New to planetary - Couple of questions

[CraigD1986]

I’m dipping my toe in the water with planetary imaging and have a couple of newb questions. Here’s my gear:

Skywatcher 190MN (1000mm f5.3) Mak Newt astrograph
ASI533MC Pro
Celestron 3x X-Cel Barlow

1) My focuser only has 40mm of travel and I can’t get enough back focus to achieve focus. Can I just buy some of these to extend the distance:

https://www.amazon.co.uk/Extension-M42x0-75mm-Telescope-Accessory-Astrophotography/dp/B0BDV5652K/ref=sr_1_4?crid=17UAT6QKJ9XGJ&keywords=T2+extension&qid=1699383479&sprefix=t2+extension%2Caps%2C88&sr=8-4

2) When using video mode in ASIAir, even when selecting the lowest resolution (which I assume is the equivalent of region of interest), the frame rate drops considerably when I start recording - to about 4 fps. Why is this? I have a 32gb Sandusky Extreme Pro micro SD card in if that makes a difference.

3) What’s the best way to focus if there is no moon? I have the ZWO EAF. Just manually by eye or focus on a star?

thanks in advance!

[Kon]

5 hours ago, CraigD1986 said:

3) What’s the best way to focus if there is no moon? I have the ZWO EAF. Just manually by eye or focus on a star?

I can only comment on this; always on the planet. Try tease some fine details, weather dependant. I check my focus for every capture unless the seeing is exceptionally stable.

[inFINNity Deck]

Hi Craig,

first about your set-up:

10 hours ago, CraigD1986 said:

Skywatcher 190MN (1000mm f5.3) Mak Newt astrograph
ASI533MC Pro
Celestron 3x X-Cel Barlow
 

Your camera has 3.76micron pixels. Theoretically you will achieve optimal conditions at a focal-ratio 3x that figure (see my white-papers on that), so at 3.76 x 3 = f/11, so the 3x Barlow may be a bit too much, will cause oversampling and longer than necessary exposure times. It is beter to go with 2x or 2.5x Barlow and resize the planet 200% afterwards.

I would add an ADC (Atmospheric Dispersion Corrector) to your equipment list (if not already present).

10 hours ago, CraigD1986 said:

1) My focuser only has 40mm of travel and I can’t get enough back focus to achieve focus. Can I just buy some of these to extend the distance:

https://www.amazon.co.uk/Extension-M42x0-75mm-Telescope-Accessory-Astrophotography/dp/B0BDV5652K/ref=sr_1_4?crid=17UAT6QKJ9XGJ&keywords=T2+extension&qid=1699383479&sprefix=t2+extension%2Caps%2C88&sr=8-4
 

Yes, that should work.

I have no knowledge on AsiAir, so I will skip that... 😉

10 hours ago, CraigD1986 said:

3) What’s the best way to focus if there is no moon? I have the ZWO EAF. Just manually by eye or focus on a star?
 

I always use a Bahtinov mask on a nearby star or, in case of Jupiter, on its moons.

Nicolàs

https://www.dehilster.info/astronomy/jupiter.php

https://www.dehilster.info/astronomy/mars.php

 

 

 

[Space Cowboy]

The vast majority of planetary imagers focus on the planet not a near by star. Atmospherics, temperature and elevation change effects focus. Bmasks are of no use.

The considered ideal focal length is 5x pixel size. An ADC will not work to it's best if undersampling the image.

Here is a great source of info on planetary imaging :

https://www.cloudynights.com/topic/812022-planetary-imaging-faq-updated-january-2023/#entry12404914

 

Edited November 8, 2023 by Space Cowboy

[bosun21]

8 hours ago, inFINNity Deck said:

pixels. Theoretically you will achieve optimal conditions at a focal-ratio 3x that figure (see my white-papers on that), so at 3.76 x 3 = f/11, so the 3x Barlow may be a bit too much, will cause oversampling and longer than necessary exposure times

I believe that for planetary it's actually 5 x pixel size. That's what i have been going by for average seeing conditions.

[CraigD1986]

Thanks for the replies. As I’m just dipping my toe in the water, I’m just making use of what I already have. If I enjoy planetary imaging, I’ll buy a planetary camera. If that time comes, how do I decide which camera to buy? ASI224MC or similar?

Edited November 8, 2023 by CraigD1986

[inFINNity Deck]

Hi Stuart and Bosun,

yes, the factor 5 is well known to me, but so far no one managed to explain why it should be used (other than because others do so). The only reason I can think of, is that the planet shows up larger in the image, but you will not gain any additional detail when going above a factor 3. Only when imaging with a perfect scope (i.e. Strehl ratio 1), perfectly collimated (is it ever?) and from outside our atmosphere, you may find it useful to go to a factor 3.7. Above that physics does not allow to gain more detail, but you will have the downside of longer exposure times.

I have discussed this before in this thread and shown what happens if we down-size and re-size the image to original size (spoiler: no detail is lost):

If you take the time to read my articles and especially the second one (linked from the first one), you will understand that indeed going much above 3 x pixel-size is rather pointless. If you want a larger planet on screen, simply up-scale it by 200%. There will be no difference in detail, but less noise due to more data and perhaps even slightly more detail because of the shorter exposure times.

Regarding the latter: suppose I am imaging using a ZWO ASI174MM with 5.9 micron pixels and do that with a C11 (my currently preferred planetary scope) at native focal length of f/10, I will achieve a theoretical resolution 0.43"/px. Today Jupiter is 49.4" is in diameter, so 115px across and thus (assuming a perfect sphere), a total surface of  (115/2)^2 x pi() = 10387 pixels. If I go to f/20 I will get a total surface of 115^2 x pi() of 41548 pixels. Now changing the focal ratio does not change the amount of light entering the telescope from Jupiter. In other words: each pixel only gets a quarter of the photons when increasing the focal ratio by a factor of two (i.e. a quadratic relationship).

So when going from a factor of 3 to a factor of 5, we have to raise the exposure time by  a factor 5/3^2 = 2.8 to get the same amount of photons per pixel, while we do not gain any additional details.

I am imaging at a factor 3.4, simply because my scope is f/10 and I use a 2x PowerMate and above camera. So my focal ratio is f/20 and camera has 5.9 micron pixels, thus factor 20/5.9 = 3.4. The optimum would have been f/17.7, but that is not easily achieved, so I stick with it, even though it comes at about 28% longer exposure times.

If you like below images, there is no reason to go above a factor 3 (two of them are resized 200% , all where imaged at a 3.4 factor).

Nicolàs

 

[inFINNity Deck]

17 minutes ago, CraigD1986 said:

Thanks for the replied. As I’m just dipping my toe in the water, I’m just making use of what I already have. If I enjoy planetary imaging, I’ll buy a planetary camera. If that time comes, how do I decide which camera to buy? ASI224MC or similar?

Hi Craig,

I never tried the ASI224MC and in general I am not too fond about OSC-cameras, but that is entirely personal (I do have a ASI290MC, but never got a decent image out of it while others have no issues with that).

The ASI224 has 3.75 micron pixels, so basically the same as the ASI533MC Pro. Thus optimal imaging is done at f/11, so 2x barlow should suffice (I would recommend a telecentric one like the TeleVue PowerMate).

Regarding the ADC: these work best at f/10 and slower telescopes, so 2x barlow should be fine (see here, although f/15 is mentioned a lower limit as well, and so is f/20). If we look at the Rolls-Royce of the ADCs, the ones by Gutekunst (you can buy a few dozen ZWOs for one Gutekunst), we learn that f/10 or f/12 and higher is recommended: "In order to get best performance from the ADC, we should use slow f/ratios, preferably above f/10 or f/12, so unless you use such a scope, a barlow should be used if we want optimum performance".

Nicolàs

[Space Cowboy]

55 minutes ago, inFINNity Deck said:

Hi Stuart and Bosun,

yes, the factor 5 is well known to me, but so far no one managed to explain why it should be used (other than because others do so). The only reason I can think of, is that the planet shows up larger in the image, but you will not gain any additional detail when going above a factor 3. Only when imaging with a perfect scope (i.e. Strehl ratio 1), perfectly collimated (is it ever?) and from outside our atmosphere, you may find it useful to go to a factor 3.7. Above that physics does not allow to gain more detail, but you will have the downside of longer exposure times.

I have discussed this before in this thread and shown what happens if we down-size and re-size the image to original size (spoiler: no detail is lost):

If you take the time to read my articles and especially the second one (linked from the first one), you will understand that indeed going much above 3 x pixel-size is rather pointless. If you want a larger planet on screen, simply up-scale it by 200%. There will be no difference in detail, but less noise due to more data and perhaps even slightly more detail because of the shorter exposure times.

Regarding the latter: suppose I am imaging using a ZWO ASI174MM with 5.9 micron pixels and do that with a C11 (my currently preferred planetary scope) at native focal length of f/10, I will achieve a theoretical resolution 0.43"/px. Today Jupiter is 49.4" is in diameter, so 115px across and thus (assuming a perfect sphere), a total surface of  (115/2)^2 x pi() = 10387 pixels. If I go to f/20 I will get a total surface of 115^2 x pi() of 41548 pixels. Now changing the focal ratio does not change the amount of light entering the telescope from Jupiter. In other words: each pixel only gets a quarter of the photons when increasing the focal ratio by a factor of two (i.e. a quadratic relationship).

So when going from a factor of 3 to a factor of 5, we have to raise the exposure time by  a factor 5/3^2 = 2.8 to get the same amount of photons per pixel, while we do not gain any additional details.

I am imaging at a factor 3.4, simply because my scope is f/10 and I use a 2x PowerMate and above camera. So my focal ratio is f/20 and camera has 5.9 micron pixels, thus factor 20/5.9 = 3.4. The optimum would have been f/17.7, but that is not easily achieved, so I stick with it, even though it comes at about 28% longer exposure times.

If you like below images, there is no reason to go above a factor 3 (two of them are resized 200% , all where imaged at a 3.4 factor).

Nicolàs

 

These images don't prove anything. I've seen far more detailed results with a C11 scope at 5x pixel scale. You are selling yourself short imaging at that focal length. Theory is all very well but in practise more detail is gained by using the longer fl, especially when focusing which needs to be tweaked every few mins to adjust for any temperature , atmospheric or elevation change.

Edited November 8, 2023 by Space Cowboy

[inFINNity Deck]

1 minute ago, Space Cowboy said:

These images don't prove anything. I've seen far more detailed images with a C11 scope at 5x pixel scale. You are selling yourself short imaging at that focal length. Theory is all very well but in practise more detail is gained by using the longer fl, especially when focusing which needs to be tweaked every few mins to adjust for any temperature , atmospheric or elevation change.

Hi Stuart,

I would love to see those images, would be great to do some FFT-maths on them.

You say that "Theory is all very well but in practise more detail is gained by using the longer fl". This is not true, see this article I published with a befriended nuclear physicist. Resolving power only depends in wavelength and aperture (see formula 2 in that article). You can have a 5000mm focal length scope, but if it is f/50 (aperture 100mm) you will never get the detail of a 200mm aperture scope, even if it is only f/10.

But we are wandering off-track. Let us try answering as best we can Craig's questions.

Nicolàs

[Space Cowboy]

The best Craig can do is read that cloudy nights planetary imaging guide compiled by the best imagers from around the globe rather than be side tracked by individuals with "alternative" ideas.

[geoflewis]

On 07/11/2023 at 19:03, CraigD1986 said:

What’s the best way to focus if there is no moon? I have the ZWO EAF. Just manually by eye or focus on a star?

As others said, I always focus on the planet. It takes time, so best to make small adjustments and observe for a few seconds, then tweak back and forth until you're confident you have the best for the conditions. Of course it is impossible in poor seeing.

Also as others have said, aim for x5 pixel size and if anything, err on being slightly over sampled than undersampled, though for my recent images I have been slightly unsampled and using ADC, so it's not a killer.

 

[Owmuchonomy]

You could do worse than watch the You Tube videos on planetary imaging by Chris Go.  An f/ of 4x to 5x  of your pixel size is the sweet spot for my ASI224 and ASI290.  If I can't get back focus I just add in a diagonal.  Happy imaging!

[inFINNity Deck]

One way to arrive at a factor of 5, is to calculate the optimal focal ratio using the spatial cut-off frequency for the shortest observable wavelength. If we consider that we record in our images wavelengths between 400nm (blue) and 700nm (red) and assume that the shortest wavelength would in theory be able to show the finest details, then the optimal focal ratio would become 2 x [pixel size] / 0.400 = 5 x [pixel size]. @vlaiv for instance uses 400nm in his calculations (as he wants to be properly sampling over the whole visible spectrum) and so arrives at 5 x [pixel size]. The issue I see with this approach is that blue light is most scattered by our atmosphere, so it is less likely that we will actually achieve the highest resolution at that wavelength, even in lucky-imaging. At the other end of the spectrum we have red light of 700nm, which would result in an optimal focal ratio of 2 x [pixel size] / 0.700 = 2.9 x [pixel size], but that of course would be the lower limit (provided we image under an ideal atmosphere, using an ideal scope). I always used green light of 540nm in my calculations (as can be seen in my papers), which results in the 3.7 x [pixel size] as a maximum useful focal ratio for planetary imaging mentioned before (one can now easily imagine that this is not entirely correct for a reddish planet like Mars). The reasons for using that wavelength in my calculations were that the Sun's continuum reaches its maximum around it, that the Baader Continuum filter has a bandpass around it (for obvious reasons), that the green filter is more or less centred around it, and that ZWO colour cameras have their maximum quantum efficiency around it (actually slightly before it).

For the article on the effect of seeing on the visibility of sunspots we simulated various telescopes under various seeing. From those simulations I later deduced the relationship between seeing and oversampling factor as roughly 1.6 x [seeing]^0.74. If we create a set-up with a C11 or an 180mm aperture Maksutov that samples optimal at zero arc-seconds seeing (regardless how we define it) and use it at a best seeing of 0.6 arc-seconds, the oversampling-factor will only be about 1.1 (so 10% oversampling), while at 0.7 arc-seconds this is about 1.2, so the ideal focal ratio will only have to be adjusted by a few tens of percent. But if the best seeing during an imaging session is 2 arc-seconds, the image will be oversampled by almost a factor of two.

Weather permitting I observe, sketch and image the Sun on a daily basis for two scientific programs and also measure the seeing during those sessions using a Solar Scintillation Seeing Monitor:

I do not measure seeing at nighttime, but can imagine that it can be a bit better than at daytime due to the absence of the Sun. Still I do not expect to image diffraction limited very often (if ever) with the C11 here in the Netherlands (due to its aperture the C11 is seeing limited above a seeing of about 0.4-0.5 arc-seconds).

I therefore firmly believe that it is for above reasons that I have yet to find a planetary image that was not oversampled (or at least not more than by 10% or 20%). But again: please post an image here that is not oversampled as I would be very interested to see it.

Nicolàs

 

[geoflewis]

34 minutes ago, inFINNity Deck said:

One way to arrive at a factor of 5, is to calculate the optimal focal ratio using the spatial cut-off frequency for the shortest observable wavelength. If we consider that we record in our images wavelengths between 400nm (blue) and 700nm (red) and assume that the shortest wavelength would in theory be able to show the finest details, then the optimal focal ratio would become 2 x [pixel size] / 0.400 = 5 x [pixel size]. @vlaiv for instance uses 400nm in his calculations (as he wants to be properly sampling over the whole visible spectrum) and so arrives at 5 x [pixel size]. The issue I see with this approach is that blue light is most scattered by our atmosphere, so it is less likely that we will actually achieve the highest resolution at that wavelength, even in lucky-imaging. At the other end of the spectrum we have red light of 700nm, which would result in an optimal focal ratio of 2 x [pixel size] / 0.700 = 2.9 x [pixel size], but that of course would be the lower limit (provided we image under an ideal atmosphere, using an ideal scope). I always used green light of 540nm in my calculations (as can be seen in my papers), which results in the 3.7 x [pixel size] as a maximum useful focal ratio for planetary imaging mentioned before (one can now easily imagine that this is not entirely correct for a reddish planet like Mars). The reasons for using that wavelength in my calculations were that the Sun's continuum reaches its maximum around it, that the Baader Continuum filter has a bandpass around it (for obvious reasons), that the green filter is more or less centred around it, and that ZWO colour cameras have their maximum quantum efficiency around it (actually slightly before it).

For the article on the effect of seeing on the visibility of sunspots we simulated various telescopes under various seeing. From those simulations I later deduced the relationship between seeing and oversampling factor as roughly 1.6 x [seeing]^0.74. If we create a set-up with a C11 or an 180mm aperture Maksutov that samples optimal at zero arc-seconds seeing (regardless how we define it) and use it at a best seeing of 0.6 arc-seconds, the oversampling-factor will only be about 1.1 (so 10% oversampling), while at 0.7 arc-seconds this is about 1.2, so the ideal focal ratio will only have to be adjusted by a few tens of percent. But if the best seeing during an imaging session is 2 arc-seconds, the image will be oversampled by almost a factor of two.

Weather permitting I observe, sketch and image the Sun on a daily basis for two scientific programs and also measure the seeing during those sessions using a Solar Scintillation Seeing Monitor:

I do not measure seeing at nighttime, but can imagine that it can be a bit better than at daytime due to the absence of the Sun. Still I do not expect to image diffraction limited very often (if ever) with the C11 here in the Netherlands (due to its aperture the C11 is seeing limited above a seeing of about 0.4-0.5 arc-seconds).

I therefore firmly believe that it is for above reasons that I have yet to find a planetary image that was not oversampled (or at least not more than by 10% or 20%). But again: please post an image here that is not oversampled as I would be very interested to see it.

Nicolàs

 

Nicolàs,

This is a very interesting discussion, which I've enjoyed following, but given the OP's opening phrase, 'I’m dipping my toe in the water with planetary imaging....' maybe it's a little too deep for this thread.....

Edited November 9, 2023 by geoflewis

[Pete Presland]

very enjoyable thread to read, love reading all the different views regarding FL and optimum imaging scales. Something i have "wrestled" with for years, without truly having a definitive answer 🙂 

In answer to question of  focus though i always focus on the planet. I stand back from the monitor and watch the planet for making tweaks as the seeing inevitably comes and goes. You going get plenty of out of focus frames most nights, hence its called lucky imaging. A term i dislike, because there is very little involved when you look at some of the images produced by planetary imagers around the world.

[CraigD1986]

Thanks for the everyone’s input but as I said, I’m trying out planetary imaging to see if it’s for me and therefore am looking to see if it’s possible with the gear I already have. If I decide that I enjoy planetary imaging, then I can worry about all of the above but for now, it’s way beyond me. I’m not looking for award winning images, just to get started.