Date
1 - 12 of 12
Should I use my BARADV for the upcoming lunar eclipse?
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Nick Iversen
I have a 130GTX and a camera with pixel size 3.76um. Is there any advantage to using a BARADV to double the size of the lunar image on the sensor? It won't increase the resolution (right?) because that depends on the aperture. But it will increase the exposure required and amplify any tracking errors. Does the answer to this question differ if I am imaging the moon at first quarter (fine crater details) or a globular cluster? I'd really like to know the mathematics involved.
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On 5/21/2021 8:55 PM, Nick Iversen wrote:
I have a 130GTX and a camera with pixel size 3.76um. Is there anyIncorrect. The resolution is the number of pixels within the image, so if the image on the sensor grows by 2X, you have doubled the number of pixels in both directions. Suppose Mare Imbrium covers a square of 500x500 pixels now, and suppose a crater within it covers 30x30 pixels. Now double the image size to cover 1000x1000 pixels, and the crater now covers 60x60 pixels. Detail that might be lost or fuzzy in the 30x30 area might become visible or sharper in the 60x60 area. Does that make sense? But it will increase the exposure required and amplifyMoon exposure should be super-short, and not require guiding at all (what would you guide on?). Most lunar imagers use "lucky imaging" on the Moon and planets. This uses software like SharpCapture to make a "movie" at 20-60 frames per second, then AutoStakkert to sort through the frames and choose the best ones, and stack them into one image. Registax can sharpen the image. You can get a lunar image with a single quick exposure, but portions of it will be distorted or out of focus because of atmosphere movement. Lunar and planetary imaging is completely different than deep-space imaging. Hope this helps. --- Mike |
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On 5/21/2021 8:55 PM, Nick Iversen wrote:
Does the answer to this question differ if I amCan't help you with the math, but AFAIK, the more pixels your target covers, the better it will be. It will indeed have higher resolution. Take it to an extreme: Would you want a 1-degree-wide image to fall on a sensor having 100x100 pixels, or one having 5000x5000 pixels? --- Mike |
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On 5/21/2021 9:36 PM, Mike Dodd wrote:
Most lunar imagers use "lucky imaging" on the Moon and planets. ThisThis is one of my lunar images acquired as described: <http://astronomy.mdodd.com/moon-02.html> Note that the individual exposures were only 1.5 milliseconds -- the Moon is very bright. Details are listed below the image, with links to the specialized software. When I wrote "SharpCapture" in my message above, I meant to write "FireCapture" which is an improvement, and is free. I'm not an expert in "lucky imaging" but I'm pleased with this image. --- Mike |
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Worsel
Nice moon image, Mike...very sharp!
Bryan |
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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 -------------------------------------------------------------------- Daniel Borcard Observatoire du Geai Bleu Clear Sky Chart: http://cleardarksky.com/c/ObGBlQCkey.html?1 Le ciel est assez grand pour que chacun y trouve sa place. --------------------------------------------------------------------  |
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Roland Christen
That's a very nice image. You must live near or on the Mediterranean coast of France.
Rolando
-----Original Message-----
From: Daniel Borcard <daniel.borcard@...> To: main@ap-gto.groups.io Sent: Sat, May 22, 2021 5:36 pm Subject: Re: [ap-gto] Should I use my BARADV for the upcoming lunar eclipse? 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
Clear Sky Chart: http://cleardarksky.com/c/ObGBlQCkey.html?1
Le ciel est assez grand pour que chacun y trouve sa place.
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 -- Roland Christen Astro-Physics |
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No, I live about 40 km straight north of Montreal :-) If you follow the link to my Clear Sky Chart (in my signature block) and click on the light pollution map you'll get an idea: Last fall I spent many nights out to image Mars. I actually managed to image its whole surface over the months, make a map and project the map onto a globe. See: Daniel -------------------------------------------------------------------- Daniel Borcard Observatoire du Geai Bleu Clear Sky Chart: http://cleardarksky.com/c/ObGBlQCkey.html?1 Le ciel est assez grand pour que chacun y trouve sa place. -------------------------------------------------------------------- |
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DFisch
Daniel, a true work of art and I am so glad your retirement coincided with a great astronomical time. I had seen this image before and did not link the author to your signature here. Exceedingly well done and creative, glad you got an award for that one. Tom
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Nick Iversen
Daniel
Thanks for that explanation. It made sense until you said "aim at an f ratio (i.e., F/D) equal to 5 times the size of a pixel in microns." Can you intuitively explain why? Failing that the maths would be a good explanation. The scope has a resolution of 1 arcsec and without a barlow that is about 1 arcsec per pixel for my camera. Your formula says I need a 3x barlow. Spreading that 1 arcsec over 3x3 pixels kind of makes sense for "smoothness." -- Nick |
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Nick Iversen
A note on tracking: a 15 second exposure is quite a long time unguided. Especially on the moon which doesn't travel at the sidereal rate. The lunar rate setting on APCC is only approximate since the lunar rate varies during the lunar cycle. And it's RA only so doesn't account for declination drift. So halving that to 8 seconds could be quite a good thing.
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Nick, Daniel You prompted me to translate a document that I have used in several talks. It presents the (simple) equations leading to the 5 x pixel rule of thumb. The original was in French. You can find the English version here: A note on tracking: a 15 second exposure is quite a long time unguided. Especially on the moon which doesn't travel at the sidereal rate. The lunar rate setting on APCC is only approximate since the lunar rate varies during the lunar cycle. And it's RA only so doesn't account for declination drift. So halving that to 8 seconds could be quite a good thing. Why on earth would you like to expose 15 seconds on the Moon? I mean, imaging the Moon is like taking a photograph of a brightly sunlit rocky landscape (it *is* a brightly sunlit rocky landscape). With the usual cameras the subexposure time is in the order of one to five milliseconds. Ideally you take thousands of them with a video camera, and this only takes several minutes at most, during which the Moon does not have to stay strictly at the same place on the sensor. Stacking programs like AutoStakkert!3 will take care of the drift afterwards. I simply set my mount to the Moon rate, and from time to time I adjust the declination manually during or between the movies. Good luck! Daniel -------------------------------------------------------------------- Daniel Borcard Observatoire du Geai Bleu Clear Sky Chart: http://cleardarksky.com/c/ObGBlQCkey.html?1 Le ciel est assez grand pour que chacun y trouve sa place. -------------------------------------------------------------------- |
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