The Lazy Astronomer

First up, you're sacrificing a lot of resolution by using a Ha filter with a colour camera, although I suspect you know and understand that already. You should expose until your sky background level swamps the read noise of the sensor. What this actually means in terms of sub length depends on many factors, including: light pollution level, optics used, sensor QE, pixel size and read noise. The formula I use for calculating optimum minimum exposure length is: Exposure = C*rn^2/P Where: C is a factor applied to determine how much additional noise will be present in the image (I use a factor of 10, which equates to 5% extra noise) rn is sensor read noise in electrons P is the light pollution rate in electrons per pixel per second All of the above is taken from a talk given a few years ago by Dr Robin Glover (Sharpcap creator). There is a video of it on YouTube. He's also made a calculator for estimating your light pollution level: https://tools.sharpcap.co.uk/

Mine are 1.25". It's interesting that you see the pattern on your Oiii, albeit fainter than Ha or Oiii, but there nonetheless - maybe due to your narrower filter passband? Also interesting that your flats are completely different to mine. I think l just assumed all 294MM's produced the same pattern, but l guess each one must be unique (I sort of like that ☺).

I have a different camera to you, but generally my flats are in the 3 - 10 second range. I'd imagine there's nothing wrong with what you've done, but the real test of whether a flat is ok or not is to answer the question: does it properly calibrate the lights? Yes = good flat, No = bad flat.

Wouldn't calibrate properly. It had a very bright central region which then overcorrected the lights, I've just tried looking to see if I still had it, but it appears I deleted it. Here's the Oiii master flat using natural light (i.e. the one that works) with just an STF in Pixinsight: To some extent you can still see a brighter circle in the centre, but with the LED panel is was much more pronounced. This is with an Astronomik 6nm filter. For reference, the Ha flat taken with the light panel looks like this (as does Sii) and both calibrate their respective lights perfectly (again, just an STF in PI):

Seperate bias frames should not be necessary as the bias signal is contained within the flat darks. For information though: the ZWO version has a minimum exposure time of 32 micro seconds, I assume the QHY one is the same. With my ZWO cameras, if I set the exposure time to 0 in the capture software, the camera will use the shortest possible exposures. Again, I assume this would work the same with QHY.

On a related note, how do you take your flats? I could not get Oiii to work with the led panel I use (Ha and Sii produce the "interesting" flats you've alluded to above, but calibrate perfectly). Had to resort to daylight flats for Oiii, but maybe there's a trick I've missed?

I think this forum needs a series of pinned posts (probably in the getting started with imaging section) to summarise things like this, and help new dso imagers get started on the right track. I'm just thinking of poor @vlaiv who must be so tired of having to explain the same things to people (including me) all the time! 😁

Aside from the fact that some users of the 294mc report issues with the l-extreme, in theory it would be better due to it's narrower passband, blocking more unwanted light and increasing contrast. There should be no issues with autofocus routines, but you'll probably have to expose for longer, so it'll take a little longer. I have the mono version of the 294, and use 10s exposures for narrowband autofocusing. Takes maybe 90 seconds to complete the routine. This is not quite right. The optimal minimal exposure time is determined by your equipment and sky conditions. Ideally, you want to expose until your sky background level swamps your camera read noise. Obviously, using narrower bandpass filters will reduce the amount of sky background which reaches your sensor, so longer exposures are needed to do that. Depending on the gain setting you use, for you this would be somewhere in the region of about 3 minutes (high gain) to about 5 minutes (unity gain) with the l-extreme. In broadband, you'd probably find you only actually need subs of around 20 - 30 seconds.

Maybe your superior tak optics have revealed that every star is in fact a close double 😁 Sorry, not very helpful...

I usually find phd2 reports a larger pa error than Sharpcap. I'm usually within 30" according to Sharpcap, but phd2 usually gives me a value of around 2 - 5 arc minutes. No problem for me - I'm happy as long as I'm getting good images. Not sure which measurement is the most accurate; but I would think phd2.

Threads facing camera is the way mine are.

If you feel like you're throwing away a high number of subs, then yeah, maybe try shorter exposure times. I usually use 30 - 60 seconds for broadband too (ZWO 294MM).

As is discussed in the video linked above, the optimum minimal sub length depends on your equipment and sky conditions. 3 minutes is probably in the right ballpark for you with the l-extreme. In broadband, you'd probably find the optimal minimum sub length drops down to somewhere around 20 seconds, however if 3 minutes is working for you in terms of mount performance, number of images affected by satellite trails, etc. then there's no burning reason to go shorter - just know that you can if you want or need to.

How dirty is extremely dirty? Can you still see the mirrors? You may be surprised to hear that dust and dirt on mirrors/lenses has a negligible effect on their performance. It's often better to just leave it, especially if you don't know what you're doing, although it is something that can be done relatively easily (as a total novice, to use your words, the most difficult part would likely be realigning and collimating the mirrors post cleaning).

10 minute subs should not be necessary with CMOS. Sure, if guiding is up to it then it won't hurt, but as long as the sub length allows the sensor read noise to be swamped by the background sky level, there is little difference in stacks of short subs or longer ones.

Back focus just relates to the distance required between the corrective optics (coma corrector, flattener, etc.) and the imaging sensor in order for those corrective optics to be performing optimally. If that spacing is slightly out, it shouldn't affect focus particularly, but star shapes will be off away from the centre of the frame. But, as you discovered, if it's significantly out, it may present focusing issues. Glad you got it sorted though 👍

In addition to GIMP, some examples of popular astro-specific post process software which supports 32-bit include (in order of cost): Siril (free), Startools, Pixinsight. On another note, 3 min subs is probably ok - what scope are you using and what gain are you using?

Have you tried flats using only natural light too? And when you say they're all the same, have you tried calibrating the lights with all of these flat variants, or are you just looking at a stretched master?

Flats temperature matched to lights, with matching flat darks. For Ha, Sii and luminance, I use a cheap tracing panel with a few sheets of white paper, target adu value of around 40% (so around 26 - 27k adu), exposures somewhere in the 3 - 10s range. For Oiii, no amount of variation in adu value, exposure time, brightness or distance of the tracing panel would get me good flats, so on an overcast day (no shortage of those in the UK 😅), I covered the scope with a white tshirt and pointed it at a plain white section of wall (in a room which already has quite soft and diffused lighting in the mid to late afternoon) and fired off some flats using the same settings as above. Worked perfectly. As I say, I haven't really tried with rgb properly yet - I'm going to get some better filters soon and I guess I'll find out what works.

Ok, well the image stats seem to be normal in that last image, so clearly there's no problem with the camera or the settings. The histogram is pushed hard up against the right hand side in those m31 images, which can only mean there's a light source shining into the scope, surely?

Any excuse for a new purchase 😁 The 294MM does calibrate well in my experience, but I feel like it's a bit of a fussy sensor. I've only really done narrowband imaging with it, and it took a little bit of fiddling to find the right settings for good flats, but now that I have that, calibration has so far been very repeatable. I'm yet to really try much broadband imaging as my current (cheapo) rgb filters have some weird reflection artifacts which do not calibrate out, but the Astronomik luminance filter I use seems to calibrate fine as well. One odd thing (and this is probably due to the cheap tracing panel I use) is that Ha and Sii calibrate perfectly, but Oiii has a brighter central region in the flat which then overcorrects the lights - nothing I tried worked and in the end I had to resort to a different light source (which worked fine).

Focus looks ok to me. If it's out, it's not out by much. When you say 120s subs were 'flooded with LP', what precisely do you mean? Did you have the subs or happen to note some of the image statistics? What gain and offset do you have set in NINA?

To be honest, it's six of one and half a dozen of the other. Both cameras would give you great results. The 294 will require a little bit more attention with calibration, but it's no more difficult than making sure you have good matched darks for your lights. Basically, do you want a square sensor or a rectangular one - that's about the only real difference.

These look very nice already, looking forward to seeing the finished result! Have you tried using the new GHS script for stretching? Takes a bit of learning and fiddling to find the right settings, but should help to control Alnitak and other bright stars without the need for star reduction later down the line.

Have you tried calibrating one of the master flats using the other? If all the dust shadows and vignetting are completely removed then you'll know there's no appreciable difference between them. Out of curiosity, what rotator are you using? Sensor orientation is the last thing I have to do manually, using my hands like some kind of common visual astronomer (I kid! 😜) - I'd love to be able to computerise it, but the rotators I've seen are just too expensive for my liking.