APO Refractor vs. SCT for DSO imaging

[Arringar]

I have been comparing telescopes for the past week or so and it has come down to a decision between a 10" Celestron SCT and a Celestron skywatcher 100mm APO refractor.

Poll time:

Leaving all else aside and focusing on optical quality for imaging alone, what would your choice be and why?

I really appreciate your feedback. Thanks!

[Chris]

Hi Arringar,

I would go for the Apo everytime when it comes to imaging alone, not to knock SCT's I think they are good all rounders, but I think they have a few things going against them for imaging, i.e I think a lot of SCT's have a focal ratio of about 10 (f/10) this is a fairly slow focal ratio for imaging and would require much longer exposure times than a short tube apo like an ED80/100 which have a focal ratio of about f/5 or 6, the lower the focal ratio the faster it grasps light. SCT's also suffer from primary mirror flop which as I learnt at Uni using 16" Meade SCT's plays havok with focus as the telescope moves, I even cited mirror flop as a source of error in my data for my dissertation. Another thing is the large central obstruction on SCT's which reduces contrast. Apo's are virtually free from false colour and abberations, I think SCT's suffer from a bit of coma towards the edge of the field although I'm not sure if they suffer false colour? Apo's are a bit more portable and require no collumnation. I think a lot of Apo's come with really clear increments on the Crayford focuser and have very fine focus controls this helps to both find perfect focus and remember where perfect focus is for infinity (although a Bahtinov mask is perhaps the most handy focussing aid).

I hope this helps,

Chris

[Arringar]

This really does help, Chris. Some very concise information regarding the focal quality is exactly what I needed.

[ollypenrice]

I think this should be on the imaging or setups discussion boards, mods?

Maybe you should do a bit more homework before jumping in. To be scrupulously honest that is my impression based on the question itself.

OK, my thoughts; the SCT with long focal length will need very accurate guiding, an accuracy well beyond what is normally at 'plug and play' level with mounts costing around £1000. The F6.3 reducer is essential on the standard SCTs and the coma corrected Edge and ACF models have some complex issues with focal reducers. (F10 is a non starter for DS imaging as explained above.) SCTs are not imaging-ready as sold. The moving mirror focus is a pain so an aftermarket rear focusser is best. I hate Crayfords and would only consider a Baader Steeltrack or Moonlite myself.

And do you really want that kind of focal length? What do you want to frame in your images? I'd go for a wider field to start with. You can model chip size-focal length on the sky by Googling for the free CCD Calculator.

The apo is a far more friendly way to start but you have picked a very slow one there, not at all the best bet for DS whare fast F ratio is so important. My own choice would be for the ED80 with dedicated focal reducer. However, there are others at reasonable prices and plenty more at unreasonable ones!!

Imaging starts with the mount, which you haven't mentioned. Don't spend any money till you are on top of all the options and have made a decision on how you plan to autoguide.

Very standard advice on here is to read Steve's book 'Making Every Photon Count' and available from FLO. It's very standard advice because it's very good advice!

Enjoy the adventure...

Olly

ollypenrice's Photos

[Chris]

Hi Arringer,

I second Olly, do lots and lots of research and for imaging its all about the mount and a short f-ratio scope. I'm imaging on a shoestring with a second hand basic RA driven EQ5, but its not ideal. With your budget I would go for something like an ED80 or megrez 72 on an HEQ5 syntrek or synscan, or if that setups a bit steep still get the HEQ5 and stick a cheap achro on it like a Startravel 80 or 102 and tweek the false colour in PhotoShop, or if your not in a windy location you could stick a 8" f/5 Newtonian on an HEQ5, Newts give you a lot of bang for your buck

Anyway enjoy researching

Chris

[Arringar]

All this advice well considered, I am in the process of getting the book you've mentioned. I've also decided to stick to a refractor for many reasons, including portability. I apologize for the AP beginner questions, I have done a lot of research but like everything else there are so many opinions and I found so much conflicting information that I was in need of some practical advise from people I could converse with to find out why they subscribed to one particular methodology over another.

I am pretty keen on this scope:

http://explorescientific.com/telescopes/102mm_ed_apo.html

As for the mount, I now have less weight to be concerned with since eliminating the larger SCT scope. I definitely plan to get an auto guider, most likely the Celestron CCD auto guider with an achromatic guide scope (I'm thinking a Meade 80mm or smaller). The mount decision is still up in the air, but I will research the ones you have mentioned above, Chris. Everyone keeps stressing that the mount is the primary concern but it seems there are other considerations before a mount can be chosen, like how much weight it can handle while maintaining stability and successfully tracking objects, right?

As for a camera, I think a CCD camera like the Mallincam is a good choice for me for starting out due to ease of use and it's light weight and small size. Where I live I will have no choice but travel out of town to have even reasonably good seeing. I've checked out some public dark sky locations that aren't too far away that I'm planning to visit regularly for viewing and imaging. I think that will make things a bit easier for me without worrying too much about trying to filter out or process out light pollution.

Any further advice you might have on the mount or any other item I've mentioned would be very helpful. Thank you all for your time and advice!

[Arringar]

Olly, your imaging work is truly amazing. Very inspiring.

[Chris]

Hi Arringar,

I've had a look at your link and an f/7 might be a good all rounder for both visual and Astrophotography if you can get a focal reducer as one thing you might want to keep in mind is that the increments for f/ratio are not linear, i.e a f/7 scope will need a fair bit more exposure time and accurate guiding than an f/5, but obviously an f/7 will be way better than the f/10 you were previously considering. I see you mentioned that you will need to travel to a dark site, you might want to consider guiding with a 50mm finder rather than a quide scope in order to save weight and bulk in transporting your kit. An equinox 66 on the NEQ3 pro is a nice little ultra portable imaging setup if weights a consideration, although I know that eq3 pro's used to have a bit of a clitch with the guide port so if you considered it you might want to make sure they corrected the glitch.

Any more questions feel free to ask, I'm always happy to talk about scopes that I wish I had lol

Chris

[Arringar]

So I bought the following and so far I am pretty happy:

http://www.telescope.com/Astrophotography/Astrophotography-Telescopes/Orion-10-f39-Newtonian-Astrograph-Reflector-Telescope/pc/-1/c/4/sc/57/p/99602.uts[/uR

http://www.celestron.com/astronomy/series/cgem-series-computerized-telescopes?cat=14

I am still looking into getting an APO in the f/6 range for further astroimaging. I took Olly's advice and did a great deal more homework and learned a lot more about what I was getting myself into before I bought anything. I went with a slightly less popular starting point and I still plan to get an APO to see what it's all about, but I am content for now. I appreciate all your advice and assistance. Thank you!

[ollypenrice]

Exciting times! The fast imaging Newts can do good stuff. They do need careful collimation and sometimes a little fettling but there are lots of threads on here and elsewhere to help with this. The scope is, I presume, also known as the GSO and Altair Astro so those threads will apply, I think. I can't actually open the link to confirm this.

Olly