Re: Should I use my BARADV for the upcoming lunar eclipse?


Daniel Borcard
 

Mike,

In this thread I see a confusion between sampling and resolution. 

What you refer to is sampling, i.e., the number of arcseconds per pixel. Sampling is indeed doubled if you double the focal length, all other things being equal.

Will your image have double resolution if you double the sampling? Well, that depends. The maximum resolution of a telescope is dependent on its diameter, with a rule of thumb like this: resolution in arcseconds = 120/diameter in mm. So a refractor with a lens 120 mm in diameter resolves 120/120 = 1 arcsecond. This means that you will be able so tell apart two points (stars, or contrasted dark features on a planetary surface) that are 1 arcsecond apart. A 240 mm scope has a resolution of 0.5 arcsecond. *No matter the electronic and computational wizardry, there is no way to overcome the maximum resolving power of a telescope*. Therefore, overampligying an image is like overmagnifying for visual observers: beyond some point expect no gain.

Now, from the imaging point of view, this means that (1) it is useless to amplify the image beyond what the telescope itself is able to resolve and (2) you must optimise the scope-accessory-camera rig to be able to project the two closest features on at least two different pixels. Enter sampling. To reach a sampling that allows you to achieve the maximum resolution of your system (with enough overhead to get a smooth image), I throw in another rule of thumb: aim at an f ratio (i.e., F/D) equal to 5 times the size of a pixel in microns. I could develop the math if there is interest.

Concretely, say you have a ZWO ASI224MC camera, with 3.75 micron pixels. The optimal sampling is approx 5 times this values, i.e. F/D = 18.75 or close to 20. If your scope is, for instance, an F/10 SCT, you will need a 2x barlow to achieve high resolution. If your scope is an F/4 newtonian, you need a 5x barlow.
The final size of the planet (or lunar feature or full Moon disk) will, of course, depend on the diameter of the scope: a 150 mm F/4 scope has a focal length of 600 mm whereas a 300 mm F/4 scope has a FL of 1200 mm. The latter will provide more resolved images *because of the larger diameter of the scope*. 

Finally, note that it is useless to enlarge the image more than this rule suggests. Indeed, the gain in image size will not translate into increased resolution (since the scope is unable to resolve finer features), and furthermore the increasing F/D number translates into darker images that must be compensated by longer subframes (more prone to poor seeing) and/or increased gain (which gives noisier subframes).

As an example here is a Mars image that I took last fall (October 6) with a 10 inch (254 mm) F/4 newtonian with a 5x PowerMate and a ZWO ASI224MC camera. And to remain in the "scope" of this group I must say that all these nights were made extraordinarily easy by my trusty AP 1200 GTO mount :-)

Daniel
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Daniel Borcard
Observatoire du Geai Bleu
Le ciel est assez grand pour que chacun y trouve sa place.
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