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Jarad, when you were looking at Saturn, which scope had more contrast? Which showed the details clearer? Diffraction is not just a matter of seeing the smallest detail. It affects contrast as well. Look at Qkslvr's images. You can say the detail is about the same in both. But are they the same? Seeing is not the issue. And it has nothing to do with whether the camera is used on this planet or for something further away. You could say diffraction affects the efficiency of being able to reproduce detail. Even making the 50mm system work at 150 line/mm is not as effective as the 16" working at 32 line/mm. The 16" is always going to be ahead. Certainly, placing the sensor that is on the 16" on the 50mm will improve things greatly for the refractor, regardless of FOV - what is the "ideal" FOV anyway (it would depend what you want to photograph). You may say contrast is not important, but you can only compensate for that so far.

I agree the difference betweeen a 50mm and 16" scope is large, but I have never heard of exceptions in the amateur astrophotography hypothosis of "sampling." If it works for one, it should work for the other. (And I do have a 50mm scope.) Perhaps it is a rule of thumb, but that is not the feeling I am getting from the folks posting here - it sound as if it were science. I know it is not science.

There is another problem. Science says it is not the pixel pitch that is the determining factor to match a sensor to optics. It is the sensor/format size. So when using an APS-C size sensor, nothing changes the requirements of the optics from a 6MP chip to a 12MP chip. Regardless of seeing, small chips require the optics to work harder.

So, can you slap any camera on any scope and get an image? Yes. Will it be the "best" image you can get? That depends. Diffraction is real. It is not simply limiting resolution. It is affecting contrast across a range of spacial frequencies.

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As it happens, in the opinions of multiple observers last night the contrast was better in my 110mm than in the 20". I believe that is primarily due to the fact that my 110mm is an APO, so the color correction is better than in the 20" achromat. The lack of chromatic aberration may not impact resolution significantly, but it certainly does improve contrast, so the image looked less washed out in my scope (despite being dimmer). In addition, poor seeing often affects larger scopes more than smaller scopes depending on the average size of the thermal cells of air.

I also understand the concepts of modulation transfer function, contrast, and acutance. As far as Qkslvr's images go... As I mentioned in more than one of my posts, none of what I have discussed applies to planetary photography where you run at much higher focal ratios. In those cases, certainly diffraction and MTF will have a significant affect on image quality. All I'm suggesting--and I believe practical experience bears this out--is that for prime focus astrophotography resolution is typically limited by seeing conditions, not diffraction. I never suggested, and I do not believe, that the images would be identical between a large scope and a small scope. Certainly there would be differences in contrast, noise, and image intensity. The larger scope wins on all these counts. I only suggested that when choosing a CCD camera for a telescope, in order to maximize spatial resolution in particular you need not take into account focal ratio--only focal length is relevant.

You mentioned in your post that seeing is not the issue and that overall image quality had nothing to do with whether the photographic system was used for terrestrial photography or for astrophotography. If that is the case, then why does the Hubble space telescope outperform much larger ground based telescopes? Isn't there an advantage to getting above the Earth's turbulent atmosphere? Isn't that Hubble's primary edge? Isn't that why professional telescopes now employ adaptive optics--to alleviate the effects of poor seeing?

The way many astronomers match a CCD to their telescope is actually quite practical: first you pick the best compromise of spatial resolution and sensitivity for the objects you wish to image (pixel size), then you pick the largest field of view you can afford! CCD's go up in cost pretty dramatically with the number of pixels/field of view.

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- Jared Willson

• TV 60iS refractor
• 80mm SV/LOMO refractor
• A-P 130 EDFS refractor
• Planewave 12.5" CDK
• A-P 900 mount
• A-P Mach1 GTO mount
• Takahashi Teegul SP travel mount
• SBIG STL-11000 C1 camera with AO-L
• Some heavily light polluted skies

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Since Takahashi makes a 60mm f/8.3 scope, let's use that.

Let's say I want to target an sampling rate (image scale) of 1"/pixel, which would give best sampling for a given OBJECT during a typical "best" seeing night for many people. In other words, 'N' number of pixels will cover that given object, which will strike the best balance between resolution vs. angular coverage. This can be accomplished with a theoretical 2.5 micron pixel camera. To get the same number of 'N' pixels covering that same object for optimium sampling with a 16" f/5 Newtonian, you'd need a 9.5 micron pixel camera.

Which fills those 'N' number of pixels faster, assuming the same QE chip? The 16" Newt, by virtue of its much greater aperture area...more photons are being delivered to the same area of the chip.

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Man Jay, I was with you up until this point Stick with me, I think I'm almost there to understanding. Doesn't FOV factor into determining how many arcseconds you are covering per pixel? Humor me, let's use my current scope and sensor:

Objective = 4.25 inches or 107.95mm
Max. theoretical resolution = 1.07"
SPC900NC pixel size = 7.5 microns
SPC900NC sensor size = 4.8mm x 3.6mm

What else do I need to plug into a formula that will calculate arcsec/pixel? If I have that formula I'll be able to use it with future scope/camera combos.

Thanks for your help!

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Curt
www.opticalsupports.com

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Man Jay, I was with you up until this point Stick with me, I think I'm almost there to understanding. Doesn't FOV factor into determining how many arcseconds you are covering per pixel? Humor me, let's use my current scope and sensor:

Objective = 4.25 inches or 107.95mm
Max. theoretical resolution = 1.07"
SPC900NC pixel size = 7.5 microns
SPC900NC sensor size = 4.8mm x 3.6mm

What else do I need to plug into a formula that will calculate arcsec/pixel? If I have that formula I'll be able to use it with future scope/camera combos.

Thanks for your help!

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The trouble with the above calculations are they don't have any real meaning because it depends on the EP used. I have a 9mm TMB (AFOV 30d), 9mm UO Orthoscopic (AFOV 42d), 9mm TMB/BO Planetary (AFOV 60d), and 9mm Nagler (AFOV 82d). They all have the same magnification but different TFOV. No matter which one you pick for your FOV calcualtion, you will get a different answer for the magnification.

I have a CCD camera of 3M pixels and a DSLR of 6M pixels, the individual pixel size is identical. Does the DSLR, with twice the area FOV, have more or less magnification over the CCD camera?

Now change the problem. I have a CCD camera of 3M pixels and a DSLR of 6M pixels, the FOV is identical because the DSLR camera's pixels are smaller. Does the DSLR, with smaller pixels, have more or less magnification over the CCD camera?

The only numbers that are consistent and repeatable are arc-seconds per pixel and TFOV. Those two values tell you what you need to know about the camera / OTA combination. From here you can argue about the correct amount of oversampling, diffraction limited, seeing limited, and everything else all you want

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