DSS failing to register any stars in lights on M81

[Penguin]

Seeing some odd behaviour here. I took 39 90s subs of M81 with my QHY153C through my SW 150P at a gain of 200 (I'm new to a CCD so I don't know the correct settings!)

The subs look ok to me, I can see plenty of stars, but when I register them in DSS, it can't find any and refuses to stack! I've tried changing the star detection threshold from the default as it suggests but that makes no difference. I'm using the same DSS settings I used on T Coronae Borealis (basically, just the recommended settings) and that one registered and stacked absolutely fine.

So what's going on, any ideas?

[Elp]

Are the stars round?

[ONIKKINEN]

Drop the star detection threshold all the way to the minimum and see if it finds any stars then, if you haven't tried that already. I found even the minimum threshold a bit too restrictive when i first switched from DSLR to an astro camera, and ultimately switched to Siril which finds hundreds of stars in images where DSS found a handful.

[Penguin]

10 hours ago, Elp said:

Are the stars round?

They're a lot rounder than they have been in the past when my PA was much worse and DSS has never had an issue finding them before. They are at least as round as those in my subs of T Coronae Borealis from the same night. So I don't think that's the issue, but thanks for the suggestion.

[Penguin]

10 hours ago, ONIKKINEN said:

Drop the star detection threshold all the way to the minimum and see if it finds any stars then, if you haven't tried that already. I found even the minimum threshold a bit too restrictive when i first switched from DSLR to an astro camera, and ultimately switched to Siril which finds hundreds of stars in images where DSS found a handful.

Yep, tried that. The minimum is 2% and it made no difference: still zero stars found. I've never heard of Siril but after a quick Google it looks interesting. I'll try it out when I get a chance.

[michael8554]

Post jpeg of one of the Lights.

Michael 

[MalcolmP]

So something changed between you 'doing' TCrB and M81
Could be either 1: on the 'scope, 2: in the camera, or 3:in DSS.

To test #3, have you still got the TCrB subs ? Go back to them, make copies** in a new folder and attempt to rereg/restack them.

As  @michael8554 said post a sample sub,, pref raw, one working from TCrB and one fail from M81

** so as to preserve originals with their stack info txt files just in case

Edited May 28 by MalcolmP

[ONIKKINEN]

7 hours ago, Penguin said:

Yep, tried that. The minimum is 2% and it made no difference: still zero stars found. I've never heard of Siril but after a quick Google it looks interesting. I'll try it out when I get a chance.

To rule out possible calibration issues try with just the lights first. If there is an offset problem between the lights and darks its possible that data is clipped and DSS doesnt know what to do with the data.

But if you want to, do try Siril as it is excellent. Its not nearly as easy to use but it will serve you well when you get the gears rolling with it.

[Penguin]

On 28/05/2024 at 16:37, ONIKKINEN said:

To rule out possible calibration issues try with just the lights first. If there is an offset problem between the lights and darks its possible that data is clipped and DSS doesnt know what to do with the data.

But if you want to, do try Siril as it is excellent. Its not nearly as easy to use but it will serve you well when you get the gears rolling with it.

Ah, it was the darks! Took those out and it stacked fine.

Too be honest they looked odd when I was taking them so I should have suspected them from the start. They looked more like light frames even though I took them with the same settings as the lights (180s at 200gain and -10deg) with the scope covered. Maybe the scope cover is not very good and it's letting some light in? Maybe I should take a library of darks with the camera off the scope and capped.

Question: Can I avoid bothering with darks by dithering (is that the term?) the scope between each sub? Maybe I should search the forums or ask a separate question for that.

Edited May 31 by Penguin
Added follow-on question

[carastro]

1 hour ago, Penguin said:

Question: Can I avoid bothering with darks by dithering (is that the term?) the scope between each sub? Maybe I should search the forums or ask a separate question for that.

To be honest I don't always bother with darks as I use a dual rig and there is overlap anyway, and I know some-one that used to dither and never did darks, so I would say if your camera is cooled and you dither you can probably get away with it.  Certainly give it a try.  

Edited May 31 by carastro

[ONIKKINEN]

1 hour ago, Penguin said:

Ah, it was the darks! Took those out and it stacked fine.

Too be honest they looked odd when I was taking them so I should have suspected them from the start. They looked more like light frames even though I took them with the same settings as the lights (180s at 200gain and -10deg) with the scope covered. Maybe the scope cover is not very good and it's letting some light in? Maybe I should take a library of darks with the camera off the scope and capped.

Question: Can I avoid bothering with darks by dithering (is that the term?) the scope between each sub? Maybe I should search the forums or ask a separate question for that.

You could get away with not taking darks when dithering, but it would still be better to take the darks. With a cooled camera its so easy this is not something that makes sense to save time on, and actually dithering will eat into your imaging time so you lose some in the end.

But yes, scope covered is not nearly dark enough most of the time and you almost certainly had light leaks. Just a few photons will ruin your darks, what you really need to do is take the camera off the scope and put it somewhere completely dark and plug it properly.  I put mine in the fridge where it will be completely dark, and it helps with cooling too.

[Penguin]

I've taken a library of darks and bias at -10deg and now my subs are stacking ok. Next issue is star trailing even though my PA is good according to SharpCap so I need to get into guiding. Another big subject!

Thanks for your advice guys.