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Mars (R-RGB) 20 September 2020


geoflewis
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I haven't posted on SGL for a couple of weeks, but thought I'd share what is my best ever image of Mars, captured between approx 3am-4am (BST) on 20 September 2020 (de-rotated timestamp is 02:30 UT). Mars diameter was 20.6" and just a few weeks prior to opposition the is phase was 97%. The slightly later (02:48 UT) 5xIR stack is also included for comparison.

Mars_2020Sept20_02303_gdbl_r-rgb.jpg.41f02a00959327f3d62a62ba814c31d1.jpg2020-09-20-0248_6-GDL-IR-Stack.jpg.ff0c5be79bbd437ebc45021a5e9179f5.jpg

The data for this image was captured through my C14 telescope with my new ASI290MM camera and Astronomic RGB filters, with a x2 TeleVue Powermate also in train. With Mars now at a decent altitude of around 40 degrees and on the advice of world renowned Australian planetary imager, Anthony Wesley, for the first time in several years I did not use the ADC.
The capture sequence was 5 sets of R-G-B-IR AVIs, each of 3 mins duration at 75 fps, however, with the R data being good, I opted to use the stack of 5xR as a luminance rather than the inferior resolution IR data.
I processed each of the 5xRGB AVIs in AS3!, taking each stack of best 30% into Registax 6 for wavelet sharpening, before de-rotating in WinJupos. The final de-rotated R-RGB stack had a further mild sharpening and colour balance adjustment in Registax 6, followed by a few aesthetic tweaks in Affinity Photo and PS, where I also added the annotation.

The image is displayed south up, with the south polar cap clearly seen. The lighter regions of Chryse and Tharsis are seen towards the bottom of the image, with the darker Aurorae Sinus, Tithonious Lacis and Solus Lacus fairly well resolved in Mars equatorial regions.

Thanks for looking.

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That's a beauty indeed!  I also ditched the ADC for my last session with the 290m (only had half an hour outside so didn't want to waste time fiddling with it) and found the results to be very nice, shan't be bothering with for for Mars for the rest of the apparition.

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8 minutes ago, CraigT82 said:

That's a beauty indeed!  I also ditched the ADC for my last session with the 290m (only had half an hour outside so didn't want to waste time fiddling with it) and found the results to be very nice, shan't be bothering with for for Mars for the rest of the apparition.

Thanks Craig. For me the on screen image without the ADC was far superior (both brighter and cleaner) to that with the ADC in train, which made attaining good focus easier. I had a few email exchanges with Anthony Wesley about optimising with/without an ADC, when using a mono camera. He's done extensive testing and concludes that it is not beneficial once Mars is up anywhere close to or above 40 degrees. To me that was counter intuitive from seeing with/without the ADC through the eyepiece, but of course that is a full spectrum, not mono view. I'll quote what he wrote to me on the subject in case it is of interest to you, or anyone else (I'm sure he won't mind).

"The idea of ADC is of course correct but in practice there are several factors in play and you have to be sure that the overall result is better than without the ADC. eg the quality of the ADC prisms is a big factor - if you're only applying a small amount of correction then this might be more significant than the dispersion.
The image offset (ie decollimation) is a significant problem for me with an f/4 scope, if you do the obvious thing and use the ra/dec controls to re-centre it then you've effectively decollimated by some amount. The correct way to handle this is to adjust your collimation controls to bring it back to the centre but this is not easy to do without touching the pointing of the scope."

I found it a very illuminating (no pun intended) discussion.

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9 minutes ago, si@nite said:

A great result Geof, love the effect red lum has on Mars, not tried it this apparition but will probably in the weeks ahead!

Yes, red being slightly shorter wavelength than IR will yield more resolution in good seeing than IR and is less destructive to the colour balance than IR when used as a luminance. Sometimes, however, the seeing is just too bad, in which case an IR luminance can help 'save' the image. If seeing is superb (rare from the UK), then maybe a G or R+G stack as lum will be even better, though I have not tested that on Mars.

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That's a super Mars image Geof... and thanks for sharing the workflow and thinking process. 

Re the ADC would be interested to know why they are thought to be unhelpful with higher altitudes. Obviously less dispersion at higher altitudes so not such an issue but if its 2 prisms that's 4 surfaces and I guess each one loses some signal and creates some reflections so that may be it. 

Also interesting that you go for a relatively lower frame rate and keep more frames than I normally do. Quality rather than quantity!

30 minutes ago, geoflewis said:

The image offset (ie decollimation) is a significant problem for me with an f/4 scope, if you do the obvious thing and use the ra/dec controls to re-centre it then you've effectively decollimated by some amount. The correct way to handle this is to adjust your collimation controls to bring it back to the centre but this is not easy to do without touching the pointing of the scope.

I'm sure he's right...... but not clear quite what he means by this? I also have an F4 newtonian scope (without CC) so take care to ensure the image is central. But I use the RA DEC controls to re-centre... not sure why thats an issue, and what does he mean by using "collimation controls"?

 

Ohhh wait...  does he mean the ADC causes an offset?? In that case recentring with RA DEC would effectively mis-centre the image. think I've answered my own question!

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17 minutes ago, Tommohawk said:

That's a super Mars image Geof... and thanks for sharing the workflow and thinking process. 

Re the ADC would be interested to know why they are thought to be unhelpful with higher altitudes. Obviously less dispersion at higher altitudes so not such an issue but if its 2 prisms that's 4 surfaces and I guess each one loses some signal and creates some reflections so that may be it. 

Also interesting that you go for a relatively lower frame rate and keep more frames than I normally do. Quality rather than quantity!

I'm sure he's right...... but not clear quite what he means by this? I also have an F4 newtonian scope (without CC) so take care to ensure the image is central. But I use the RA DEC controls to re-centre... not sure why thats an issue, and what does he mean by using "collimation controls"?

 

Ohhh wait...  does he mean the ADC causes an offset?? In that case recentring with RA DEC would effectively mis-centre the image. think I've answered my own question!

Thanks @Tommohawk. I will confess to riding on the coattails of others. Niall MacNeill is another Aussie, with whom I've struck up a friendship over the last few years, also a superb planetary imager (you can see some of his work on Astrobin at https://www.astrobin.com/users/macnenia/). Like me Niall has a C14 and with many nights of clear skies and superb seeing he's also done a fair bit of testing. Plus he's also had many discussion with Anthony Wesley. Niall wrote to me that the advice he had from Anthony is that it is better to have an exposure time that just freezes the seeing and get as many photons per frame as possible; so less frames but higher quality ones (NB he believes that is also Damian Peach’s advice).  Anthony advocates a 20 ms exposure time which, if the data transfer rate is not constrained, gives 50 gives fps.  However, Niall decided to do some experiments of his own.  On a night of fair seeing where he would see the difference he imaged Jupiter at 40 ms (25 fps), 20 ms (50fps), 13 ms (75 fps) and 10 ms (100 fps). The results at 13ms (75 fps) seemed to be optimum for his equipment (NB he's using a different camera), so in the absence of any testing of my own, that is what I've started using with my new ASI290MM. (BTW I could never get close to that fps with my old ASI120MM-s).

Regarding your question about the ADC "de-collimating" and required use of collimation controls, yes, I think your own answer about the image being offset is what I understand, but this is new learning for me, so I'm not totally sure that I understand it.

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Thanks Geof.

I'm just playing around with last nights Mars data and one obvious disadvantage of using very short exposures is that you end up with massive files. I have 126GB I'm trying to manage by decanting it over to an external drive. It's slow and painful!

In the past I've always aimed for as short an exposure as possible for best "freeze the seeing" effect and assumed the noise resolves by virtue of having more frames - but I'm assured by others who know better that this isnt the best approach. Yesterday I did 5  x 3 minute videos or Mars with a modest 35/50 gain and 1.5ms, but I also did one 3min video with 3.5ms - and this definitely looks better. So this seem to prove the point.

I will experiment again and try and find the optimum.

PS at the moment I'm testing an Omegon 385, rather than using my ASI290, so this may have a different optimum exposure.

Edited by Tommohawk
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58 minutes ago, Tommohawk said:

I'm just playing around with last nights Mars data and one obvious disadvantage of using very short exposures is that you end up with massive files. I have 126GB I'm trying to manage by decanting it over to an external drive. It's slow and painful!

I have a similar problem as my SSD only has about 115MB of free storage; I made the mistake of buying a laptop with only 250GB SSD + 1TB HD, not realising that the SDD is the boot drive so over half of it has all my progs on it. I was previously using a 600x600 ROI in FC which at 3mins gives a file size of 4.867GB for each AVI. After reading about the cut out option on the FC forum I had a play with that for this image. I kept the ROI of 600x600 to maintain good fps and a safety margin to not lose the image outside of the ROI, but then placed a 400x400 cut out around Mars to reduce the size of the files being written to the SSD. The new file sizes are 2.266GB, so half the size which really helps with my storage. Even then I still captured over 80GB, so still need to take of to the Sata drive for processing. I didn't see any reduction in capture performance, so will stick with that routine in future.

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Sounds good Geof. I wasn't aware of the cut out option in FC - but how is this different from just a smaller ROI if you're only writing the central 400x400 area?

As it happens the Omegon 385 wont run on FC, so I've had to learn Toupsky. It works well, although some aspects not as slick as FC. 

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2 minutes ago, Tommohawk said:

how is this different from just a smaller ROI if you're only writing the central 400x400 area?

ROI effectively stops down the chip size, so anything outside the ROI is not recorded, which allows for both fast FPS and smaller file sizes. Cut out is a 'save to HD' only feature where all of the chip (or whatever ROI) is being read, but on downloading, the file size only contains the region of the cut out. If you have accurate tracking and/or auto guiding then just using a smaller ROI will have the same effect, but I'm not auto guiding, so need a little addition play room around the planet, so as not to lose it outside the ROI. Mixing the 2 tools seems to give me the best of both worlds - so far anyway, but then I've only used it the once....🤔🤞

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2 hours ago, Tommohawk said:

Ohhh wait...  does he mean the ADC causes an offset?? In that case recentring with RA DEC would effectively mis-centre the image. think I've answered my own question!

Yes that is the issue.  We collimate our scopes on-axis, and if you then move a defocused star to the edge of the field you see that the donut is no longer concentric as it is now off-axis and miss-collimated. 

When the ADC is adjusted it shifts the image and if that image goes off the sensor you have to move the scope to get it back on, but then the target is now off-axis and so miss-collimation and other aberrations may come into play (coma, field curvature etc.) Three ways you could combat this:

  • You can use a bigger sensor to help avoid moving the scope
  • Collimate the scope at the off-axis position (could check for this position during set up as the ADC levers position, and hence the position of the corrected image, should be predictable)
  • Use a scope with low off-axis aberrations

Also, my own ZWO ADC adds a touch of astigmatism to the image, only a little, and on low planets when the dispersion is large, the astig. is the lesser of two evils.  Currently Mars is only exhibiting small amount of dispersion requiring a tiny ADC adjustment and on this target the astig. the ADC introduces is worse than the dispersion it's corrected, which may explain why I got a better image without the ADC last time out.

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2 hours ago, CraigT82 said:

Yes that is the issue.  We collimate our scopes on-axis, and if you then move a defocused star to the edge of the field you see that the donut is no longer concentric as it is now off-axis and miss-collimated. 

When the ADC is adjusted it shifts the image and if that image goes off the sensor you have to move the scope to get it back on, but then the target is now off-axis and so miss-collimation and other aberrations may come into play (coma, field curvature etc.) Three ways you could combat this:

  • You can use a bigger sensor to help avoid moving the scope
  • Collimate the scope at the off-axis position (could check for this position during set up as the ADC levers position, and hence the position of the corrected image, should be predictable)
  • Use a scope with low off-axis aberrations

Also, my own ZWO ADC adds a touch of astigmatism to the image, only a little, and on low planets when the dispersion is large, the astig. is the lesser of two evils.  Currently Mars is only exhibiting small amount of dispersion requiring a tiny ADC adjustment and on this target the astig. the ADC introduces is worse than the dispersion it's corrected, which may explain why I got a better image without the ADC last time out.

Thanks Craig, that's a great description of what's happening with the ADC and has really helped to clear the remaining fog (well most of it) from my brain, after discussing the issue with Anthony Wesley.

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Hi Geoff

Great image, well done. Also interesting discussion re ADC and exposure times. I tend to use around 10mS exposures with the ZWO  224MC colour camera. With a small RO!, I get 140 fps on Mars at f/22 on the C14. Of course you still need the ADC with a colour camera even at 40 degrees. I always collimate on screen so the whole image train is included in the adjustments.

Best regards

Peter

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7 minutes ago, astroman001 said:

Hi Geoff

Great image, well done. Also interesting discussion re ADC and exposure times. I tend to use around 10mS exposures with the ZWO  224MC colour camera. With a small RO!, I get 140 fps on Mars at f/22 on the C14. Of course you still need the ADC with a colour camera even at 40 degrees. I always collimate on screen so the whole image train is included in the adjustments.

Best regards

Peter

Thanks Peter,

Yes, from my discussion with Anthony, collimation on screen through the ADC is the only way to correct for the offset and I agree that using an ADC is a must with a colour camera. Different cameras will very likely optimise at different exposure times, but it makes complete sense to me that you need to be just fast enough to freeze the seeing and then maximise the number of photons captured, without risking blur through the planets rotation. I think with Mars, 4 min AVIs/SERs is about the maximum without risk of blurring when imaging with the C14, but I've stayed well inside that at 3 mins per AVI.

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3 hours ago, nfotis said:

I am surprised that you had to use a 2x Barlow with the C14.

So, would a Luminous 2.5x Barlow be usable with a C9.25 XLT? Or should I go for a 1.5x Barlow?

(I am thinking about the ASI462 camera myself)

N.F.

 

 

The x2 Powermate is a bit high for my new camera, but it is what I have. A good rule of thumb is that you want the effective focal ratio of your system to be about x5 the pixel size of your camera in microns. The ASI462 has 2.9 micron pixels (same as my ASI290), so you want to aim for a focal ratio of approx F15. The C9.25 is F10, so a x1.5 Barlow would be a good fit, but you can go a bit higher in good seeing. I think the x2.5 Barlow would be too much though. Also be aware that the ASI462 is a colour camera, so will need to factor in the use of an ADC, so see the above discussion about using those. Also with the ADC between the Barlow and sensor, your x1.5 might turn out to be greater magnification. It is one of the reasons that I use the x2 Powermate, as that does not suffer from additional magnification when including an ADC in the optical path.

Good luck.

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