Advice for lunar imaging?

[rnobleeddy]

I'd like to get in to lunar imaging this summer, as a way to have something to target whilst the nights are lighter. I was hoping someone could help with some advice on kit.

I already have a either a 200PDS (1000mm) or 250PDS (1200mm) Newtonian available and a QHY163M. FOV for the camera/250PDS is below. The camera should manage a little over 20fps at full res/8-bits.

I've been trying to get a little more scientific in the way I approach DSO, and so my questions are around what is most important as I move to lunar imaging.

- Is there anything fundamentally wrong with the plan to use this kit? I realize the pros don't use Newtonian's but I'm guessing this is primarily because there are more practical longer focal length scopes, and a higher f-ratio isn't an issue for lunar work? 

- This camera, with the 1200mm scope, leads to a resolution of 0.65" per pixel. I understand the principle of lucky imaging and have seen suggestions that lucky might mean as low as 0.1" , but not sure what's a sensible expectation? Assuming I'm under-sampling for lunar imagine, would a barlow or powermate be a sensible option?

- As the moon doesn't suffer from much rotation, is the lack of fps an issue, or can I just record for longer? I guess this also comes down to a question of whether it's better to collect more frames at a lower FOV? I don't know what's considered top of the range for lunar imaging, but I'd guess over 100fps is easily achievable, but probably with a smaller chip.

 

Obviously I'll be trying it out anyway, and I'm keen not to spend too much extra, primarily because my view is blocked to the south (below approx 30 degrees) and so Jupiter/Saturn are generally not visible and so whilst I might get mars sometimes, I can't really do a lot of planetary imaging. That said, if a cheapish longer focal length scope or a new camera would help a lot, I wouldn't rule them out.

 

Edited April 2, 2021 by rnobleeddy

[CraigT82]

What kind of images do you want to achieve? Close in shots of individual features or full disk?

 

[rnobleeddy]

3 minutes ago, CraigT82 said:

What kind of images do you want to achieve? Close in shots of individual features or full disk?

 

Yeah, should have mentioned that. 

I guess I'm looking for close in shots or high-res mosaics. I've had a few goes at capturing the full disc and processing with autostakkert/registax already with OK results.

[vlaiv]

Here are some guidelines to help you out.

- Newtonians have diffraction limited field that is way smaller than the sensor you are planing on using, so if you want to use that camera (or that is your only choice) - you'll probably want to use ROI - region of interest.

- ROI helps with achievable FPS (smaller ROI enables for higher FPS because less data needs to be transferred over USB link, but there is a limit). You are right about the Moon allowing for much smaller FPS rates, but the Moon does change over the course of the night. It is not as obvious but it does change both size and orientation as well as position with respect to the Sun - which in turn changes level of shadows. I have not researched this matter extensively but I'll suggest that you keep your sessions limited to maybe half an hour?

- Smaller ROI means you won't be able to do full disk at once - that is what mosaics are for - so research how to shoot and stitch those

- You can use lower sampling rate than critical sampling rate - but I would advise against going over critical sampling rate as it really does not do anything for image quality and only lowers SNR. You can calculate F/ratio that you need for given pixel size by using following formula:

F_ratio = 2 * aperture * pixel_size / wavelength

F_ratio = 2 * pixel_size / wavelength

(aperture is already in focal ratio, formula with aperture is for focal length instead)

Use same units of length for all quantities (say micrometers for all).

(reference: https://en.wikipedia.org/wiki/Spatial_cutoff_frequency, https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem)

use Barlow lens to achieve close to critical sampling rate if you want the most resolution out of your images

- Use narrowband filter to tame atmosphere. People often use Ha filter or IR pass filter or Baader Solar Continuum 540nm filter when doing lunar with mono camera

- Don't use histogram to set your exposure, in fact don't look at histogram at all. Set your exposure length below 5-6ms to beat the seeing

- Use very high gain settings - as that lowers the read noise. Read noise is one of your worst enemies here

- Do proper calibration (means take darks, flats and flat darks)

Diffraction limited field of Newtonian is given by following expression:

https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

With F/5 scope that gives 1.389 mm from optical axis or 2.77mm diagonal. You need to select your ROI to that size (or that size amplified by barlow lens if you use one).

 

Edited April 2, 2021 by vlaiv
Error in formula

[rnobleeddy]

4 minutes ago, vlaiv said:

Here are some guidelines to help you out.

- Newtonians have diffraction limited field that is way smaller than the sensor you are planing on using, so if you want to use that camera (or that is your only choice) - you'll probably want to use ROI - region of interest.

- ROI helps with achievable FPS (smaller ROI enables for higher FPS because less data needs to be transferred over USB link, but there is a limit). You are right about the Moon allowing for much smaller FPS rates, but the Moon does change over the course of the night. It is not as obvious but it does change both size and orientation as well as position with respect to the Sun - which in turn changes level of shadows. I have not researched this matter extensively but I'll suggest that you keep your sessions limited to maybe half an hour?

- Smaller ROI means you won't be able to do full disk at once - that is what mosaics are for - so research how to shoot and stitch those

- You can use lower sampling rate than critical sampling rate - but I would advise against going over critical sampling rate as it really does not do anything for image quality and only lowers SNR. You can calculate F/ratio that you need for given pixel size by using following formula:

F_ratio = 2 * aperture * pixel_size / wavelength

Use same units of length for all quantities (say micrometers for all).

(reference: https://en.wikipedia.org/wiki/Spatial_cutoff_frequency, https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem)

use Barlow lens to achieve close to critical sampling rate if you want the most resolution out of your images

- Use narrowband filter to tame atmosphere. People often use Ha filter or IR pass filter or Baader Solar Continuum 540nm filter when doing lunar with mono camera

- Don't use histogram to set your exposure, in fact don't look at histogram at all. Set your exposure length below 5-6ms to beat the seeing

- Use very high gain settings - as that lowers the read noise. Read noise is one of your worst enemies here

- Do proper calibration (means take darks, flats and flat darks)

Diffraction limited field of Newtonian is given by following expression:

https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

With F/5 scope that gives 1.389 mm from optical axis or 2.77mm diagonal. You need to select your ROI to that size (or that size amplified by barlow lens if you use one).

 

Thanks Vlad - very helpful as always. I will do the maths later and see how it looks!

[CraigT82]

In your shoes I'd probably use a 2.5x or 3x amplifier. For hi res work you need to make sure scope is as collimated as you can get it and that it is cooled and thermally stable. Consider fans and/or wrapping of the tube. Also need to consider what you're shooting over and try to avoid houses or car parks. Shooting over fields is better but may not be possible for you. 

Try to image when the moon is up high, I've got my best images up above 45 degrees although nice results can be had when lower. 

If seeing is so-so then use red or IR filters. Good seeing might call for a green filter and excellent seeing might support a blue filter for the finest resolution your scope can give. 

During capture you want to ignore the histogram as Vlaiv says. You dont want any blown out highlights so keep an eye on the rims of craters etc. The image will look dark in the preview when properly exposed, but with processing a lot of detail can be teased out of the shadows.

Focusing is critical so spend time on that. Don't bother with bhatinov masks or any focusing aids... get the preview image zoomed in and watch the fine details carefully as you rack the focuser in and out through the focus point. Make small adjustments and observe the image carefully after each adjustment. I would say that motorised focus is essential. 

You don't need huge capture sessions really, I find that 5000 frames captured and stacking maybe 500 best frames yields good results. 

Hooe that helps, good luck!

 

[Owmuchonomy]

All good advice above.  I'm not as clever as those guys so I use a simple formula whereby the ideal focal length is 5x your chip pixel size.  So if your pixel size is 5 microns you need to aim for f/25; for 3 microns, f/15.  That said there are other factors I also find critical, predominantly defeating the seeing conditions.  That is all about collecting frames during the best seeing so I aim for at least 80fps and use an IR pass filter where possible.  As @vlaiv says aim for very fast shutter speeds and a high gain setting.  I use flat frames but never bother with dark frames for lunar with my cameras (ASI174MM and ASI290MM).  Lunar imaging is very rewarding so have a go and enjoy the results.  Below is an example using the 'widescreen' format 290 chip.

[vlaiv]

1 hour ago, Owmuchonomy said:

 I'm not as clever as those guys so I use a simple formula whereby the ideal focal length is 5x your chip pixel size.

That is actually quite right. In theory that is "the best" focal ratio to use as it relates to above formula if we plug in 400nm wavelength as shortest wavelength of interest (just realized that I gave wrong formula above - now corrected, if we use focal_ratio then aperture should not be on left hand side). In any case focal_ration = 2 * pixel_size / 0.4 = 2 * 2.5 * pixel_size = 5 * pixel_size (as 0.4 is reciprocal of 2.5).

In practice - you can use x4 which would be equivalent of using 500nm wavelength in above formula. This is because blue part of spectrum is heavily impacted by atmosphere and blurs the most (blue part of spectrum is most susceptible to dispersion).

[rnobleeddy]

On 02/04/2021 at 12:50, vlaiv said:

Diffraction limited field of Newtonian is given by following expression:

https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

With F/5 scope that gives 1.389 mm from optical axis or 2.77mm diagonal. You need to select your ROI to that size (or that size amplified by barlow lens if you use one).

 

Would it be worth using a coma corrector here? 

Quote

- You can use lower sampling rate than critical sampling rate - but I would advise against going over critical sampling rate as it really does not do anything for image quality and only lowers SNR. You can calculate F/ratio that you need for given pixel size by using following formula:

F_ratio = 2 * aperture * pixel_size / wavelength

F_ratio = 2 * pixel_size / wavelength

(aperture is already in focal ratio, formula with aperture is for focal length instead)

Use same units of length for all quantities (say micrometers for all).

(reference: https://en.wikipedia.org/wiki/Spatial_cutoff_frequency, https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem)

 

Thanks. Based on this, with this camera, a 2x barlow would probably suffice for Ha wavelength. I'll have a think though - a decent 2" barlow isn't cheap so I may consider something like a asi178mm-mono, as I also could also use it as a 2nd guide camera to prevent swapping kit as much.

[rnobleeddy]

On 02/04/2021 at 16:35, CraigT82 said:

In your shoes I'd probably use a 2.5x or 3x amplifier. For hi res work you need to make sure scope is as collimated as you can get it and that it is cooled and thermally stable. Consider fans and/or wrapping of the tube. Also need to consider what you're shooting over and try to avoid houses or car parks. Shooting over fields is better but may not be possible for you. 

Try to image when the moon is up high, I've got my best images up above 45 degrees although nice results can be had when lower. 

If seeing is so-so then use red or IR filters. Good seeing might call for a green filter and excellent seeing might support a blue filter for the finest resolution your scope can give. 

During capture you want to ignore the histogram as Vlaiv says. You dont want any blown out highlights so keep an eye on the rims of craters etc. The image will look dark in the preview when properly exposed, but with processing a lot of detail can be teased out of the shadows.

Focusing is critical so spend time on that. Don't bother with bhatinov masks or any focusing aids... get the preview image zoomed in and watch the fine details carefully as you rack the focuser in and out through the focus point. Make small adjustments and observe the image carefully after each adjustment. I would say that motorised focus is essential. 

You don't need huge capture sessions really, I find that 5000 frames captured and stacking maybe 500 best frames yields good results. 

Hooe that helps, good luck!

 

Thanks. How far away does the ground matter? I have a clear view over fields to to the East, and have a couple of hundred meters to the south, so hopefully I'll have opportunities. 

Any recommendations for a barlow? My current camera would need a 2" barlow, so it's probably worth considering a smaller planetary cam  + a 1.25" barlow as well. 

[vlaiv]

1 minute ago, rnobleeddy said:

Would it be worth using a coma corrector here? 

I don't think so, but can't be 100% sure.

Problem with corrective optics like coma correctors is that they do fix coma further away from optical axis - but they often "mess up' things on axis. Optics often ends up not being diffraction limited.

This is not an issue for DSO / long exposure astrophotography as star sizes are dominated by other factors like seeing and mount performance - but it will be detrimental for lucky imaging where you want the most sharpness that you can get.

For example - simple 2 element coma corrector is known to introduce spherical aberration. Even very expensive CCs don't have nice spot diagrams.

Here is example from SharpStar quite expensive coma corrector:

As you can see - spot is smaller at 13 and 18mm away from optical axis than on axis (both geometric and RMS radius of spot diagram).

7 minutes ago, rnobleeddy said:

Thanks. Based on this, with this camera, a 2x barlow would probably suffice for Ha wavelength. I'll have a think though - a decent 2" barlow isn't cheap so I may consider something like a asi178mm-mono, as I also could also use it as a 2nd guide camera to prevent swapping kit as much.

You don't need 2" barlow if you are going to use only central portion of the sensor - you can use 1.25" barlow. If for example you have diffraction limited field of 3mm - then using x2 barlow will turn that into 6mm and using x3 barlow will turn that into 9mm.

All well within 20+ mm of clear aperture of 1.25" barlow.

In fact, if you want to utilize larger field - you can get special barlow. APM has coma correction barlow and while not cheap - it is considered one of the best barlows.

https://www.teleskop-express.de/shop/product_info.php/info/p5662_APM-Comacorrected-1-25--ED-Barlow-Element-2-7x---photo---visual.html

However, that barlow will require some experimentation on your part. Barlow elements can be moved closer and further away from sensor to vary their magnification. This is very handy as you can dial in exact F/ratio that you want to image at. With this barlow - there is "working distance" as it works as coma corrector. It is designed for F/4 scope and x2.7 magnification - which means that working distance is defined:

If you change working distance - you'll change magnification factor, but you'll also change coma correction of it. You'll have to experiment with size of the field you want to image to see if this barlow will correct that field at your wanted magnification.

Other than that - you can choose any simple barlow that has removable element that you can then position at wanted distance from sensor with different extension tubes - and you can calculate (or measure) magnification of it.

For example:

https://www.firstlightoptics.com/barlows/baader-classic-q-225x-barlow.html

But if you want the best barlow, then no doubt it is this one:

https://www.firstlightoptics.com/barlows/baader-vip-modular-2x-barlow-lens-125-and-2.html

 

[rnobleeddy]

On 02/04/2021 at 12:50, vlaiv said:

Diffraction limited field of Newtonian is given by following expression:

https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

With F/5 scope that gives 1.389 mm from optical axis or 2.77mm diagonal. You need to select your ROI to that size (or that size amplified by barlow lens if you use one).

 

 

On 04/04/2021 at 16:49, vlaiv said:

You don't need 2" barlow if you are going to use only central portion of the sensor - you can use 1.25" barlow. If for example you have diffraction limited field of 3mm - then using x2 barlow will turn that into 6mm and using x3 barlow will turn that into 9mm.

All well within 20+ mm of clear aperture of 1.25" barlow.

 

Realize I'm dragging up the distant past, so feel free to ignore! 

I'm considering some choices on scope/camera and came back to this thread - still working with Newtonians but one day I'll get a decent quality longer focal length refractor. I don't think my 60mm F6 APO refractor will be much use for lunar work

I'm trying to work out how big a camera it's worth using for lunar imaging with a Newtonian and have two questions:

- why does the the x2 barlow double the size of the diffraction limited field rather than quadruple it?  Assuming a 2x barlow moves from F5 to 10, based on the links you quote and for example here it appears that the diffraction limited field scales with F squared? I imagine I've just made a very dumb mistake in my logic here!

- Less info is available on whether I need to stop exactly at the edge of the diffraction limited field? Everything is a compromise (e.g. less panels in a mosaic) and so I was trying to find an estimate of how quickly being outside of the diffraction limited becomes the limiting factor in the quality of the resulting image.

 

[vlaiv]

10 hours ago, rnobleeddy said:

- why does the the x2 barlow double the size of the diffraction limited field rather than quadruple it?  Assuming a 2x barlow moves from F5 to 10, based on the links you quote and for example here it appears that the diffraction limited field scales with F squared? I imagine I've just made a very dumb mistake in my logic here!

Because barlow lens does not change geometry of the mirror - it simply enlarges image at focal plane.

Geometry of the mirror is responsible for creating aberration, and while we see light as being F/10 - it was still produced by F/5 mirror and not F/10 one. Barlow changes light beam only after it has been bent by F/5 mirror and therefore aberration has already been "imprinted" in it.

Similarly - if we take fast F/5 achromat and add barlow - we won't make chromatic aberration be equivalent to F/10 achromat of same aperture. We will have same level of CA only magnified in focal plane.

10 hours ago, rnobleeddy said:

- Less info is available on whether I need to stop exactly at the edge of the diffraction limited field? Everything is a compromise (e.g. less panels in a mosaic) and so I was trying to find an estimate of how quickly being outside of the diffraction limited becomes the limiting factor in the quality of the resulting image.

You don't need to stop right at the edge, but what will happen depending on where you stop is very complex topic.

Coma is asymmetrical aberration. That is somewhat problematic when we try to do frequency restoration in the end (try to sharpen image).

When we do lucky imaging, we end up with image that is somewhat blurry after stacking and we need to sharpen it up. Better SNR we achieve and less blurry image is to start with - better sharpening results we will get.

We are actually able to "sharpen" up even diffraction limited image. Here is a graph that represents inherent "sharpness" of telescope:

Black line is ideal perfect unobstructed aperture - red looks like about 25% central obstruction or maybe 1/4 wave spherical aberration.

Ideal image with perfect resolution for given sampling rate would have constant line at Y=1 - it would not fall down to 0. When we sharpen up image - we take above graph and try to "straighten" it - try to raise that line to stay at constant Y=1.

For example, sharpening is able to correct for spherical aberration of the telescope. This is why people make excellent planetary images with SCT telescopes that have rather large central obstruction - over 30%.

In fact, I took this image of Jupiter with rather fast newtonian that has spherical mirror - it was with 130mm F/6.9 spherical mirror:

But it is rather sharp and detailed image for such a scope, isn't it? This is because sharpening corrects even for blur caused by spherical aberration.

However, with coma, things are different, look at this image:

Look at the shape of coma blur - next to being blurred, it is also "smeared" in one direction and if you correct the blur - there will still be some "smear" left.

In above MTF diagrams - some aberrations have two lines on their graph and some have single line. Coma, astigmatism and seeing in above graphs have two lines - this means that they are asymmetrical aberrations. Seeing here is just a single moment of seeing and in stacked image - it turns in more or less symmetrical aberration - simply because we stack many subs each having different seeing PSF - and it averages out to "round" shape.

Back to understanding why limiting to diffraction limited field is important:

- coma is asymmetric aberration - that means that sharpening will not fix it properly - you'll be able to deblur it somewhat, but there will still be some level of "smear" left

- coma depends on distance from optical axis - that means that single panel will have different level of coma blur in center and on edge (unless you keep it constrained to diffraction limited field - which in theory should have same - minimal level of coma blur). We don't yet have selective sharpening algorithms - that will apply different levels of sharpening in center and at the edge. This in turn means that you'll either over sharpen the center or under sharpen the edge for same amount of sharpening.

Result in any case is image that does not have equal sharpness with zones of blurriness where panels join.

This is still possible with mosaic images even if you have perfect optics - as seeing conditions can change between panels and you end up with some panels being sharp and some being less sharp due to changing seeing - but there is nothing we can do about that - and it is transient in nature - meaning next time it might not happen again.

In the end - it is good to understand how fast coma grows with height of field and we can use spot diagram to represent that.

Look at B ) part of the image above / top right. It shows different F/ratio newtonians at center, 2.1 mm and 3mm of axis. Circle represents airy disk and in theory we want to keep all the dots inside airy disk. This will slightly enlarge airy disk - but not by much (airy disk is interference pattern that forms when all rays come to single point and above spots are different rays hitting in different spots. They have minimal phase shift due to not hitting in same place and this will somewhat change interference pattern as well).

In any case - at 2.1mm off axis at F/5 we have slightly larger spot diagram than airy disk, but at 3mm - it is twice as large - so aberration grows rapidly. In fact - in left top part of the image - you can see airy disk and first ring and corresponding spot diagram.

For F/5 newtonian at 2.1mm off axis - we have same spot diagram as F/8.15 newtonian at 6mm of axis and Airy pattern looks like classical case of miscollimation - and you know how soft image is when there is a bit of miscollimation.

Bottom line - I would try not to exceed diffraction limited field if at all possible, but you can get larger sensor and simply use ROI and experiment. Take larger and smaller panels and see what sort of difference you are getting - then settle for best compromise in panel size and sharpness.

HTH

 

[rnobleeddy]

12 hours ago, vlaiv said:

Because barlow lens does not change geometry of the mirror - it simply enlarges image at focal plane.

Geometry of the mirror is responsible for creating aberration, and while we see light as being F/10 - it was still produced by F/5 mirror and not F/10 one. Barlow changes light beam only after it has been bent by F/5 mirror and therefore aberration has already been "imprinted" in it.

Similarly - if we take fast F/5 achromat and add barlow - we won't make chromatic aberration be equivalent to F/10 achromat of same aperture. We will have same level of CA only magnified in focal plane.

You don't need to stop right at the edge, but what will happen depending on where you stop is very complex topic.

Coma is asymmetrical aberration. That is somewhat problematic when we try to do frequency restoration in the end (try to sharpen image).

When we do lucky imaging, we end up with image that is somewhat blurry after stacking and we need to sharpen it up. Better SNR we achieve and less blurry image is to start with - better sharpening results we will get.

We are actually able to "sharpen" up even diffraction limited image. Here is a graph that represents inherent "sharpness" of telescope:

Black line is ideal perfect unobstructed aperture - red looks like about 25% central obstruction or maybe 1/4 wave spherical aberration.

Ideal image with perfect resolution for given sampling rate would have constant line at Y=1 - it would not fall down to 0. When we sharpen up image - we take above graph and try to "straighten" it - try to raise that line to stay at constant Y=1.

For example, sharpening is able to correct for spherical aberration of the telescope. This is why people make excellent planetary images with SCT telescopes that have rather large central obstruction - over 30%.

In fact, I took this image of Jupiter with rather fast newtonian that has spherical mirror - it was with 130mm F/6.9 spherical mirror:

But it is rather sharp and detailed image for such a scope, isn't it? This is because sharpening corrects even for blur caused by spherical aberration.

However, with coma, things are different, look at this image:

Look at the shape of coma blur - next to being blurred, it is also "smeared" in one direction and if you correct the blur - there will still be some "smear" left.

In above MTF diagrams - some aberrations have two lines on their graph and some have single line. Coma, astigmatism and seeing in above graphs have two lines - this means that they are asymmetrical aberrations. Seeing here is just a single moment of seeing and in stacked image - it turns in more or less symmetrical aberration - simply because we stack many subs each having different seeing PSF - and it averages out to "round" shape.

Back to understanding why limiting to diffraction limited field is important:

- coma is asymmetric aberration - that means that sharpening will not fix it properly - you'll be able to deblur it somewhat, but there will still be some level of "smear" left

- coma depends on distance from optical axis - that means that single panel will have different level of coma blur in center and on edge (unless you keep it constrained to diffraction limited field - which in theory should have same - minimal level of coma blur). We don't yet have selective sharpening algorithms - that will apply different levels of sharpening in center and at the edge. This in turn means that you'll either over sharpen the center or under sharpen the edge for same amount of sharpening.

Result in any case is image that does not have equal sharpness with zones of blurriness where panels join.

This is still possible with mosaic images even if you have perfect optics - as seeing conditions can change between panels and you end up with some panels being sharp and some being less sharp due to changing seeing - but there is nothing we can do about that - and it is transient in nature - meaning next time it might not happen again.

In the end - it is good to understand how fast coma grows with height of field and we can use spot diagram to represent that.

Look at B ) part of the image above / top right. It shows different F/ratio newtonians at center, 2.1 mm and 3mm of axis. Circle represents airy disk and in theory we want to keep all the dots inside airy disk. This will slightly enlarge airy disk - but not by much (airy disk is interference pattern that forms when all rays come to single point and above spots are different rays hitting in different spots. They have minimal phase shift due to not hitting in same place and this will somewhat change interference pattern as well).

In any case - at 2.1mm off axis at F/5 we have slightly larger spot diagram than airy disk, but at 3mm - it is twice as large - so aberration grows rapidly. In fact - in left top part of the image - you can see airy disk and first ring and corresponding spot diagram.

For F/5 newtonian at 2.1mm off axis - we have same spot diagram as F/8.15 newtonian at 6mm of axis and Airy pattern looks like classical case of miscollimation - and you know how soft image is when there is a bit of miscollimation.

Bottom line - I would try not to exceed diffraction limited field if at all possible, but you can get larger sensor and simply use ROI and experiment. Take larger and smaller panels and see what sort of difference you are getting - then settle for best compromise in panel size and sharpness.

HTH

 

Awesome, thanks @vlaiv - that's exactly what I was looking for. As I guessed, the barlowed F-ratio vs diffraction limited field size was just a misunderstanding on my part.

Doing some comparisons will be relatively easy because I can do a full frame (of the 4/3 camera I have) in far fewer panels than the diffraction limited version.