What Scope For Astrophotography?

[tonyh66]

Im wondering, what is the best type of scope for astrophotography?

Is is a refractor? are newtonians no good?

Is it dependant on what you want to image?

All advice is welcome.

[NickK]

I'd probably go against the grain and say that your mount is the most important then your scope.

You'll get some defraction patterns off the spiders on newts or SCTs. The result will be 4/6/8 lines coming off bright stars. Refractors don't have this problem.

The width of what you're attempting to image is important - both in deciding if you're going to just use single shots or mosaics, and, resolution (mount tracking, telescope resolving ability and the position/CCD sensor area size and pixel size of the CCD).

[x6gas]

My understanding is that it's basically down to the focal length of the scope. In order to have a reasonable tube length, Newtonians like ours generally have shorter focal lengths than refractors or catadioptrics which means that the image at the eyepiece is less magnified in Newts than longer focal length scopes.

For prime focus photography that means that - with no EP magnification - the image projected on your sensor is smaller so not so good for planetary imaging, but better for wide-field nebulae.

With any long exposure photography you obviously need a good, stable mount and good tracking.

I hope I've got this right - I am sure someone will correct me if not!

[tonyh66]

Im aware that tracking accuracy of the mount is probably most important, which is why a HEQ5 with tracking is a minimum I guess. Diffraction spikes aren't necessarily a negative and look good in some photos. I guess im most interested in Nebulae with the occasional Lunar mosaic. The planets are good to look at but I think imaging them needs a whole lot of money on a big scope and top end cameras. An image or animation of a wobbly little spot the size of a smilie doesn't do it for me.

[Dave-Ryan]

It's actually cheaper to image planets. First of all your mount doesn't need to be as acurate as you only need to take very short exposures and secondly a cheap simple web cam will give good results. You will need a long focal length telescope but this doesn't have to be very expensive.

[jnc71106]

I have seen some stunning pictures on here from people using the skywatcher 190mn

[ollypenrice]

Newts will give diffraction spikes, the number corresponding with the number of veins holding the secondary. Usually this is four. SCTs and Maks will not give these, nor will refractors.

Here are the key points:

Focal length determines image scale. For a wide view, you need a short focal length and for a close up view you need a long. Many of us have a range of focal lengths, starting with camera lenses and ending when the money runs out!

A focal length of around 600mm frames the Orion Nebula with Running Man nicely on a DSLR chip. To get M101 to fill the frame comfortably, you probably want about 1.3 metres. Small galaxies and planetaries cry out for 2 metres and more.

There is a 'but' and it is a big one: long focal lengths need very, very accurate autoguiding on big, accurate, expensive mounts. They also need nights of good seeing and low wind. This is expert, not beginner, territory.

Next comes focal ratio. The f ratio alone, not the size of the telescope, determines the exposure length. Exposure time on extended objects goes as the square of the f ratio. A five min sub exposure at f5 would need a whopping 20 minutes at f10. Don't go there! This is the fastest imaging rig we have, a 200mm telephoto lens in front of the red CCD camera. Not as big as the telescopes but significantly faster. It can image well at f3.2. I would not want to image at anything slower than F7.5 and faster is better.

Why does this matter? Because astrophotos have exposures measured in hours at the best of times. How many clear nights do you get? How good is the tracking of the mount? To track beyond ten minutes needs significantly better polar alignment, etc.

Then there are the mechanical considerations. With refractors there usually aren't any. You plug and play. With anything else you may lose time to 'issues' but not insurmountable ones. Things like mirror flop, flexure in the focus, collimation, all need attending to with non refractors. However you can use all sorts of sytems. I would say that, as delivered, SCTs are woefully incomplete from a deep sky imaging standpoint.

So I choose refractors out to a focal length of a meter but second the post above that says start with the mount. Get a German equatorial, minimum HEQ5, and you are ready to start in the dark art of deep sky imaging.

And life will never be the same again...

Olly

[NickK]

Newts will give diffraction spikes, the number corresponding with the number of veins holding the secondary. Usually this is four. SCTs and Maks will not give these, nor will refractors.

VC200L will give spikes from my understanding (it uses a spider and doesn't hold the secondary using the corrector lense). ok, ok, I'm being anally pedantic!

I think Olly much better positioned (geographically and in equipment) to be authorative on this subject compared to me!

Astrophotography is just like coffee:

1. Freshest beans = clearest, darkest nights and a supply of them on a regular basis!

2. The best flat burr grinder you can afford results in the most uniform coffee grounds without heating it and evapourating the oils = best mount.. tracking and precision placed on the most stable tripod/peir with the best alignment.

3. Coffee machine (oddly people start with this, professional coffee geeks start with 1,2..) = telescope.

[ollypenrice]

VC200L will give spikes from my understanding (it uses a spider and doesn't old the secondary using the corrector lense. ok, ok, I'm being anally pedantic!

I think Olly much better positioned (geographically and in equipment) to be authorative on this subject compared to me!

Now the VC200L is not really an SCT - to be even more pedantic (in a frivolous and amiable way!) The one-armed, hard-drinking Herr Schmidt invented the glass corrector plate and with advice from Walter Baader devised a way of grinding them. If the optical system does not have a Schmidt corrector plate is it an SCT?? Who cares, good point!

I like the coffee analogy. Knowing what I know now I would start with an autoguided GEM and camera lens, gradually working up the focal lengths as I gained skill in mastering all the nonesense!

Best,

Olly

[brianb]

Yes .... the mount is about 90% of the equation, you really need to get to grips with equatorial mounts; the longer the focal length you intend to use, the better the mount needs to be. Portable mounts (EQ6) are probably good up to 1000 mm at best, even with guiding. Whilst Olly has a point about starting out with a camera lens, this is a bit esoteric & good glass can be very expensive; personally I think the popular ED80 f/7.5 is a good introduction, but I'd be interested to see if anyone is using the TS 65mm f/6.5 quadruplet which has a flat 44mm diameter imaging circle and what should be an excellent focuser at a very reasonable price.

[NickK]

I assume that as F number is a ratio between the focal length and aperature that once you have your focal length, then can calculate the aperature size required for the intended F number.

Ie a long focal length with a low F number equates to a very large aperature thus large primary & secondary which then equates to a large cost for the scope and the mount to be able to accurately track the very heavy telescope sat on it - pushing cost further up.

[ollypenrice]

I assume that as F number is a ratio between the focal length and aperature that once you have your focal length, then can calculate the aperature size required for the intended F number.

Ie a long focal length with a low F number equates to a very large aperature thus large primary & secondary which then equates to a large cost for the scope and the mount to be able to accurately track the very heavy telescope sat on it - pushing cost further up.

Yes. In amateur terms you are going to have to accept slower f ratios as your focal length rises or your aperture is going to get seriously out of hand!

Brian is quite right, I think, about a metre or a bit more as defining the comfort zone of the EQ5 or 6 and to exceed that, which many people do succeed in doing, involves coaxing and some tweaking. I also agree that the ED80 is the obvious choice but that this new quadruplet wants a going over.

Olly