8" SCT for DSOs?

Kirkus (Kirk) · #1 30-08-2009, 08:54 AM

I know I keep posting totally amateur questions, but I'm still just a bit confused about focal length and focal ratio. I want to see if I have this correct (before I buy a second scope). I'm this close to pulling the trigger on an 8" SC.

I currently have a 6" SN with a focal length of 762mm and a focal ratio of f/5. If I understand correctly, this would be better than the 8" SC for a lot of DSO photography because of the fast f-ratio and the wider field of view due to the short focal length.

The 8" SC, which has a FL of 2032mm and f-ratio of f/10 would be better than the 6" SN for planetary photography because of the longer focal length - thereby bringing the planet closer, even though it's an f/10 scope, the brightness of the larger planets isn't as critical with the slower f-ratio.

Is this correct?

If my understanding is right, I'd need to start applying a focal reducer to image some of the larger DSOs with the 8" SC. Though it might do a better job on the smaller ones that would fit in the 8" SCs FOV.

I guess what I'm trying to figure out in my mind is whether or not it's worth it for me to make the purchase of an 8" SC to image Jupiter, Saturn, Mars, Venus, and smaller DSOs.

sheeny (Al) · #2 30-08-2009, 09:23 AM

Your thinking is on the right track, Kirk.

I have an 8" SCT and many year ago I used to own an 8" Newtonian. The 8" SCT does have a longer focal length, but you can add a focal reducer to it to increase FOV and make it faster.

Basically a Newtonian will give you better contrast than an SCT because it has a smaller secondary obstruction. So you will see a lot of both planetary and DSO specialists using Newts - the planetary guys will use barlows and TV PowerMates to extend their focal length.

The SCT is more portable than a newt and can be a bit more versatile depending on what you went to do. E.g. with a couple of focal reducers and a Powermate you can have one scope that changes from f/3.3, f/6.3, f/10 to f/25 say. If you got one with fastar or hyperstar you would get something like f/1.8! The SCT also has more focal range so I can achieve higher dispersion rates out of say a Staranalyser 100 spectrometer

(which I curently enjoy

) and it's possible to use an off axis guider without having to modify the scope. But the trade off is you will never get the contrast you get out of a Newt... that may not mean a lot until you get to the expert end of DSO and planetary imaging and yet a lot of expert imagers use SCTs (e.g. Damian Peach).

The SCT will do the stuff you say you want to do

. The planetary stuff will be relatively easy to achieve - the DSO's might be more difficult or disappointing depending on the mount you have. To do DSO's justice you will need to guide, which probably means a heavier mount (say EQ6 or G11) and guide scope unless you tackle using an off axis guider.

Al.

Robert9 (Robert) · #3 30-08-2009, 10:05 AM

Hi Al, That's the most concise analysis I've yet come across. I also want to break into a bit of astrophotography with my Nexstar 6SE. I understand the limitations of a 6" SCT, but would appreciate any advice on how best I might make use of it - focal reducers, type of camera etc.
I hope I'm not hijacking the post. If so, I'm sorry, but I think my question adequately continues the subject.
Robert

Kirkus (Kirk) · #4 31-08-2009, 02:14 AM

Thanks for the reply Al. Good to know I'm catching on, sort of. I hadn't considered the issue of contrast.

I've intentionally not considered a larger newtonian because of the weight issue. I have an lxd75 mount which is already a little overloaded with the 6" SN, camera, and guide scope. I can successfully achieve longer exposures (420 seconds is the longest I've shot due to light pollution) but I fear the extra weight of a larger Newtonion would simply be too much. Therefore, the lighter SC.

Thank you for your explanation.

therockfrog (James Petersen) · #5 01-09-2009, 03:08 PM

I have an 8" sct and use it for DSO Galaxies and planets and use my 111 refractor for nebulae and some galaxies. With some good guiding, DSO's can be imaged very well with 8" sct's as, of course, can planets.

Gallifreyboy · #6 01-09-2009, 06:16 PM

Quote:

Originally Posted by Robert9

Hi Al, That's the most concise analysis I've yet come across. I also want to break into a bit of astrophotography with my Nexstar 6SE. I understand the limitations of a 6" SCT, but would appreciate any advice on how best I might make use of it - focal reducers, type of camera etc.
I hope I'm not hijacking the post. If so, I'm sorry, but I think my question adequately continues the subject.
Robert

I used to have a nexstar 6se until I became interested in astrophotography. The mount is not up to it unless you want to stick to moon shots. I sold the 6se and invested in an EQ6 and have a good quality APO refractor now. I started with a canon 1000d (since modded)learnt how to polar align and you can take it from there. The 6se OTA I think will fit on an EQ6 but an SCT is a difficult place to start astrophotography compared to a refractor. Good luck.