What aperture scope do I need to get good images?

[Venera9]

Hi

I am just starting out with astronomy. I am thinking about buying a scope. This is some way off yet so this question is really for discussion. But I'm interested in what kind of images one can get with say a 6" v. an 8" scope. For what it's worth I'm looking at a Celestron Schmidt-Cassegrain NexStar 6" or 8". Obviously I'm aware that there are all sorts of other variables including experience and so on. But I'm just trying to get a general idea of what is possible. I've been looking at some images in the gallery here  - of Jupiter and some deep space objects including a Galaxy - which look pretty good. But there is no scope/camera information with them. Could someone just point me to a few planetary images and a few deep space images and say "this was taken with such and such a scope" and this with "such and such"? That would be a fantastic help.

Thank-you

Venera9

[Steve Ward]

Try "Astrobin" ..... http://www.astrobin.com/

The images there all include the shooting technical cards with the info ... 

[russellhq]

Look up images from an ed80. It's aperture is 80mm and gets some great images.

[Venera9]

Thanks Steve

That's a very useful site. I found I could search for specific scopes / objects. Based on some preliminary research what I noticed was that even with a 4" aperture people are getting good shots of Jupiter, confirming that skill is a very important factor. On the whole the images of Jupiter taken with a 4" or 6" scope were a little smaller (Jupiter itself) than ones taken with an 8". I guess the more light gathering capability translates into higher resolution images? I looked also at Galaxies - here most of the images were taken with an 8" scope anecdotally suggesting to me that for deep-space astro-photography one needs an 8" scope? (For information I was limiting my searches to the Celestron NexStar range). 

Thanks again,

--Justin 

[foundaplanet]

There is a book called 'Making Every Photon Count' available from First Light Optics, the link to their page can be found in the site header. The book is the best place to start before you make any decision on gear.

There is an imaging section, if you scroll down the forums you will see it and often you will see the gear used listed with the images.

I only dabble with a decent mount a DSLR and a zoom lens.

Astro imaging can get very involved and can eat money so, it is worth doing lots of research to understand what it is you are considering getting yourself in to. It can be very rewarding and it can be very frustrating, even to very experienced imagers.

Unlike visual observing it is not aperture that is king with Astro Photog, it is the speed of the thing that counts, the f number you see in the specs....eg. f4.8 f5.3 f6 etc. The lower the number the faster the scope is at putting the photons onto the the camera sensor. Many people use small refractors for AP with 80mm ed's being very popular, plenty of people use Newtonians to though. Sct's generally tend to be the scope of choice for planetary imaging and rarely used for deep sky imaging (galaxies and nebula etc).

The mount is the most important piece of gear for AP....and mostly the Skywatcher HEQ5 is considered to be the starting point.

This will be worth a look..but seriously, keep asking questions and reading.

[Venera9]

Thanks foundaplanet

That - the article explaining the difference between a for mount and an equatorial mount, is very interesting. So. This is the point of the mount and why that is so important, I see. Obviously I have a lot to learn; that's already a good start.

Ta!

--Justin 

[Chris]

I agree with the above, it would make things a lot clearer for you if you picked up a  copy of Making Every Photon Count.

At the moment you seem to be a bit hung up on aperture for imaging. Aperture is king for visual work but for imaging we tend to talk more in terms of focal ratio and focal length. The book can explain it better than I can but basically scopes with fast focal ratios are more suited to long exposures of deep sky objects like galaxies whilst scope with very long focal lengths are more suited to planetary imaging. 

I could try and explain further but I'm rubbish at explaining stuff

Chris

[RikM]

The first thing to think about is 'what do you want to image?' as Chris says, different objects need different equipment.

Then think about the mount. This needs to be rock steady.

For planetary imaging you will be using very short exposure times, so you can get away with an altaz mount so long as it is sturdy and has motor tracking capability.

For deep sky objects, nebulae etc. you will need very long exposures because these things are faint. To achieve very long exposures you will need a very sturdy mount, a good polar alignment and eventually most people add an autoguider.

After that comes the camera.

For planetary imaging you will be working with a fast frame rate 'video' type camera capturing many hundreds to thousands of individual frames which software will then sift through and produce a composite of all the best bits from the individual frames. That way you get a highly detailed image rather than the smeary/blurry image you would have if you let the atmosphere get its own way. Because planets are small and you have thousands of frames, you don't necessarily want a very large sensor.

Deep sky imaging is all about signal to noise. You want a high sensitivity and low noise. Most nebulae are pretty large so a big sensor chip is desirable. The downside is that high sensitivity + low noise + big chip = high cost. If you don't have several thousand pounds to spare you will need to compromise. You could have a big chip with lower sensitivity and more noise = DSLR, or a smaller chip with low noise and excellent sensitivity (Atik 314L+ or Starlight Xpress SXV-H9). A smaller chip is not necessarily a big problem as we shall come onto next.

The last thing to think about is the telescope.

Planets need large aperture (for optical resolution) and long focal length (for good image scale / pixel resolution) SCT's have the right set of features for this. 8", 9.25", 11" are the common sizes.

Nebulae and galaxies require fast optics and good colour correction ( the telescope should focus all colours of light together) aperture (taken on its own) is almost irrelevant for deep sky imaging. It is important only that it gives you the focal ratio (f/7 or less) at the focal length you want to use. Why should you care about focal length? Focal length is what gives you the image scale / area of sky you can fit in your image. Very many deep sky objects are quite large. If you have a large chip camera, you can cover a lot of sky with a fairly large telescope. If you have a camera with a smaller chip, you can cover the same area of sky if you use a shorter focal length telescope (to imagine this, you could think of it as lower magnification, though magnification is meaningless in imaging). Galaxies are a particular challenge because the still require fast, apochromatic telescopes but apart from a couple of whoppers, they are all pretty small so need long focal length as well. To maintain the fast optical speed, you are talking about a substantial telescope.

If your interest is deep sky imaging please take the advice given in the previous posts and get a copy of Making Every Photon Count it really will make it all clear and could save you a fortune by avoiding the wrong equipment.

[Bg7pep]

mmm.....Brains......

is right !

The last thing to think about the telescope!

Eq mount  +  dslr can do a lot of things in deepsky with a camera lens !

please forget my weak english 

[swag72]

You have already been given good advice here - Definitely get the book, loads of us have it and it really is something of an imagers bible. Then decide what you want to image as Solar system and Deep Space have very different requirements.

Don't buy anything until reading Making Every Photon Count at least twice!! Then you will understand what you need and why.

Probably one of the most common scopes in use for DSO's is an ED80, it's short focal length is an ideal partners for the HEQ5. If you go over to the imaging section, people generally put their equipment in the post with the image. You can get a real feel for what works.

Lastly as an aside really, be prepared for frustration and a massive time commitment. The good imagers out there will spend many hours after they have the data processing it. You will see that some people take images that have many, many hours worth of exposures. If you want to produce top class images, then you need to invest a lot of time and money. Often there's no indication from imagers in these sorts of posts how it doesn't stop once you have the data - That really is just the start of it, many hours later an image pops out