Darks, Flats, Bias with dslr and lens?

[jimjam11]

I understand the purpose behind these frames, but how do people actually take them? If I recall correctly the focus is not supposed to be altered before taking these frames but I always find I struggle to get the lens cap on without moving the focus slightly. This is made even worse if I have a lens hood on the lens which seems essential to try and combat dew.

[nmoushon]

Darks - You can put the lens cap on and take a picture with the exact same settings as your lights. Focus isnt as important here since the hot pixels will show up whether your image is in focus or not. But it is important for the other two so if your having trouble keeping focus while putting the lens cap on then do them last.

Bias - Turn the speed of your camera to the fastest setting your camera has. Mines is 1/4000 sec on a 450D. And just start taking pictures. Usually around 40 is the normal. Since this is takes no more than a minute or two.

Flats - Theses are a bit tricky. A common way is to open note pad on your laptop and point the scope at the screen. Make sure to move the scope only through the mount and not touch the camera and how is oriented on your scope or flats will not work correctly. Once you have that set up you change your camera mode from M to AV. This setting will auto pick the correct exposure time so all you need to do is take the pictures. This also doesnt take long. The hard part is doing it correctly. There are other more precise way of doing it but I find that this works pretty darn good for my amateur self. Once I get my own Obsy on better gear then I might switch methods.

[jimjam11]

Thanks, I will try that. I am getting good results with lights/darks but flats/bias ruin my images!

[jimjam11]

The bias frames are the ones I cannot understand. Reading the DSS help it suggests temperature isnt important and doesnt mention focus. I therefore took my bias frames indoors before I started imaging but they ruin my images and somehow cause trailing...

[jgs001]

I've always struggled with flats with a camera lens... I think, because the shutter speed always ends up just wrong and the refresh rates cause me problems, so to date, I've not used flats with lenses. The method described by nmoushon works fine with my scopes, I suspect as the focal ratio is slower, the shutter speed is longer and this averages out the refresh rate oddities . I don't normally use bias frames with just lights and darks.

[jimjam11]

So perhaps I wont get the benefit of bias frames unless I use flats as well?

[jgs001]

They shouldn't make any real difference as far as I'm aware... The Bias frames are subtracted from all the other frames... The darks are then subtracted from the lights. As the darks contain the Bias also, when you subtract the darks from the lights (well DSS does anyway), the bias is automatically removed. That's how I've understood it. Bias frames are required if you use flats (and I know I should use flats, but I have to overcome the issue of the refresh rate weirdy I alsways hit).

[JB80]

I'm still not entirely sure about flats with a lens either, I did try the computer screen method but didn't have much luck and I tried the twilight sky but again I'm just not sure if I was on the right track. Most of the info regarding flats is while using a scope, I guess the same principles should apply.

When taking darks I flick it over from the MF setting to AF setting to put the lens cap on as it kind of locks the focus slightly to avoid changing the focus like you mention.

Don't forget though to switch it back to MF once the cap is on before taking the darks incase the AF decides it wants to chime in.

[jimjam11]

When taking darks I flick it over from the MF setting to AF setting to put the lens cap on as it kind of locks the focus slightly to avoid changing the focus like you mention.

Don't forget though to switch it back to MF once the cap is on before taking the darks incase the AF decides it wants to chime in.

Good idea, I will try that.

[jgs001]

Focus and aperture settings is only critical for flats. Exposure time and temperature is critical for darks. I often remove the camera from the scope, fit a lens and then shoot the darks. It manes I can pack away the scope gear whilst capturing the darks.

[jimjam11]

Focus and aperture settings is only critical for flats. Exposure time and temperature is critical for darks. I often remove the camera from the scope, fit a lens and then shoot the darks. It manes I can pack away the scope gear whilst capturing the darks.

It they suggest a tee-shirt over the scope for the flats why is focus important (assuming aperture doesnt change)?

[jgs001]

If you adjust the focus, then the position and effect of anything inside the optical train could/will have moved/changed. Flats are to deal with things like dust bunnies (dust on the sensor) and other oddities through the imaging kit. In a scope, holding something over the objective, be it a t shirt or a flat panel or laptop, doesn't affect the focuser.

[jimjam11]

Good point. I was thinking of the flats purely in terms of vignetting.

I assume the bias frames can be captured once and used a reference for future imagining sessions because they dont appear to be temperature or focus dependent?

[jgs001]

I have one set of Bias frames... and I'm not sure I've ever created a new set. They are not temperature or focus dependant. Just make sure, if you're shooting them during the day, that you cover the viewfinder. It's possible, although not likely, that light can leak in through the viewfinder, during normal usage, this can't happen, as you're eye would normally be to the viewfinder. (I have tested this bit out, and only when shining a torch directly into the viewfinder, on a long exposure, did I get any light leak, but...)

[jimjam11]

Thanks, I will try to take a set of bias frames tonight and keep them for future use.

[jgs001]

What you'll find is, that when DSS completes the stacking, you'll have a master_bias frame for the specific ISO. Just grab that, and keep that in your image library... add it to the master group in DSS.. then group 1 will appear, and you can add your lights and darks to that... this gives you the leeway for stacking multiple nights worth of data together... by using multiple groups... the bias in the master group are applied to all the images.

[jimjam11]

Thanks,

[pel]

Great discussion gents. I wasn't sure about Bias frames before.