First dedicated astro cam after several years of DSLR - advice please!

[edarter]

Hi All,

I went to the PAS 2024 over the weekend and finally purchased my first dedicated astro camera - an Altair Astro 26C (Sony IMX571). So now I'm excited to try it, but a bit apprehensive as the documentation that comes with it is a bit more sparse than I would have imagined. I've just worked out the spacer I need to purchase to get the right distance to my Baader MPC Mk3 but a few thoughts have occurred to me, so advice on any of the following would be great!

Focussing - I usually use live view at 10x zoom on the screen on the back of my EOS 600D coupled with a Bhatinov mask on the main scope. Whats the approach with a dedicated camera? is it live view in NINA or sharpcap on the computer screen instead? (I don't have an auto focuser)

Correct Gain - How do I find out what is the correct gain to use? I did find another thread on here about the AA 26C which I think suggested gain of 200 as thats when the S/N ratio changed significantly. I would like to understand how to come about this though, do I run a sensor analysis in sharpcap? If so does that have to be done with the camera/telescope together and focussed or can that be done on the camera alone on the bench?

I use NINA - long shot but does anyone know if this camera is plug and play with NINA?

Is there anything else I need to be aware of in making the jump from DSLR to dedicated camera?

Many thanks
Ed

[ollypenrice]

For focus,  your capture software should give you a Full Width Half Max reading on either a chosen star or an average across the chip.

https://en.wikipedia.org/wiki/Full_width_at_half_maximum

This means that it looks at the bell curve of brightness going from one side of the star, through the middle, to the other side and it measures the stellar width half way up the curve. It can't accurately measure from one side of the full stellar image to the other because the faintness of the edge of the star makes it impossible to be sure just where it starts and ends. If using this FWHM facility in your software, the star must not be saturated. If it is, the top will be chopped off top of the curve and this will then have a flat top. To avoid saturated stars you can choose fainter ones or shorten the exposures, but 3 second-plus exposures average out the seeing and give you more stable FWHM readings. Do expect them to vary, though, at each reading and keep the lowest reading in mind.

In practice, I would use the B-Mask in live view on your bright alignment star, frame up the image and then check FWHM just before starting the run. The thing about FWHM is that you'll find a focus star without having to leave the target whereas, with a B mask, there might not be a suitable one in the frame. So... initial bright star focus with B mask and then FWHM on the target region after that. Check focus regularly - as ever.

Olly

 

Edited March 25 by ollypenrice

[Vroobel]

A back focus of the 26C is 17.5mm. I can recommend a ZWO Filter Drawer v2 as I think you will want to use filters. It's 21mm long, so you need any 16.5mm spacer which I believe the AA provides together with the camera, like ZWO does. 17.5 + 21 + 16.5 = 55 which is exactly what the Baader MPCC Mk III needs. Please remember about a 2.5mm ring provided with the MPCC. 

Edited March 25 by Vroobel

[edarter]

5 minutes ago, Vroobel said:

A back focus of the 26C is 17.5mm. I can recommend a ZWO Filter Drawer v2 as I think you will want to use filters. It's 21mm long, so you need any 16.5mm spacer which I believe the AA provides together with the camera, like ZWO does. 17.5 + 21 + 16.5 = 55 which is exactly what the Baader MPCC Mk III. Please remember about a 2.5mm ring provided with the MPCC. 

Hi, I've never understood what that ring is for? I've always used the CC at the full 48mm width with the DSLR and removed the ring. Will I need it for the 26C then?

[Vroobel]

Only with the 2.5mm ring attached you can have the proper 55mm back focus. I can only conclude that it's attachable/detachable to get a longer back focus if you need to add device(s) needing a longer optical path. I also didn't use the spacer in the beginning, but I had bad coma in corners/edges. Recently, after some break, I used my Netonian again with the MPCC Mk III together with the spacer and with my ASI2600MC and I had nice and flat field. 

[PhilB61]

I also have the AA26C, I downloaded and installed the software and drivers from the Altair website, don't forget you get 12 months free Sharpcap pro license also. It works perfectly with NINA not a single issue, except if you try using a too long USB3 cable, I just use USB2 which is plenty fast enough for DSO imaging. The 50mb files take about 1-2 secs to download, this could be an issue if you wanted to do high frame rate planetary imaging but then you probably wouldn't choose this camera for that. I use gain 100 with high conversion mode selected for all my images, very happy with the results so don't really see the need to experiment with other settings. 

[PhilB61]

For focusing with a B mask, in the NINA imaging tab select a 3-5 sec exposure and tick the loop button, then zoom in on a prominent star and adjust focus whilst the images loop on screen

[edarter]

13 minutes ago, PhilB61 said:

I also have the AA26C, I downloaded and installed the software and drivers from the Altair website, don't forget you get 12 months free Sharpcap pro license also. It works perfectly with NINA not a single issue, except if you try using a too long USB3 cable, I just use USB2 which is plenty fast enough for DSO imaging. The 50mb files take about 1-2 secs to download, this could be an issue if you wanted to do high frame rate planetary imaging but then you probably wouldn't choose this camera for that. I use gain 100 with high conversion mode selected for all my images, very happy with the results so don't really see the need to experiment with other settings. 

Didn't realise about sharpcap, that's a shame as I only renewed a few weeks ago.

I currently can't register on the AA site, some error message comes up which they are looking in to, I did get a disk with it but would assume the site is the only place to get updated drivers etc.

[woldsman]

I use the NINA image tab (on loop) for manual focussing - you can zoom in. Much nicer than that 10x live view, you'll find. A 3 second exposure for broadband works well with a Bahtinov  mask. 

Procedure is rough focus (get the HFD down to a low figure), slew to a focus star, then fine focus.  If the camera driver is ASCOM compliant, NINA should recognise it (it does my ZWO and ATIK cameras, likewise the DSLR too as I've a DSLR ASCOM driver). 

I use the Baader Mk III coma corrector and it works fine with CCD, CMOS and DSLR cameras. Reported issues I think stem from a) misunderstanding the measuring point for back distance (see diagram); b) not using the 2.5mm ring (+1 Vroobel);

c) not having the right camera for the scope which causes guiding/tracking issues, affecting stars in the corners. 

Not sure what gain is on your camera. Maybe 100 to start with!? Don't forget offset  as otherwise taking darks will cause calibration issues. I think the offset should be 50 or so. 

Edited March 25 by woldsman

[PhilB61]

Forgot about offset, I use 250 (the gain and offset scale for ZWO versions is different). Cuiv the Lazy geek does a good video on how to determine the best value.

[edarter]

39 minutes ago, woldsman said:

I use the NINA image tab (on loop) for manual focussing - you can zoom in. Much nicer than that 10x live view, you'll find. A 3 second exposure for broadband works well with a Bhatinov mask. 

Procedure is rough focus (get the HFD down to a low figure), slew to a focus star, then fine focus.  If the camera driver is ASCOM compliant, NINA should recognise it (it does my ZWO and ATIK cameras, likewise the DSLR too as I've a DSLR ASCOM driver). 

I use the Baader Mk III coma corrector and it works fine with CCD, CMOS and DSLR cameras. Reported issues I think stem from a) misunderstanding the measuring point for back distance (see diagram); b) not using the 2.5mm ring (+1 Vroobel);

c) not having the right camera for the scope which causes guiding/tracking issues, affecting stars in the corners. 

Not sure what gain is on your camera. Maybe 100 t start with!? Don't forget offset though as otherwise taking darks will cause calibration issues. I think it offset should be 50 or so. 

 Thanks for the tips regarding NINA.
I too had forgotten about offset! Thanks for the reminder. And thanks for the above diagram, I think its refreshed a memory as to why I removed the 2mm ring as well. I'm using a 130PDS telescope, and they are renowned for focus tube protrusion in to the OTA, especially when using a DSLR. My solution was to get the CC as far in to the focus tube as possible and to utilise a low profile DSLR adaptor. That way the DSLR is literally only 2mm out from the end of the FT. If I were to fit this ring I don't think that would allow the CC to go all the way inside the FT as its a wider Dia?? If that is the case then I presume I just need to include the extra 2mm in the spacer I order?

[ONIKKINEN]

I would not bother with the bahtinov mask at all, really the HFR readings from NINA are all you need and the bahtinov mask is an extra step with no benefit. As long as you arent completely out of focus and stars are visible you get a reading and can take it from there. You can mark your focuser drawtube in rough focus, so you dont need to guess when setting up on subsequent nights and can jump straight to the actual focus part.

I use 3s exposures looped in NINA and focus manually based on the HFR reading. After you have used your kit for a few nights you get an idea on what kind of HFR to expect, so it really doesnt take too long to find the smallest HFR. NINA will also plot your subs when you image through a sequence, so you can see if focus has gone out and when refocusing is necessary.

As for offset, make sure you have enough. Too much is not an issue but too little is, my RisingCam IMX571 requires at least 600 offset, so i just left it at the default of 768 and never looked back. You can measure the required offset by taking a bias frame (same gain as intended imaging gain) and seeing if the min value is above 0. If it is 0, increase until it isnt.

[woldsman]

5 hours ago, edarter said:

 Thanks for the tips regarding NINA.
I too had forgotten about offset! Thanks for the reminder. And thanks for the above diagram, I think its refreshed a memory as to why I removed the 2mm ring as well. I'm using a 130PDS telescope, and they are renowned for focus tube protrusion in to the OTA, especially when using a DSLR. My solution was to get the CC as far in to the focus tube as possible and to utilise a low profile DSLR adaptor. That way the DSLR is literally only 2mm out from the end of the FT. If I were to fit this ring I don't think that would allow the CC to go all the way inside the FT as its a wider Dia?? If that is the case then I presume I just need to include the extra 2mm in the spacer I order?

I also use the Baader cc mk iii with a 130pds but changed the focuser. Yes, if you add 2mm that should compensate. 

[edarter]

I've just done a sharpcap sensor analysis on it and the results are as follows. Does this mean that the best gain to use is 300 (significant drop in read noise)

[ONIKKINEN]

Yeah 300 seems like the optimal value. No real point in going with a higher gain as read noise really doesn't go down that much but you lose a lot of full well capacity.

[edarter]

31 minutes ago, ONIKKINEN said:

Yeah 300 seems like the optimal value. No real point in going with a higher gain as read noise really doesn't go down that much but you lose a lot of full well capacity.

Actually, that's a good point... full well depth is significantly lower at 300 than at 100, is that figure of 16k ish still OK?

 

Apologies for all the questions, lots to learn!

[ONIKKINEN]

9 minutes ago, edarter said:

Actually, that's a good point... full well depth is significantly lower at 300 than at 100, is that figure of 16k ish still OK?

 

Apologies for all the questions, lots to learn!

16k is more than enough for even the brightest targets (maybe M42, M31 and the like). This looks like the same gain as ZWOs gain 100 on the 2600MC, or gain 100 on my RisingCam IMX571, I'd say look no further, that's your go-to gain (300 that is).

I struggle to think of a scenario where you would actually need the 50k full well enough to justify the much higher read noise. Actually i dont think there is one, since with 2.4x the read noise you would need to expose 2.4^2 = 5.76 times longer to swamp read noise by the same amount as you would be with gain 300, so the 3x full well capacity is kind of meaningless.

Edited March 25 by ONIKKINEN

[woldsman]

According to the chart, optimal gain looks to be 290. You might want to repeat the analysis to check the result is constant and invariant to cooling temperature. Bear in mind that NINA and Sharpcap treat gain differently. On NINA I think you can select between low conversion and high conversion gain. Sharpcap reports results using HCG. This is applicable to Altair cameras with dual gain. Not sure if your camera is dual gain though - worth checking.  I think the Sharpcap data may require high conversion gain in NINA which may  involve selecting HCG. There is a NINA page dealing with this issue here:

https://nighttime-imaging.eu/docs/master/site/troubleshooting/altair_dualgain/

HCG is the default but if for some reason you selected LCG you would need to change it back again. Anyway this note may be irrelevant if your camera is not dual gain - it’s just that Altair make cmos cameras that are.

Edited March 26 by woldsman

[edarter]

1 hour ago, woldsman said:

According to the chart, optimal gain looks to be 290. You might want to repeat the analysis to check the result is constant and invariant to cooling temperature. Bear in mind that NINA and Sharpcap treat gain differently. On NINA I think you can select between low conversion and high conversion gain. Sharpcap reports results using HCG. This is applicable to Altair cameras with dual gain. Not sure if your camera is dual gain though - worth checking.  I think the Sharpcap data may require high conversion gain in NINA which may  involve selecting HCG. There is a NINA page dealing with this issue here:

https://nighttime-imaging.eu/docs/master/site/troubleshooting/altair_dualgain/

HCG is the default but if for some reason you selected LCG you would need to change it back again. Anyway this note may be irrelevant if your camera is not dual gain - it’s just that Altair make cmos cameras that are.

Thanks for the heads up Woldsman. this camera does indeed have dual gain so I will double check what was used, though it sounds from what you say that Sharpcap would default to that high gain? I didn't realise that gain changed between applications though, I knew it did between camera manufacturers, but assumed that if optimum gain was determined in Sharpcap that it would be the same in NINA (albeit making sure HGC is turned on to be 100% like for like). Reading that article, does this mean that if 290 is the optimal gain as reported by Sharpcap I should set it as 145 in NINA?

Edited March 26 by edarter

[woldsman]

Yes, high conversion gain = Sharpcap reported gain. So provided HCG is turned on in NINA, all is fine.  (THIS IS INCORRECT -see below). 

Edited March 26 by woldsman

[PhilB61]

From what I understand 100 gain at HGC is essentially the same as 300 gain in LGC mode, noise wise but without the hit in well depth. From a discussion I read including some posts from the Sharpcap author the high/ low conversion factor is 3 

[woldsman]

9 minutes ago, PhilB61 said:

From what I understand 100 gain at HGC is essentially the same as 300 gain in LGC mode, noise wise but without the hit in well depth. From a discussion I read including some posts from the Sharpcap author the high/ low conversion factor is 3 

 

I did some reading and what I said earlier wasn't correct.  Its LCG that is equated not HCG. I think that:

NINA 100 LCG = Sharpcap 100 gain

NINA 100 HCG = Sharpcap 200 gain

Don't have a dual gain  Altair camera! Reading NINA's instruction carefully looks the way to go. 

Edited March 26 by woldsman

[AstroMuni]

On 25/03/2024 at 12:11, edarter said:

Correct Gain - How do I find out what is the correct gain to use? I did find another thread on here about the AA 26C which I think suggested gain of 200 as thats when the S/N ratio changed significantly.

One way is to look at the graphs provided by the vendor. As the ZWO ASI2600mc is using same IMX571 chip the graphs for this would be similar to the Altair version. These show a significant drop in read noise at Gain 100 when HCG kicks in.

[Vroobel]

I cannot see any similarity in e.g. the read noise and dynamic range graphs. I use both ASI2600MC-Pro and ASI2600MM-Pro with gain 101. I thought that the cameras using the same sensor should work similarly, but apparently it's more up to the firmware than the sensor. 

[edarter]

17 hours ago, AstroMuni said:

One way is to look at the graphs provided by the vendor. As the ZWO ASI2600mc is using same IMX571 chip the graphs for this would be similar to the Altair version. These show a significant drop in read noise at Gain 100 when HCG kicks in.

I'm struggling to get hold of documentation at the moment, the altair website requires a login and that's not working for so.e reason. Being investigated apparently but does leave me a bit sparse on docs for this camera / chip combo. They have just sent me the latest software though while I wait.