Bias / Dark frames; Very basic question

[davedownsouth]

Hi all, first off, very sorry for this incredibly basic question, I am very new to this DSO imaging game .

So, I am waiting on a suitable mount to get into this properly, but I am keen to practise with my 80mm refractor on the Nexstar mount; so short exposures only at the moment. Last night I had a play with getting some images of M82, and I could very easily see the galaxy on the screen with a 5 second exposure and the gain set high (obviously not good for imagining). So, taking 10 second exposure is giving me some images which I am happy, with, so next step is to work out how to stack, but reading making every photo count, I need to take some Dark & Bias frames first (when I have got all three types I will try to work out how to use DSS). So tonight my imaging will be very basic, I will probably take 40 Luminance shots at 1x1 and 15 RGB shots at 2 x 2; all shots with just10 second exposures. From reading the book, I think I understand Dark frames and basically I have to take these at the same time as the proper photos, using the same gain / exposure / focus settings, but with the end cap screwed on the scope (will ten shots be enough?). The Bias frames do not need to be taken at the same time (?), but do the settings need to be similar in terms of Gain? The books states that the exposure time does not matter, but I am already on a short exposure, so how will this be different from the Dark shots. Any quick answers would be good as I am hoping to be out in a field in 3 hours time ,

Cheers,

Dave

[johnrt]

Darks need to be taken at the same temp or a close as possible to the lights, but with the endcap screwed on - match the settings and length of exposure to the lights on your camera. Bias are not temperature dependant, but again match the settings on your camera and take the shortest possible exposure you can. Take lots and lots of both, 50+ and then stack em up!

[davedownsouth]

So the Bias shots do need to be at the same gain?

[johnrt]

So the Bias shots do need to be at the same gain?

Yes match the camera settings. You just don't need to match the temp.

[Leveye]

You don't need 50+ for both Dark and Bias frames using RAW files from a dslr. I have tested this over and over. If i only have 25-45 lights than all i need is half of that to stack with. 25 of each is more than sufficient and will save on the shutter count of the camera.

[johnrt]

You don't need 50+ for both Dark and Bias frames using RAW files from a dslr. I have tested this over and over. If i only have 25-45 lights than all i need is half of that to stack with. 25 of each is more than sufficient and will save on the shutter count of the camera.

Well that depends on how much you want to perfect you calibration frames which are going to be apllied multiple times, infact to every sub, before you produce a final image. Poor calibration frames will add noise rather than remove the signal we are looking to remove. The number of calibration frames is also not linked to the number of light frames you are stacking.

Now I'm not saying that 25 isn't enough, they'll a job, but when you can shoot bias and if you can temp match your darks, at your leisure, why not make the best calibration frames possible?

What are we  trying to do with bias and dark frames? We are trying to isolate the fix pattern noise caused by the bias and thermal signal in the camera chip that are present in every sub. To do this we need to sperate the bias and thermal signal from any other random noise present.

This leads us down the signal to noise ratio path and the rule of diminishing returns.

You will still see good improvement up to 100 frames before your returns diminish, but dependent on the noise levels and properties of the camera you may not need to rely so heaviiy on noise reduction.

[davedownsouth]

Thanks for the input.  I have bottled it a bit and have chosen not to put my filter wheel on tonight   .  I have taken 50, 20 second shoots of M81, a fair few of which I will be throwing away as 20 seconds is pushing it past the limit for my mount I think..  I have just set it running with the cap on to take 50 Darks (just about enough time to make a coffee ).  I wil then go and change to an exposure to 1 second and run 50 Bias.  I think it is going to take me alot longer than these combined times to work out how to process them in DSS .  Wish me luck

[davedownsouth]

Okay, you have to squint a bit, but you can just make out M81 .  Think I have got a long way to go, but it is a start at least

[ollypenrice]

Do you know about stretching the linear image as delivered by the camera? If not check out Rob's clear explanation. It is an essential step in astro imaging. http://www.middlehillobservatory.co.uk/articles-primers/Levels%20and%20curves.htm

Olly

[davedownsouth]

Do you know about stretching the linear image as delivered by the camera? If not check out Rob's clear explanation. It is an essential step in astro imaging. http://www.middlehillobservatory.co.uk/articles-primers/Levels%20and%20curves.htm

Olly

Thanks Olly, heard people mention stretching in other threads, but not got to that level yet .  Will have a look through the stuff in your attached link later, but probably not tonight as it is meant to be clear again  and I want to go out and play with the new scope  .

[ollypenrice]

Thanks Olly, heard people mention stretching in other threads, but not got to that level yet .  Will have a look through the stuff in your attached link later, but probably not tonight as it is meant to be clear again  and I want to go out and play with the new scope  .

You can stretch when it's raining. In a nutshell, all the wonderful nebulae that you see on the DS imaging board just look like fields of stars on a dark sky as they leave the camera. But the picture is in there, as you'll see.

Olly

[davedownsouth]

Hi Olly,

Thanks a lot for the link, I had a quick go at stretching the photo in photoshop, and although it now looks a bit 'grainy' / washed out, you can certainly see the spirals better .  As you say, something to play with properly when it is raining .

[ryandsimmons]

That is a great picture. I did over an hour of subs last night and didn't get as good. What are the skies like near you?

[ryandsimmons]

That is a great picture. I did over an hour of subs last night and didn't get as good. What are the skies like near you?

[davedownsouth]

That is a great picture. I did over an hour of subs last night and didn't get as good. What are the skies like near you?

This was taken from my back garden on a fairly good seeing night (moon just up, but hidden behind the house).  I took the image below of M82 on Saturday night from a fairly dark sky site.  It was ~30 images (all 20 second exposures) stacked in DSS, keeping the best 70% (Darks and Bias used but again no Flats).  Image has not been touched post stacking, as stretching it / playing with the contrast / other things did not seem to improve it at all.  Either M82 is alot brighter / easier to image than M81, or the dry skies really helped??