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UPGRADING SCOPE

gonzostar

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gonzostar

gonzostar

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Hi i have a 8"celestron sct avx scope. I am getting some decent images with the camera. So i am thinking is it worth upgrading to a 9.25 or a 11" SCT. I am based about 3 miles from the city but less then a mile from the sea. So the sky can be dark enough for dso which is my interest

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Peter Drew

Peter Drew

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I don't think many imagers would recommend the SCT's for DSO imaging, they are great for lunar and planetary. Usual first choices for DSO's are a SW 130PDS or a 80mm-100mm APO refractor as focal length and  correction are more important than just aperture.   :smiley:

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RobertI

RobertI

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Hi i have a 8"celestron sct avx scope. I am getting some decent images with the camera. So i am thinking is it worth upgrading to a 9.25 or a 11" SCT. I am based about 3 miles from the city but less then a mile from the sea. So the sky can be dark enough for dso which is my interest

Can you tell us a bit about the camera you are using and show some examples of images you have taken to date? It will show people your level of experience and allow them to give you the best advice possible. it may be that you shouldn't rush to change anything quite yet, although Peter is absolutley right in what he says about the scope people normally use for imaging DSOs.

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brantuk

brantuk

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I agree - all good advice above. For the price of a 9.25" or 11" Sct you should really be looking at some decent quality glass doublets or even triplet refractors for dso imaging. Aperture is more important for observing - whereas for imaging some of the most important factors are good tracking, wide angles, and short f-ratios. I'd suggest a read of "Making Every Photon Count" to appraise yourself of dso imaging stuff. :)

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nmoushon

nmoushon

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You can image just fine with a SCT but the focal lengths are very long. The biggest problem you face is your mount. I think you have the AVX mount. This will not be a suitable mount for supporting a 9.25s weight and focal length for imaging DSOs. You might get away with it if you are only imaging lunar and planets via video imaging. Definitely not a 11" for visual or any type of imaging. The 11" is almost twice the weight of a 9.25" and the FL of 2800mm is just crazy long for that mount. 

I would suggestion sticking with the 8" and focus on getting imaging mastered with that. You can buy a reducer/flattener to shorten up the FL. This will also making guiding easier. If you havent started guiding I would do so and use an off axis guider and not a guide scope. The SCT will be good for imaging galaxies and PNs. The long FL makes it harder to image nebulas but not impossible. 

If you decide you want even wider FoV for imaging large nebula then go with the above suggestions of a 80mm ish refractor. You can get a really good one for the price of a 9.25 or 11" SCT. Plus all the need extra bits. 

Decide on exactly what kinf of DSOs you prefer to image and that will help you decide on the type of scope. 

PS: Another option is to upgrade to a 8" EdgeHD SCT for better imaging. If you dont have the edge version already.

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gonzostar

gonzostar

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Thankyou everyone for your constructive feedback. I have certainly a lot to think about. 

Robert and nmoushon i have a canon 450d  eos camera and use a astronomik CLS   filter. I dont have the edge Also i do use a focal reducer. 

How do attach photos onto this website? 

I have some average images  of some of the brighter DSO such as m13, m57 cigar and eagle nebula. Havent quite mastered the autogiuder as yet.  But would be great to get a more experienced opinion

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Dave In Vermont

Dave In Vermont

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For DSO's using an SCT, a focal-reducer will be your best friend. With these you want top-quality - not who's cheapest. I'm outfitted to drop my F/10 SCT to a 3.3. Here's a link to an excellent overview of focal-reduction and How-To. Built around the MallinCam's, the principles remain the same regardless of the camera:

http://www.mallincam.net/uploads/2/6/9/1/26913006/focal_reduction_for_dummies.pdf

Enjoy your choice of SCT!

Dave

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ollypenrice

ollypenrice

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Thankyou everyone for your constructive feedback. I have certainly a lot to think about. 

Robert and nmoushon i have a canon 450d  eos camera and use a astronomik CLS   filter. I dont have the edge Also i do use a focal reducer. 

How do attach photos onto this website? 

I have some average images  of some of the brighter DSO such as m13, m57 cigar and eagle nebula. Havent quite mastered the autogiuder as yet.  But would be great to get a more experienced opinion

With this camera, which has small pixels, cannot be binned 2x2 and is uncooled, the last thing you should be doing, in my view, is adding more focal length.

Use this calculator to work out your pixel scale on the sky. http://www.12dstring.me.uk/fov.htm

Without reducer you are already at 0.5"/PP and the reducer only gets you to about 0.8. This is very high resolution in the optics/camera department but can your seeing and your mount actually allow that resolution to be acheived? I doubt it, personally. Going for even finer resolution would surely be a waste of time? You just won't acheive it, I don't think.

If sticking with a DSLR I would want to reduce the focal length to something more realistic and speed up the F ratio to get enough light onto the pixels in the time available before the noise builds up too much in the uncooled camera. DSLRs are best used in short focal length, fast F ratio optics - which is pretty much the opposite of an SCT.

Olly

Edit: a shorter focal length will also make sufficiently accurate tracking much, much easier to realise. Even with an autoguider 0..5 arcsecs per pixel is premium mount territory.

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Uranium235

Uranium235

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When i fiqure out how to download photos on this this site then i can show you what i have taken so far. Also you mentioned uncooled camera, does this mean i have to stick it in the fridge before observation?  :laugh:

Nope, uncooled means that your camera will suffer from a thermal build-up over the course of the session, and hence increasing the amount of noise in the image. Not so bad if its -10o outside, but any other time of the year its going to be a problem. This can be negated to a certain extent by the use of darks, but even that isnt a perfect solution to thermal noise.

Also, with the SCT youre still at over f6 (when reduced) - and being that DSLRs are a bit "deaf in the ear" so to speak, an SCT isnt quite shouty enough for your camera. So to get any sort of decent signal level you need to either expose for longer (sub length as well as amount), get a faster telescope (f5 or below), or get a CCD camera. Choosing a shorter focal length will also dramatically increase the ease of guiding.

Its not how big your telescope is, its what you do with it that counts ;) (well.. when it comes to imaging anyway!)

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