Gina

The mount hit the end stop so I had to do a meridian flip and then readjust the framing to match the previous frames. Decided to increase exposures to 90s too. Here's a 90s sub g500 t-30°C 3nm OIII just flipped and rotated left as linear, full frame.

Had a look outside and the clouds have gone (for now). No sign of the moon on the eastern horizon yet. Just turned down the cooling set point to -30°C and the cooling has reached that with no problem - it's cold out tonight! I could see a reduction in the noise in the image. I think the OIII exposure is alright at 60s though could possibly be increased a bit but I'll leave it at that for now. I expect to need more exposure for SII.

Still getting some clouds passing over so these subs will need going through with the Blink tool in PixInsight. I could do with my all sky camera operating so that I can see what the sky looks like from indoors but I only have one Windows laptop and that's capturing NB ATM. I must have another go at Linux and the Raspberry Pi 3 with INDI and KStars which I now see has image capture without external capture software.

Changed to OIII filter and refocussed - now capturing subs. Here is the first OIII 60s sub, flipped and rotated only then saved in PNG format. Here's the full size image.

Some cloud about now but it's supposed to clear a bit later.

I only captured a few subs like this so I'm going for the full 100 before changing filter. I guess the OIII will be next. At least when the moon comes up it will be something like 90° away from the area of sky I'm imaging and with 3nm filters, it should be alright. I may stay up most of the night if it stays clear - I would love to grab the full set of NB images at this FOV, then I can go back to the 135 lens for the next session. There are several possibilities for the next few sessions. The Cygnus Loop NAN & Pelican Cygnus Wall IC1396 the one that contains the Elephant's Trunk Andromeda Galaxy M31 - depend on there being no moon

Clear tonight and capturing images Widefield Cygnus area with 55mm lens. Frame alignment done and currently imaging Ha. I'll be changing to OIII or SII shortly. Here's a 60s g500 t-25°C 3nm Ha sub.

Here is the histogram before stretching.

Calibrated master flat for 55mm lens and 1.25" 3nm Ha filter up against camera body. 100 subs each of biases, darks and flats. Firstly as is - just resized and saved in PNG format and secondly fully histogram stretched from black to white.

" a squadron of pigs coming-in over ASDA " - I love it

Just back home from a meeting and saw the sky was clear and the stars were out - moon not up yet - so I opened up the observatory and set everything up for some imaging. Got to the point where I had an image on the preview screen and the camera cooling nicely when all of a sudden the image disappeared. Went outside and looked at the sky and - guess what - yep!!! cloudy! Grrr... Still, I enjoyed the meeting and can't be in two places at once

Looks like there could be some clear sky tomorrow night with luck but still time enough to change completely! Moonrise is around 11pm so could have some time with a dark sky Maybe getting to the time to think about OIII and SII - the 3nm SII filter came today so I now have a full set of 3nm NB filters I might see about doing the Cygnus area in three wavelengths. Actually, I think I've covered pretty much all I can do at this time with what DSOs are available in Ha. There is some vignetting with the f1.8 lens and 1.25" filter even right up against the body of the camera (inside the adapter ring). Probably sorted out with calibration with flats though but I wouldn't want to move the filter further away from the sensor so a manual filter change will be in order.

Yes, 40°C below ambient or -30°C absolute whichever is higher. And yes, the tool for measuring the camera sensor characteristics is from PixInsight. I now have the appropriate captured frames so now I have to find the reference and set things going I also have the flats for the 55mm lens rig and matching master dark and master bias to calibrate with. After that I can process all the data I captured last night. I'm never short of things to do

My camera can barely get down 35°C below ambient let alone 40-45! Don't know why but if the absolute limit is -30°C it doesn't really matter. For planetary I bought an ASI185MC camera but the only real difference would be that it's a colour camera - cooling wouldn't be necessary. The pixel size is almost identical - 3.75 v 3.8 microns. A mono camera of the same pixel size will give better resolution as the colour needs sets of 4 pixels per coloured pixel. So the 1600 will beat the 185 for planetary for resolution. I'm not sure whether RGB filtering would be possible due to planetary rotation. It would with three cameras and three scopes of course but that's out of the question for the very long focal length required.

The manual doesn't show the noise below -20°C as shown in my next clip below, but my understanding is that thermal noise decreases all the way down to absolute zero, though the camera will cease to function well above that I've certainly seen a reduction in noise between -20°C and -25°C at the gain I've been using. The general rule is that the noise doubles with every 6°C rise in temperature, or thereabouts.

I got that info from the ZWO manual. Here's the relevant part of the manual - note the last line of what I've snipped :-

If I'm right the S/N depends on the total exposure time of all the subs but it is limited by the read noise which is reduced overall by using a large number of short subs because the signal is additive whereas the noise increases by the square root of the number of subs. Optimum is attained where the read noise equals the thermal noise which in turn depends on the temperature. The ZWO CMOS sensors have a lower read noise compared with thermal noise than CCD sensors and hence benefit from more and shorter subs than CCD cameras. Calculations on the theoretical amount of thermal noise is beyond me but I have read that there are ways of measuring it for the camera concerned. I am going to look into this further and see what I can get for my ASI1600MM-Cool at a reasonably achievable set-point temperature. So far this seems to be -25°C but I should be able to go lower when winter comes. The limit for this camera is said to be -30°C so I'm already approaching that.

At present I'm using a constant gain of 500 but this is something I want to investigate. It's very unlikely that I have picked on the best gain setting by mainly luck and a little judgement. A first thought is to match the Full Well with the ADC. So we have Full Well of 20Ke and ADC of 12 bit. 12bit = 4096 ADU giving a gain of 20,000/4096 = 4.88 electrons per ADU. The graph shows that a gain of 0dB gives 5 e/ADU so maximum dynamic range in each sub occurs with a gain of a little over 0dB, though I suspect that the graph is approximate and related to the gain that matches FW to ADC. However, this is not "the whole story". Image integration results in adding the data per pixel from all the frames stacked. If we work in 16bit unsigned arithmetic the resultant ADU must not exceed 2^16 = 65536 which is 65536/4096 = 16. In other words, if we were to use 0dB gain we would saturate the whites with 16 frames of exposure giving 65536 somewhere in the frame using optimum exposure per sub. If we stack say 96 subs we can increase the gain by 6x = 7.78dB or to put it the other way round, a gain of 8db gives a ratio of 6.3 corresponding to a stack of 101 subs. So for 100 subs we can use a gain of 8db. All the above assumes we can use an exposure that will attain full well for a few of the pixels in the image (ignoring stars which we allow to saturate). In the case of the data I have obtained for the Cygnus area with the 55mm lens, I estimate that 60s will just saturate some pixels in the Crescent Nebula - the brightest DSO in the FOV. Now if I were to reduce the gain to 8dB compared with 50db I would need 42dB of extra exposure = 19953 times. This would clearly be ridiculous. That's enough for one post - I'll look at this from a practical point of view in another post.

A heavy mist has rolled in now so I guess that's it for tonight! Oh well, I've done quite well - can't grumble I'll shut up shop then and go to bed. Processing will be tomorrow.

Here is a 90s sub flipped and rotated and saved in PNG format, otherwise untouched. This is the straight linear image as captured, just converted from FITS to TIFF, flipped and rotated then changed to PNG for upload.

90s is just about alright. Though the Crescent is just saturating on the brightest part - the rest of the image is alright. Integration of these subs will show the best the camera is capable of with its 12bit ADC. I think this is the reason this camera is better with many shorter subs than fewer longer ones.

Actually 120s is just a touch too much so I'm going for 90s subs.

240s is too much so I'll go for collecting 120s subs for the rest of the night or until the sky gets poor/bad.

Thank you for finding and posting that Huw Here's a 120s = 2m exposure.

Looking at the histogram it would seem that there's plenty of headroom to increase the exposures quite a lot further. OTOH it's more a question of saturating certain areas. Might just try a series if increasing exposures rather than just long runs at the same exposure.