10 scope of any sort. Even with a f/6.3 reducer, the Field of View is pretty small.

Using a separate guide scope, you will get differential flexure which will lead to egg shaped stars for exposures of longer than about 3 minutes. Even at f/6.3, using an off axis guider is a pain. You need a sensitive guide camera to find a guide star on all occasions.

I love my C11 on CGEM + 50mm guide scope for the moon, planets, comets, minor planets, supernovae, exoplanet transits, and photometry. It is not very good for "pretty picture" deep sky astrophotos. But luckily that is not my thing.

-Ivan

[lepton3]

Grant,

The C9.25 has a f/2.5 primary rather than the usual f/2 primary, so it has a slightly smaller secondary and the f/2.5 mirror is potentially easier to figure than the deeper f/2 design. The down side is the tube is longer (almost the same length as the C11).

The C9.25 has a sort of reputation as being in a sweet spot in terms of design tradeoffs for SCTs. Many people note that for visual observing, the image quality seems better than the usual for SCTs of similar aperture.

But for many imaging situations, sheer light gathering power dominates, and then (a good quality) C11 performs better.

Of course, being mass produced it's always possible to get a mediocre example at any size!

-Ivan