View Full Version : Light gathering question

2007-Aug-07, 06:37 PM
I have recently gotten back into amatuer astronomy after being out of it for years and my renewed interest has sparked a question.

The power of an optical telescope is based on the amount of light it is able to collect, hence a 16" scope is more powerful thenan 8" scope. So my question is , can you approxamate the results of a 16" scope by using 2 8" scopes with a CCD setup and combining the input from the 2? I understand that the actual setup would not be as simple as described, but shouldnt you be able to approximate the same amount of light capture of a larger telescope with mthe comined capture of multiple smaller telescopes. Thanks, and hope this question isnt to obvious to some people.

My first guess is that the answer is "no" probably due to effects related to resolution. but i defer to anyone who has more knowledge on this.

Amber Robot
2007-Aug-07, 08:17 PM
In terms of collecting area two 8 inch telescopes are not equal to one 16 inch telescope, because the area goes as the square of the radius. In terms of the imaging resolution, a 16 inch telescope will have better resolution than an 8 inch one, in the diffraction limit, but two telescopes can do better, regardless of their aperture, if there is a large baseline between the two telescopes and their signals are combined interferometrically.

2007-Aug-08, 01:26 AM
It depends on what you want to do and where you are doing it. Down here on earth at many locations while a 16" scope in theory has twice the resolution of an 8" (forget 2 8" it can't be done by typical amateur techniques and even the pros have barely begun to get it to work at optical frequencies). In practice, on many, if not most, nights the atmosphere will limit them to the same resolution. But the 16" has the advantage of 4x brighter image at any given power. Thus it needs only 1/4th the exposure to take a shot of Jupiter or the moon. That gives it a better chance of freezing seeing and getting a better image. The difference isn't equal to the cost difference however! This is why the 8" is so common.

For deep sky photos using a CCD a 8" scope will easily outperform what you can see visually in a 16" scope. It isn't even close. The eye can see only what photons hit it in about 1/10th of a second. The CCD sees what hits it over the period it is working. So while an 8" scope captures only 1/4th the photons per second that a 16" scope can, after about 2/5ths of a second the CCD has captured as much light as the eye. But the brain is far better at digging a signal out of noise than a computer so you will need several seconds of signal to equal what you see.

For deep sky photography the 8" has a lot of advantages over the 16". It needs far less in the way of a mounting, it takes in 4 times the field of view at a given focal length and is far cheaper. So you won't find very many amateurs working a CCD with a 16" scope but you will find a lot using a 4" refractor. For most deep sky objects you can substitute a longer exposure time for loss of light from the smaller aperture and save a ton of money and trouble (guiding a big scope is far more challenging than guiding a small one).

Visually the old saw "There's no substitute for inches" carries some truth. For deep sky CCD work that isn't necessarily the case. I know a lot of imagers working with 8" scopes that can easily equal or often outperform those using 16" and larger scopes. The guy behind the system is far more important than the size of the equipment.

2007-Aug-08, 01:03 PM
Thanks for the enlightening input guys, very interesting topic, i will have to read some more material on it.... so i guess i have to accept my official geekdom, in that i love to learn just for the fark of it! :lol:

2007-Aug-08, 01:16 PM
Since you gentlemen seem to be well versed in this material perhaps you can comment further on this...

For background i am currently using and old 4" meade i got from a friend. but we were discussing telescopes and CCD's and their applications for imaging both deep sky objects and planets. I know alot of the info i am asking about is answerable through various book on astronomical imaging (ordered one today!) so i appreciate any input. So my actual question is as follows: I understand the explanations as given in the previous comments, but image the following scenario. I have 2 8" telescopes such as an orion XT8 and have them setup with CCD's for image caputre. I will use these two telescopes to image saturn at the same time and then combine all of the images into 1 using the normal astro software.

The point of the exercise here is to gather more light in given period of time then any 1 smaller scope could do. Once i combined the two sets of images from the telesopes would there be any apreciable difference? This is just a thought experiment i have had bouncing around in my head, but i would guess that using this additional imagery from the second telescope you should be able to improve the end result image in that you have more light "data" to use for image processing and interpolation. I would also guess that you might be able to slightly improve resolution through image analysis using this method, althought probably nothing significant. Ultimatly i just need to do more research into astro CCD imaging.... thanks for the responses!

Amber Robot
2007-Aug-08, 03:42 PM
The point of the exercise here is to gather more light in given period of time then any 1 smaller scope could do.

If there were some reason why you couldn't just expose longer on the smaller scope then this might be one way to get more data.

2007-Aug-09, 02:11 AM
The point of the exercise here is to gather more light in given period of time then any 1 smaller scope could do. Once i combined the two sets of images from the telesopes would there be any apreciable difference?

In theory yes it would sort of work. But the difficulties of doing it in practice and the gains there from just don't make it worthwhile. The more time on an object the less noise up to a point. What that point is depends on many factors but the gains aren't linear. So by combining images from two scopes does help but not as much as you think. It would be beetter if one took the color data while the other took the luminosity data. But the scopes aren't identical, the cameras won't be taking the same field of view with the same orientation so you will have the added problems of matching the image size and orientation without losing much field of view. Software can handle this but it is an added step. If portable you have to scopes to accurately polar align again taking from your imaging time.

Better yet would be to use one scope to take one object and the other to take a different object. The gains would be much greater because time on one object improves it by the square root and isn't linear at all as you seem to be assuming. This would be a far more efficient way of doing what you want, doubling what you can do in the same time. But trying to keep track of two computers running two scopes likely will take a lot of organization. You will have lots of windows to watch on the computers. It certainly won't be relaxing!!

The learning curve for this is steep, very steep. Far steeper than the ads would lead you to believe. I still screw up after a couple weeks off due to clouds. Just too many things to remember. I need a pilot's check list to go through to make sure every box on every screen is checked the way it should be. Or I missed one and waste a lot of time. Trying to do it for two at one time would drive me to insanity. But you're welcome to try. But master one scope and one CCD before attempting it! A high quality 4" scope on a good mount would make an excellent starting system. So you may have all you need to get started.

Also if you want to gather 4 times the light in the same time just bin your CCD 2x2. It does exactly that! There's no free lunch as you have half the resolution. Often that's no loss at all as the atmosphere limits your resolution anyway. For an example see my post of M81 chowing down on Holmberg IX that was taken exactly that way.

Good luck!