First proper attempt at M81/M82

[Adam1234]

Hi all, I took advantage of a clear sky on Sunday and got just over 2 hours data on M81 & M82 at ISO 800.

Lights 65 x 120s

Darks 15 x 120s

50 x flats 

50 x bias

Here's my current version of the image, I may do some extra processing over time to see if I can improve the image further and will probably add extra subs from future imaging sessions.

 

This was also the first time I've tried taking flat frames. Probably didn't do the flats correctly as I still had quite a gradient across the image but I think the vignetting on the original uncropped image was reduced. Perhaps most of the gradient was caused by the light pollution from the 3 street lamps outside my garden... I managed to remove the worst of the gradient using this photoshop method by Astrobackyard.

 

 

For the flat frames I used the t shirt method, but instead of waiting until dawn as I've heard is recommended, I did this at the end of my session and positioned my scope upwards and balanced my laptop screen on top with a full white screen. Put camera in AV mode and ensures the histogram was between 1/3 and 1/2 from the left. Not sure what exposure times we're looking for here though? They seemed very short. I've heard to aim for 0.5s, so if that's correct maybe the brightness of my laptop screen could have done with being a bit dimmer.

Any tips on taking flat frames will be much appreciated!

Adam

 

[ebdons]

Hi, I normally aim for 2-3 sec flats, in stark labs I aim for a peak on the graph, just left of middle, also look at the flat image sample and see if there are any objects/dust holes etc on it as that would mean you are getting near to the right flat setting, it depends also on filter choice. and iso setting, but I found that when using a newt and an sx825m the exp can be shortened quite a bit, or dimmed to get that 2- 3 sec exp length, when using a 80 apo, I can get the exp sorted easier even when using a OSC. other folks on here will know more. ps normally I use a mono with filters so don't know if this is relevant? and you may have to dim the screen more than you think? Ton

[Adam1234]

Thanks Ton, next time I'll try dimming the screen more and aim for 2 - 3s and see what effect that has. I suppose I could on the same night, collect some flats at different exposures and see what affect this has on my images.  

In my case I was using no filters, just stick dslr and iso 800.

 

[alacant]

Hi. Nice image.

I'm with @ebdons on the 2s+ flat frames (FF). Sometimes the screen refresh rate interferes with the shutter causing banding in the flat FF. You can check by stretching a few of the flat frames.

BTW, the FF will not remove gradients but will go a long way to even out the image when you do come to process the gradient.

HTH

[ebdons]

Yep, there are guides out there about how to take flats bias darks etc and how many, but I found it a struggle when I first started a few years ago, but now can do them almost without any preview needed, only issue I sometimes have is as alacant mentions about the shutter spec, and also whether rolling shutter on some osc cams, and if possible I turn this shutter option off which helps the image consistency of the flats. but you may to play about a bit, and you don't even need clear skies to practice. clear skies soon hopefully.

[Adam1234]

Thanks guys! Hopefully I get some better flat frames next time.

I've got the Optolong L-Pro Light Pollution broadband filter on it's way from FLO so hopefully that will help to reduce some of the gradients caused by light pollution.

What's your opinions on reusing the same flat frames for multiple nights imaging? Obviously i take it the best option is to take new ones for each night. 

[alacant]

16 minutes ago, Adam1234 said:

flat frames for multiple nights imaging?

If you have not rotated the camera relative to the telescope between sessions, that's fine and probably what most of us do.

If you find new dust has settled on the sensor, only then would you need to redo the flat frames. It only takes a few minutes anyway as you only need 12 or so of them.

HTH

[Adam1234]

20 minutes ago, alacant said:

If you have not rotated the camera relative to the telescope between sessions, that's fine and probably what most of us do.

If you find new dust has settled on the sensor, only then would you need to redo the flat frames. It only takes a few minutes anyway as you only need 12 or so of them.

HTH

That's brill, I try to keep my camera parallel to the OTA anyway.