C8 vs C6 for DSO astrophotography

[James]

I have been looking at telescopes and mounts for deep sky astrophotography, and for my budget, I have found two good deals:
C8 SCT f/10 and C6 SCT f/10 (C8 is 500$ more expensive)
mount: advanced VX (possibly a CGEM/CGEM II/EQ6 if possible)
guide camera: QHY5L-II monochrome
imaging camera: Pentax k100D (will upgrade into ASI 294MC pro)
skies: Bortle 9 (nearest 'dark' sky site 1 hour away,Bortle 4)
focal reducer: Celestron f/6.3 focal reducer
So, my question is, would the two scopes have any big difference, and would the 500$ gap be worth it?

[Graeme1858]

Hello James

The extra 2" will make a difference. If you can afford the $500 then definitely go for it.

Welcome to the forum.

Regards

Graeme

[JayTee]

With both scopes being f/10 they will yield the same level of brightness for a given object using the same amount of time on the object. What the extra 2" gives you is better resolution. BUT what really should determine which OTA you buy is dictated by which mount you choose to buy! For AP, you should always keep your payload weight (OTA, guide scope, cameras, etc) around 50% of its max payload. For that reason, plus personal experience, I would not put an 8" SCT on an AVX mount, but I would put it on the CGEMII or the EQ6.

You really can't go wrong with either OTA if you put it on the right mount.

Cheers,
JT

[James]

thanks for the replies! I would go for the 8" SCT If I can afford it!

[JayTee]

thanks for the replies! I would go for the 8" SCT If I can afford it!

Don't forget, it will require a more expensive mount to yield decent AP results.

Cheers,
JT

[James]

I have been wondering about another thing, would the CGEM and CGEM II have much of a difference (CGEM II is 800$ more expensive)

[JayTee]

I have been wondering about another thing, would the CGEM and CGEM II have much of a difference (CGEM II is 800$ more expensive)

Think of the CGEM as a 2012 Corolla and the CGEM II as a 2018 Corolla. Same car (mount), slightly different look, but there is a new motor and new transmission in the 2018 version (better/more precise motors and better meshing gearbox). For that reason, the CGEM is considered a very good mount for visual use but falls down when used for AP especially when you get close to your max allowed AP payload (no more than 60% of the mount's max). This is why the CGEM II is a better mount for AP and why it is around $500 more expensive. Plus it's got a "jazzy" new look!

Cheers,
JT

[James]

I'll try saving for the CGEM II. thanks!'
I recently searched for the FoV and found out Andromeda cannot be photographed with a C8 on APS-C sensor while C6 can should I go for the C8 still?

[sdbodin]

M31 is, by far, the largest galaxy in the sky. Most are less than a tenth this size. So, if all you want to image is Andromeda, then get an Redcat 250, anything else will require a much longer focal length.

Clear skies,
Steve

P.S. I image galaxies at 2500mm efl and have not run out of them yet.

[JayTee]

As Steve said, M31 is HUGE (6x bigger than the full moon)!!! If you want to thoroughly investigate what your imaging scale would be using your data and real-world DSOs, go to this site and put in your particulars. https://astronomy.tools/calculators/field_of_view/

I already did this for you so you could get an idea of what you are up against. AP is NEVER easy, and often times it isn't even fun, BUT it is always challenging!
The first image is using your Pentax K100D and the two scope you are interested in, the second image is replacing the Pentax with ASI 294MC

If you look at my signature you'll see that I have two AP scopes (this is actually pretty standard for APers). The RC is for small objects - like every other galaxy and the 80mm APO is for the larger objects like M31 and M45.

Cheers,
JT

[James]

is it a good Idea to keep a C6 as the main scope for most DSOs and an additional 60-80mm apo for M31 and 45

[JayTee]

Yes, but not necessarily acquired at the same time. I got my 80mm APO 3 years before my 6" RC. The stuff in the sky is not going anywhere so you have time.

Cheers,
JT

[James]

I was also thinking about a hyperstar however I would need to go with the C6 AVX in that case. would that be a good combination

[JayTee]

Remember that with Hyperstar, the camera is placed in front of the mirrors. Even with today's small cooled CMOS Astrocams, they will still be bigger than the secondary mirror in that 6" SCT. Meaning that they will block light coming into the OTA. IMHO Hyperstar should not be considered for OTAs smaller than 9.25". A 9.25" OTA's secondary mirror is big enough to hide the camera making Hyperstar advantageous.

Cheers,
JT

[James]

does it cover enough for the dawes limit to be over 1 arc-second?

[JayTee]

It depends on the profile of your camera. If you try to put a DSLR in front of a C6 you're going to get some crazy diffraction patterns and a lot of light loss. If you use a small circular cooled CMOS camera then it may work.

Cheers,
JT

[James]

does a ZWO ASI 183MC cool qualify as a small circular cooled CMOS camera?

[JayTee]

You need to look at the diameter of the camera and the diameter of the secondary mirror holder for the C6. I don't have either of those numbers readily available to me. If the ASI 183 MC Pro has a diameter equal to or less than the diameter of the C6 secondary mirror holder then you should be good. I bet you could even get away with 1-2mm bigger than the secondary mirror holder without too much diffraction.

Cheers,
JT

[James]

the diameter of the secondary mirror holder is 56mm
hyperstar diameter is 71.1 mm
the camera diameter is 78mm
in these dimensions would there be large diffraction errors?

[JayTee]

Nice job on getting those numbers. I can't advise you any further than I already have. Even though you are in South Korea, I would suggest you call Starizona in Tucson Arizona and ask them your questions and pick their brains over this setup. I recommend them because they sell the Hyperstar system and they should have all those answers. tel:1+ 520-292-5010

Cheers,
JT