question about multiple sessions

[StuartT]

Sometimes my scope is one one side of the pier when I shoot one sequence and then on the other side when I shoot a second night. This means that one set of images is rotated 180 degrees from the other.

If I am combining the two nights in Astro Pixel Processor do I need to tell it that there is a difference between sets? Or is it possible to rotate one set of FITS by 180 before I start stacking?

Thanks

[Shimonu]

APP will figure it out. I think the multi-session option in APP is mostly so you can apply your calibration frames per session.

[StuartT]

Just now, Shimonu said:

APP will figure it out. I think the multi-session option in APP is mostly so you can apply your calibration frames per session.

oh really? so if I add the two sets of lights as two different sessions it will figure out one set is the 'wrong' way up? Cool!

 

[CloudMagnet]

I would be suprised if that caused a problem. Even if you are the same pier side over multiple nights, it wouldn't be uncommon for the images to be a few degrees off anyway The software is clever enough to figure it out. Its similar to platesolving in a way.

[Clarkey]

I use APP and you need to specify which lights and calibration frames are for which session. For darks you can specify all sessions and for which filters. Obviously, if you are using calibration frames for multiple sessions it would not matter. The orientation is not important.

Edited September 24, 2021 by Clarkey

[StuartT]

1 hour ago, Clarkey said:

I use APP and you need to specify which lights and calibration frames are for which session. For darks you can specify all sessions and for which filters. Obviously, if you are using calibration frames for multiple sessions it would not matter. The orientation is not important.

I wasn't really thinking about the calibration frames. I assume those are unaffected by a meridian flip. 

What I was asking was whether or not I have to rotate the images from the second night through 180 degrees so they match up with the images from the first night. But it sounds like Astro Pixel Processor is clever enough to do that on its own

 

[Shimonu]

3 minutes ago, StuartT said:

I wasn't really thinking about the calibration frames. I assume those are unaffected by a meridian flip. 

What I was asking was whether or not I have to rotate the images from the second night through 180 degrees so they match up with the images from the first night. But it sounds like Astro Pixel Processor is clever enough to do that on its own

 

The calibration frames are not affected by meridian flips but your flats might not be identical between sessions.

[StuartT]

16 minutes ago, Shimonu said:

The calibration frames are not affected by meridian flips but your flats might not be identical between sessions.

right. good point. I tend to do all my calib frames the next day anyway

[The Lazy Astronomer]

Unless you have a particularly dusty environment, or break down the imaging train after each session, you should be able to reuse flats multiple times. I hate the faff of taking flats (and associated flat darks) so I usually like to get a couple of months use out of them. 

Your original question has already been answered, but just to add to it: astro stacking software analyses images to determine patterns in the stars and uses that information in order to align them to a given reference frame. Individual lights can be in literally any orientation and as long as the software can find enough matching star patterns it will align them all to the reference as part of the stacking process. 

It's weird how it's something we don't really think about, but it is extremely clever!

[StuartT]

7 hours ago, The Lazy Astronomer said:

Unless you have a particularly dusty environment, or break down the imaging train after each session, you should be able to reuse flats multiple times. I hate the faff of taking flats (and associated flat darks) so I usually like to get a couple of months use out of them. 

Thanks. Would you say it's ok to re-use all the calibration frames from a previous night? I did unscrew the camera, but everything else was left in place

[The Lazy Astronomer]

41 minutes ago, StuartT said:

Thanks. Would you say it's ok to re-use all the calibration frames from a previous night? I did unscrew the camera, but everything else was left in place

Afraid not. To reuse flats, the entire imaging train must be undisturbed between nights.

[Clarkey]

17 minutes ago, The Lazy Astronomer said:

Afraid not. To reuse flats, the entire imaging train must be undisturbed between nights

I will not reuse flats as I have to bring the scope indoors between sessions (unless the weather is very well behaved). Even that can cause movement in the dust within the imaging train as I have found out from previous experience.

[StuartT]

35 minutes ago, The Lazy Astronomer said:

Afraid not. To reuse flats, the entire imaging train must be undisturbed between nights.

ok, so would it be better to ;

a) use flats from a previous sequence (with a slightly disturbed imaging train)
b) use no flats at all

[rnobleeddy]

3 minutes ago, StuartT said:

ok, so would it be better to ;

a) use flats from a previous sequence (with a slightly disturbed imaging train)
b) use no flats at all

I'd guess a, but assuming it's easy to do, try both and stretch both images and see which one looks best. 

[The Lazy Astronomer]

1 hour ago, StuartT said:

ok, so would it be better to ;

a) use flats from a previous sequence (with a slightly disturbed imaging train)
b) use no flats at all

Depends how many dust motes you're contending with - if you've got none, then you could probably use the previous flats with no problem. If you've got quite a few, the potentially you'll be introducing more artifacts unless you were very lucky and managed to precisely line the camera up exactly as it was before you removed it.

If you find the flats are making things worse, you might be able to model a synthetic flat in post processing (try a Google/Youtube search for some how to's with whatever software you use).

[StuartT]

2 hours ago, rnobleeddy said:

I'd guess a, but assuming it's easy to do, try both and stretch both images and see which one looks best. 

ok, so here is a direct comparison.

a) is the one using previous session flats and has a pretty terrible gradient. b) is only stacking the light frames - no calibration frames at all. This is just as they both came out of APP so none of the usual post stack fiddling I normally do. I'd say that b) wins

a)

b)

 

[Dazzyt66]

Agree with that - nice one! 

[StuartT]

and now after my Photoshop adjustments. I think the one with the calib frames looks a little over-processed. I think I prefer the subtler look of the one without any calib frames

With calib frames

Without calib frames

[rnobleeddy]

6 hours ago, StuartT said:

ok, so here is a direct comparison.

a) is the one using previous session flats and has a pretty terrible gradient. b) is only stacking the light frames - no calibration frames at all. This is just as they both came out of APP so none of the usual post stack fiddling I normally do. I'd say that b) wins

a)

b)

 

I guess what's why I shouldn't guess! 

 

My Newt has visible vignetting which I think might dominate my unprocessed subs, but these look great without flats!

[StuartT]

yes, I'm beginning to wonder if calibration frames are all they're cracked up to be.

I think I might try my next few targets without any and see how I get on

[Dazzyt66]

8 hours ago, StuartT said:

yes, I'm beginning to wonder if calibration frames are all they're cracked up to be.

I think I might try my next few targets without any and see how I get on

I guess it all depends on what you want from your images. If, like me, you are just interested in getting something nice to look at, rather than winning awards/earning money from, then go for whatever works best for you. Following this thread I've redone some of my most recent images without calibration frames and had a bit of a mixed bag tbh, but I think from now on I'll do both versions to check what gives me best version. I only use bias and flats anyway so its no extra time really to knock up 30 of each after each session. Good thread! 👍

[StuartT]

4 hours ago, Dazzyt66 said:

I guess it all depends on what you want from your images. If, like me, you are just interested in getting something nice to look at, rather than winning awards/earning money from, then go for whatever works best for you. Following this thread I've redone some of my most recent images without calibration frames and had a bit of a mixed bag tbh, but I think from now on I'll do both versions to check what gives me best version. I only use bias and flats anyway so its no extra time really to knock up 30 of each after each session. Good thread! 👍

interesting that you don't use darks. I wonder what the order of importance of the various types of calibration frames is? i.e. which type are the most important to get?

[tooth_dr]

12 hours ago, StuartT said:

yes, I'm beginning to wonder if calibration frames are all they're cracked up to be.

I think I might try my next few targets without any and see how I get on

You have some nice data there, and without calibration data looks good.  You have some badly out of focus stars at the top of the image so I wonder did you keep a dodgy sub or two?

Personally I find calibration frames (dare I say it) essential for post processing.  I dont think you are dithering either between exposure - there is some walking noise evident also in the images - I find calibration really helps here if you prefer not to dither.

Are you taking your flats correctly?  If you are getting such negative impact with the flats is it possible that your arent taking them correctly or if you are taking them correctly then the focuser and or camera has moved between taking them and the lights?  Light leakage would be something else to look at.  I can see how it would appear that on the surface calibration data is doing harm, but I dont think I've come across a situation where it's harmed my data (talking specifically about set point cooled data, as with DSLRS its different) - unless the calibration data was wrong ie temp, duration, wrong camera data used etc.

What do you think?

[ollypenrice]

Colour gradients probably won't be corrected by flats. I find, certainly with OSC cameras, that they seem to derive from the chip. I recently processed some OSC data from a pre-flip session and a post-flip session. The images from each session had a red-green imbalance which changed sides at the flip.

Your second image is certainly better. Very good indeed. Its top left corner is flat and it has less noise in the dark section around the bottom, half way along. In Ps you might try going Image-Adjustments-Selective Colour-Reds and dropping the cyans in red by moving the top slider left. This is a demon trick for getting Ha signal to shine.

Olly

[Dazzyt66]

12 minutes ago, StuartT said:

interesting that you don't use darks. I wonder what the order of importance of the various types of calibration frames is? i.e. which type are the most important to get?

I just made a judgement - a few people say that darks don’t add as much to an image as would bias and flats especially when shot at ‘optimal’ iso for the DSLR - in my case 1600. But hey, I’m only after average/good rather than ‘magazine quality’. I’ve found flats to be the most beneficial for removing dust etc. As with everything it will come down to personal preference in the end and what you perceive as acceptable.