C8 astrophotography advice needed

[SaffaSwede]

Hello all. I need your help. I'm trying to get into astrophotography with a vintage Celestron C8 (1980s orange tube) that my late father passed down to me. I've had it serviced, bought a ZWO AM5n mount + TC40 tripod and am trying to get to grips with the image train now. The fine team at FLO advised me that the C8 is not good for DSO without the F6.3 Focal Reducer. So, assuming I get this and connect it directly to the back cell, I have exactly 105mm to the image sensor. Luckily, Celestron (and other brands) have this covered with their 50mm T-adapters (https://www.firstlightoptics.com/adapters/celestron-sct-t-adapter.html) and camera mounts. However, these are M42. So my question to you is that not going to cause vignetting to an APS-C size sensor? Does anyone have experience with astrophotography with a C8? 

[Elp]

What do you intend to image?

Are you going to autoguide? (Needed for DSO, especially at that focal length and with a harmonic drive mount).

Vignetting is likely to occur, you take and apply flats to your lights to remove it (as long as it's not complete black) and any dust motes within the imaging train.

[SaffaSwede]

I am going to autoguide, yes. Would a M48 image train not be better then? 

[Elp]

Yes, same applies if you intend to use filters, they generally have to be larger than the imaging circle the imaging sensor occupies.

[SaffaSwede]

Thank you very much Elp!

[CraigT82]

Bigger is usually better in the imaging train as bigger is usually stiffer. 

With regards to vignetting this will also depend on the distance between the restriction and the sensor. Closer to the sensor will cause less vignetting and further will cause more.  But generally vignetting will be corrected by good flats. You may find yourself cropping images anyway which will help with vignetting issues. Images from all but the best imaging scopes will be a bit iffy around the edges. 
 

BTW I’m pretty sure I have one of those Celestron M42 T adaptors lying around in the garage,  if you message me your address I’ll send it to you if you don’t already have one. 

Edited July 17 by CraigT82

[woldsman]

It’s possible to deep sky image with a C8 but there are significant challenges. There is an argument for either just doing visual or lunar/planetary imaging or selling the scope and buying a refractor or a reflector even.  Challenges:

1) Guiding: a good mount (EQ6?) is needed and off axis guiding probably required too.

2) Long cool down time and proneness to dewing - practical solutions needed.

3) Good seeing conditions needed to benefit from resolution.

4) No real solution to image shift and mirror flop.  Hutech used to sell an accessory to lock the mirror which reduces image shift but this requires installing a Crawford focuser on the back. Long exposure astrophotography over a whole evening will run into image shift issues with a focus system that moves the mirror due to gravity. You probably have to refocus throughout the night. 

5) The C8 will need regular collimation.

6) Reducer required due to the long focal length and also a camera with a sampling rate that matches the scope - a crop sensor DSLR may not be a good match. 

7) With a vintage C8 you are also working with lens coatings that are less good than what is available now. 
 

I think quite a few people (me included!) have thought about taking on the challenge as vintage C8s come up at reasonable prices. It could be fun to have a go and kudos to those that have produced images. I went for another option in the end mainly due to 4) but good luck if you take this on.

Edited July 17 by woldsman

[happy-kat]

I looked on astrobin using C8 as the imaging telescope and there were lots of images to look at including DSO