Can I combine more than one session?

[StuartT]

Last night I got this of the Eastern Veil Nebula (NGC6992), but I clearly need more data. If the sky is clear tonight I plan to shoot this target again. Can I combine data from two nights? Do I need to bear anything in mind if I do that? Thanks

(although my framing isn't ideal, so I may just start from scratch)

Edited September 6, 2021 by StuartT

[Seelive]

13 minutes ago, StuartT said:

Last night I got this of the Eastern Veil Nebula (NGC6992), but I clearly need more data. If the sky is clear tonight I plan to shoot this target again. Can I combine data from two nights? Do I need to bear anything in mind if I do that? Thanks

(although my framing isn't ideal, so I may just start from scratch)

Yes you can but for large targets (in relation to the FOV) the position of the target within the images needs to be very close between sessions otherwise you'll just end up with an even smaller usable FOV.

Edited September 6, 2021 by Seelive
Typo

[wimvb]

Combining data from several sessions is quite common, but there are a few things you needto pay attention to.

1. Try to keep the camera attached to the scope. If this isn't possible, align the sensor edge with RA and DEC. This makes reframing easier. To align the sensor, start a 30 s exposure. About 5 s in, press the RA button on the hc, with slew rate at 1x sidereal, until the exposure ends. This will create a star trail. After the exposure view the result. Turn the camera into the trail. Repeat until star trails are parallell to the sensor's edge.

2. Use platesolving software if youcan. If you can't, note the position of brighter stars near the frame edge, and align these as close to the previous sessions position as possible.

3. Allow for mismatch between sessions and keep the main target clear from any edge.

 

Btw, if I were you, I'd reframe the veil and reshoot, or use the data you have and create a mosaic.

[StuartT]

many thanks for the advice. Given the slightly off framing, I shall start over I think.

It's quite hard to frame these objects tho as the nebulosity doesn't really show up well in the trial images.

[wimvb]

40 minutes ago, StuartT said:

many thanks for the advice. Given the slightly off framing, I shall start over I think.

It's quite hard to frame these objects tho as the nebulosity doesn't really show up well in the trial images.

I agree. There are three options.

The first is that you download a sub to your computer and stretch it hard. Use this stretched test image to reframe. In this case the brightest stars in the upper half of your image need to be close to the very top of your frame, if you want to catch the whole arc.

The second option is to download an image with the same object from the net and use this as a reference. Again, use stars as a guide.

The third option is to use a stellarium program to frame the target. I frequently use the online resource aladin. You need to know your approximate field of view (easy to get from a processed image such as the one in your first post).

https://aladin.u-strasbg.fr/AladinLite/

And of course, you can go all in, and control your gear by computer, plate solving etc.

 

Edited September 6, 2021 by wimvb

[StuartT]

11 minutes ago, wimvb said:

I agree. There are three options.

The first is that you download a sub to your computer and stretch it hard. Use this stretched test image to reframe. In this case the brightest stars in the upper half of your image need to be close to the very top of your frame, if you want to catch the whole arc.

The second option is to download an image with the same object from the net and use this as a reference. Again, use stars as a guide.

The third option is to use a stellarium program to frame the target. I frequently use the online resource aladin. You need to know your approximate field of view (easy to get from a processed image such as the one in your first post).

https://aladin.u-strasbg.fr/AladinLite/

And of course, you can go all in, and control your gear by computer, plate solving etc.

 

Thanks. I do have platesolving and my mount is steered from the laptop (I use NINA). But there is still quite a bit of trial and error involved, I find. But thanks for the tips.

I think what I'll do is platesolve and sync in NINA then click on the spot I want in Stellarium and Ctrl-1 to go there.

Edited September 6, 2021 by StuartT

[The Lazy Astronomer]

16 minutes ago, StuartT said:

Thanks. I do have platesolving and my mount is steered from the laptop (I use NINA). But there is still quite a bit of trial and error involved, I find. But thanks for the tips

FYI, with NINA you can give it an image and get it to plate solve on that.

Alternatively (and this is what I do): use Stellarium/CdC/whatever to select a target and import the coordinates to NINA, then just leave NINA open between sessions. All you need to do then is click slew to target in the framing assistant.

You can also save a sequence with the target information and load it back up next time... So many options!

[edarter]

3 hours ago, The Lazy Astronomer said:

FYI, with NINA you can give it an image and get it to plate solve on that.

Alternatively (and this is what I do): use Stellarium/CdC/whatever to select a target and import the coordinates to NINA, then just leave NINA open between sessions. All you need to do then is click slew to target in the framing assistant.

You can also save a sequence with the target information and load it back up next time... So many options!

This is exactly how I use NINA. I choose a target in Stellarium, import in to the framing assistant in NINA and then frame it up as I want it. Then import that as a new sequence in to the sequencing tab and set up the other paramaters. When the sequence is run NINA will platesolve, adjust the scope position and when framed up as you set up in framing assistant it will start taking the 'real' images.

[StuartT]

10 hours ago, edarter said:

This is exactly how I use NINA. I choose a target in Stellarium, import in to the framing assistant in NINA and then frame it up as I want it. 

Thanks. How is this bit done exactly? (I get the rest of it)

[markse68]

I had an issue last time i tried combining data across sessions. I plate solved each time so the target was in centre of frame but there was a slight camera rotation between sessions. This caused some weird pattern noise as the rows and columns of pixels caused a moire effect when combined. It looked worse than not bothering so i didn’t 😁. So i think getting camera rotation perfect is quite important and the advice to keep camera attached is the best if you can (I can’t)

Mark 

[edarter]

14 hours ago, StuartT said:

Thanks. How is this bit done exactly? (I get the rest of it)

Here you go, very straightforward. I see your in Oxford, only a few miles from me! Hope you're able to make the most of the clear skies again tonight.

 

[StuartT]

14 hours ago, markse68 said:

I had an issue last time i tried combining data across sessions. I plate solved each time so the target was in centre of frame but there was a slight camera rotation between sessions. This caused some weird pattern noise as the rows and columns of pixels caused a moire effect when combined. It looked worse than not bothering so i didn’t 😁. So i think getting camera rotation perfect is quite important and the advice to keep camera attached is the best if you can (I can’t)

Mark 

Yes, I leave everything out between sessions (thank god for Telegizmos - changed my life!). And I don't mess with the optical train at all, so everything is exactly the same and I only need one batch of flats.

42 minutes ago, edarter said:

Here you go, very straightforward. I see your in Oxford, only a few miles from me! Hope you're able to make the most of the clear skies again tonight.

Thank you. Yes indeed. Beautiful night tonight. Managed to get another couple of hours on Veil Nebula to add to last night's data. NINA is the best! So easy to match up multi-night sessions, I discovered tonight. Accurate down to a few arcsec 😃

[edarter]

7 hours ago, StuartT said:

Yes, I leave everything out between sessions (thank god for Telegizmos - changed my life!). And I don't mess with the optical train at all, so everything is exactly the same and I only need one batch of flats.

Thank you. Yes indeed. Beautiful night tonight. Managed to get another couple of hours on Veil Nebula to add to last night's data. NINA is the best! So easy to match up multi-night sessions, I discovered tonight. Accurate down to a few arcsec 😃

Lol, that's my target as well!  Nice and high in the sky.

Yep, save the sequence and just load it up again next time. Only thing you have to re-do is camera rotation but I leave my DSLR on the scope so even that is not an issue.