taking subs on different nights will different positions matter

[Wirral man]

i just need to clear up a worry i have now i understand that the more data the better for good images, so i was to take say 1 hours worth each night would it matter if the dso is in a different part of the frame each night as my mount can alter slightly with its target acquisition each time i hunt for targets hope someone can clear this up for me thanks.john 

[alacant]

No, but get it close otherwise when you stack the light frames there will be margins around the edge which you will need to crop. HTH.

[vlaiv]

It indeed maters, you should aim to position your mount so that target is roughly in the same part of the frame each time, and that your camera is oriented the same way (rotation of camera).

When you stack subs, only common pixels between sessions will have improvement in SNR, and if you deviate too much between sessions in your position and orientation you will need to crop out a lot of image which will impact composition and final image size.

This is where plate solving helps a lot. There are other techniques to make sure you have proper alignment (it does not have to be perfect, but closer - the better).

[Redscouse]

Mate, you and me must have the same train of thought! 

I was out last night grabbing some lum images. Managed to take 17x300s before I had to call it a night, darn that work in the morning thingy!!  I was about to ask the same question here! 

I have a little add-on to your question; Can we use the darks from previous nights also?

Anyhow, great question!
I think I'm going to follow you, might save me asking a question or two!  

Pete.

[Redscouse]

Sorry to jump in to OP's thread, I have a related question.
Can we use the darks from the previous night or do I need to take fresh ones every time?
(same target; not sure that matters though?) 

Pete.

[SteveNickolls]

29 minutes ago, Redscouse said:

Can we use the darks from previous nights also?

Hi,

You ought to be able to reuse dark frames with a few proviso's- the same camera, same exposure time and ISO, oh and temperature needs be similar too. You also need to take enough of them to matter, too few and you could end up adding noise to the final image. 

Cheers,
Steve

[tooth_dr]

59 minutes ago, Redscouse said:

Sorry to jump in to OP's thread, I have a related question.
Can we use the darks from the previous night or do I need to take fresh ones every time?
(same target; not sure that matters though?) 

Pete.

And sometimes we dont use darks at all, if using a DSLR for example.

[Redscouse]

41 minutes ago, tooth_dr said:

And sometimes we dont use darks at all, if using a DSLR for example.

 So you're telling me I've wasted hours previously taking darks with my Canon1300D? Are darks not needed for that?

I have a ASI178MM now, but I do still use my DSLR sometimes.

 

1 hour ago, SteveNickolls said:

Hi,

You ought to be able to reuse dark frames with a few proviso's- the same camera, same exposure time and ISO, oh and temperature needs be similar too. You also need to take enough of them to matter, too few and you could end up adding noise to the final image. 

Cheers,
Steve

I generally take darks about 25% of my lights. 
This stack i've just added 17 Lum to is now 39L 11R 11G 11B 22D.

[tooth_dr]

17 minutes ago, Redscouse said:

 So you're telling me I've wasted hours previously taking darks with my Canon1300D? Are darks not needed for that?

 

I dont use darks for my DSLR images any more, I just bias and flats.  But you need to dither between exposures.

[SteveNickolls]

2 hours ago, tooth_dr said:

 So you're telling me I've wasted hours previously taking darks with my Canon1300D? Are darks not needed for that?

It really depends.

If you use a EQ mount and you don't dither and you can get reasonably consistent temperatures from your light frames then matching dark frames should help you. There is a rule of thumb that noise doubles with every 6 degree C rise in temperature. How to tell the temperature of your light frames? I rely on what the exif data from BYEOS provides in terms of temperature for what it's worth. By diverse means such as controlling the length of pause between light frames I find temperatures can be kept within a degree or two. 

However when I used an Alt-Az mount its zig-zag motion meant it was less accurate tracking and there was field rotation to consider during exposures. Then I went from taking dark frames to not bothering and couldn't see a difference. The images being moderately poor hid a lot. Light pollution too plays a huge part in degrading images in this regard. So you see it really depends.

Actually the best part of imaging is finding out for yourself. Try comparing stacked images with and without dark frames-can you see any difference? As you progress you may well need to take dark frames up until you begin guiding and have the ability to dither.  

Hope this makes sense.

Cheers,
Steve

[tooth_dr]

I use two identical cameras 40d side by side, one reads 4 degrees warmer on the exit data!

[Wirral man]

14 hours ago, Redscouse said:

Mate, you and me must have the same train of thought! 

I was out last night grabbing some lum images. Managed to take 17x300s before I had to call it a night, darn that work in the morning thingy!!  I was about to ask the same question here! 

I have a little add-on to your question; Can we use the darks from previous nights also?

Anyhow, great question!
I think I'm going to follow you, might save me asking a question or two!  

Pete.

haha great minds think alike bud take it yr a red man?

[Wirral man]

what is dither?

[tooth_dr]

57 minutes ago, Wirral man said:

what is dither?

http://www.skyandtelescope.com/astronomy-blogs/astrophotography-jerry-lodriguss/why-how-dither-astro-images/

 

http://dslr-astrophotography.com/dithering-optimal-results-dslr-astrophotography/

[Redscouse]

13 hours ago, Wirral man said:

haha great minds think alike bud take it yr a red man?

I am indeed.  

13 hours ago, Wirral man said:

what is dither?

In simple terms, between shots the mount moves a couple of pixels so any noise etc. is never in the same place in the images.
That's the way I understand it anyway.

[ollypenrice]

The key thing is to orientate your chip along RA and Dec unless you have very good reason not to. (Some objects may oblige you to shoot at an angle but most don't.) You can have RA as the long side or Dec as the long side depending on the target, but do yourself a favour and choose one or the other. That means you can come back to a target in the future and add to it easily without losing most of the frame.

How to align the chip along RA and Dec? Easy: take a short sub (5 seconds) while slewing in one axis at a medium speed. This will create star trails showing your current camera angle. Rotate the camera and repeat the test till the trails are horizontal of vertical and that's that. Reframing is ridiculously easy if you do this. You just look at the stars around the edges. If you like faffing around with software you can plate solve. If, like me, you are stone age you just look at the starfield.

Olly

[The-MathMog]

1 hour ago, ollypenrice said:

The key thing is to orientate your chip along RA and Dec unless you have very good reason not to. (Some objects may oblige you to shoot at an angle but most don't.) You can have RA as the long side or Dec as the long side depending on the target, but do yourself a favour and choose one or the other. That means you can come back to a target in the future and add to it easily without losing most of the frame.

How to align the chip along RA and Dec? Easy: take a short sub (5 seconds) while slewing in one axis at a medium speed. This will create star trails showing your current camera angle. Rotate the camera and repeat the test till the trails are horizontal of vertical and that's that. Reframing is ridiculously easy if you do this. You just look at the stars around the edges. If you like faffing around with software you can plate solve. If, like me, you are stone age you just look at the starfield.

Olly

Don't know why I never thought of that. I literally just added some more subs to a target, taken two days after, with the subs being almost aligned but not perfectly, hence why the stars now have double diffraction spikes.. Thanks for the idea  

[vlaiv]

23 minutes ago, The-MathMog said:

Don't know why I never thought of that. I literally just added some more subs to a target, taken two days after, with the subs being almost aligned but not perfectly, hence why the stars now have double diffraction spikes.. Thanks for the idea  

Double diffraction spikes happen for a different reason - not sensor orientation. It is related to how ota is aligned to RA/DEC rather than sensor. If you have newtonian that you use to image with - don't rotate it in it's rings between sessions and spikes should be aligned regardless of slight rotation of sensor between sessions.

[The-MathMog]

1 minute ago, vlaiv said:

Double diffraction spikes happen for a different reason - not sensor orientation. It is related to how ota is aligned to RA/DEC rather than sensor. If you have newtonian that you use to image with - don't rotate it in it's rings between sessions and spikes should be aligned regardless of slight rotation of sensor between sessions.

Ahh yeah, you're right about that. I haven't had the scope out of the rings for a long time, but it must've slightly rotated in some way either way during the assembly/disassembly. Might need to slightly tighten the rings. The difference between the spikes ARE very small, but noticable.

Still a great advice