Choosing a Reflector

[wornish]

Want to go for smaller deep sky astrophoto targets so thinking about adding a reflector to use when my Esprit 100 refractor won't cut it.  It is a great scope but only 550mm focal length.

These are on my short list.

Celestron  Edge HD 9.25 OTA. Focal Length 2350
Meade F/8 ACF 10" OTA.          Focal Length 2032
iOptron  Photron RC10 OTA.    Focal Length 2000

Am I expecting too much from my mount AZ-EQ6-GT to handle this focal length ?
Anyone got any comments on which of the three is the best bet ?

Any other OTA I should consider?

Thanks for any input.

Edited August 22, 2019 by wornish

[RT65CB-SWL]

I admit I do not do astro-photography [at present] or DSO's; but I think many would say go for the Ritchey-Chretien. Before I purchased my Celestron C6/SCT, I was considering a Ritchey-Chretien purely for visual and advised they are purely or better for imaging.

[vlaiv]

Depends on your intended imaging resolution. What camera do you plan to use it with?

If we are to go by your signature, meaning ASI1600, then I would choose following, if your mount is capable of being guided to about 0.5" RMS:

10" RC + Riccardi x0.75 FF/FR. This will give you working focal length of about 1500mm and if when you bin your subs in software, around 1"/px. For that sort of resolution, you will need good seeing and good guiding.

8" RC + above reducer will be more forgiving as it will give you 1200mm reduced, and at 3.8um binned x2, that results in 1.3"/px.

[ollypenrice]

Vlaiv above hits the nail on the head.

1) Detail in small objects (or any objects!) depends on image scale in arc seconds per pixel. But...

2) You can make up an optical system which can image at a thrilling resolution like 0.5 arcseconds per pixel but...

3) You would need incredibly stable seeing to resolve at this detail and you would also need a mount which could track, under guiding (encoder or stellar) at half that, so 0.25 arcseconds RMS. A good EQ6 is more likely to deliver 0.5"RMS under guiding meaning an image of scale of 1 arcsecnd per pixel. And that's a good one. Don't bank on it.

Enter the real world. None of the above is likely to be possible. So what is possible at your site? Firstly I'd try to ascertain your guide RMS in arcseconds. PHD2 will give you this provided it knows the pixel size and focal length of your guide setup. Double your guide RMS to get an estimate of your best useful image scale in arcsecs per pixel. You'll probably find that this value is very variable from night to night. It certainly is here and that's at 3000 feet on an Alpine hillside. Sometimes we can resolve at 0.9 arcsecs per pixel. Sometimes it's three times worse. (Then we just shoot colour.)

I would urge you to forget telescope specifics and to think in terms of these fundamental numbers. I'm a pragmatic imager but there are some basic numbers which cannot be ignored.

Olly

[gorann]

If you go for an EdgeHD, I think the 8" would be a better match for your mount, almost half the weight of the 9.25", and the scope is also about half the price. The light gathering of the 9.25 is only 30% more. If you want to see what can be achieved with that 8" scope, have a look at the astobin page of Jason Guenzel:

https://www.astrobin.com/users/Thirteen/

[wornish]

Thanks for the responses so far.  What I am learning is do the numbers first , which I  intend to do, hence this post.

I plan on using the camera I already have i.e. the ASI1600MM.  My location here in Cheshire has Bortle 4 - 5 Sky when the clouds eventually clear, which does happen occasionally if my memory serves me well!

I think my PHD guiding  is just above 1"/pixel so far without any real tuning but might be wrong.

 

[ollypenrice]

21 hours ago, wornish said:

Thanks for the responses so far.  What I am learning is do the numbers first , which I  intend to do, hence this post.

I plan on using the camera I already have i.e. the ASI1600MM.  My location here in Cheshire has Bortle 4 - 5 Sky when the clouds eventually clear, which does happen occasionally if my memory serves me well!

I think my PHD guiding  is just above 1"/pixel so far without any real tuning but might be wrong.

 

If your guide RMS is about an arcsecond you are really not going to resolve details beneath 2"PP. That's in broad agreement with what many UK imagers say about useful resolution. I've never imaged in the UK so I can't comment on that from experience.

Olly