Light, Dark, Flat, Bias frames with DSLR

[Alveprinsen]

I've searched the forums and found a good post on this, however; I still have a question.

the post I found was from 2009, so I thought it more apropriate to start a new one.

Light, Dark and Flat frames I get... No problem.

But the Bias frames.. Those are another story.

I watched a youtube tutorial on bias frames, and from what I understood, these were best taken at dawn, as the sun came over the horizon, with the camera connected to the scope, poining straight up. Exposure times between 1-2 sec.

Also, on DSLR there is the question concerning ISO.

Does one have to take dark, flat and bias for all ISO values that will be used, as well as under all temperatures, and all exposure-times that will be used?

Is it correct that dark and flat frames stored on the computer for later use needs to be replaced by new ones about once a month, while the Bias frames every 3-5 months?

I'm just a tiny bit confused.

Alveprinsen.

[RikM]

Bias frames can be taken at any time, the camera doesn't even need to be connected to the telescope at the time. Just cover the lens or scope, and shoot a set of exposures at the shortest exposure time possible (1/4000 sec on my Canon). These can be reused for months at a time.

Darks can be reused if you know and can match the temperature (±5°C), exposure time and ISO to the light frames. These can also be reused for quite a while.

Flats should be done fresh every time you reset the camera onto the focuser. These should also match the ISO setting of the lights if you are using DSS for stacking as It seems to work better that way. You should point the telescope at an evenly illuminated light source (can be the sky with a bit of white material like a T-shirt, stretched over the aperture), then check the histogram on the camera and set the exposure time so the peak in the histogram is 1/3 to 1/2 the way across. From your description of the Youtube vid, sounds like they were taking flat frames rather than bias.

[Alveprinsen]

Flats should be done fresh every time you reset the camera onto the focuser. These should also match the ISO setting of the lights if you are using DSS for stacking as It seems to work better that way. You should point the telescope at an evenly illuminated light source (can be the sky with a bit of white material like a T-shirt, stretched over the aperture), then check the histogram on the camera and set the exposure time so the peak in the histogram is 1/3 to 1/2 the way across. From your description of the Youtube vid, sounds like they were taking flat frames rather than bias.

Ah, I obviously used the wrong term... bias, instead of flat.

About the flats though; if I take them straight up towards a clouded but illuminated white sky, at the correct temperature ofc... What about focus?

And please, explain to me this histogram in the camera, and the "1/3" to "1/2"... one third to one half of... what exactly? Excuse my ignorance, I am quite new at this.

Alveprinsen

[jgs001]

The simplest way of shooting flats with a dSLR, is to set the camera to Av mode, make sure exposure compensation is turned off, and fire away. I shoot one, and check the histogram to make sure it's as Rik said, then fire away.

When shooting flats, do not adjust anything about the optical system, or you will change the specifics and the flats either won't work, or instead of reducing effects, will introduce others.

[Alveprinsen]

The simplest way of shooting flats with a dSLR, is to set the camera to Av mode, make sure exposure compensation is turned off, and fire away. I shoot one, and check the histogram to make sure it's as Rik said, then fire away.

When shooting flats, do not adjust anything about the optical system, or you will change the specifics and the flats either won't work, or instead of reducing effects, will introduce others.

If I do like RikM suggests in the post above, that is t-shirt stretchover against the sky, or simply against a white sky, do I connect the camera to the scope before doing this, or can I do it with an ordinary lens only?

[jgs001]

You have to have the entire optical system setup in exactly the same way as you did when you were shooting the lights. Even a slight change in the orientation of the camera, or shift of focus, will cause problems with the flats.

[Alveprinsen]

You have to have the entire optical system setup in exactly the same way as you did when you were shooting the lights. Even a slight change in the orientation of the camera, or shift of focus, will cause problems with the flats.

Wow..

This means going out one night, shooting the pictures, then NOT disconnecting the camera or anything, waiting for daytime, and then going out to take the flats? ... that sounds quite... insane..

Would it be possible to stretch a white T-short over the lens of the Scope after finishing the photographing for the night, shine a flashlight at the front of the scope and snap the shots?

I've got a LED Lenser flashlight that I can adjust both the width of the cone of light as well as the strength of the light. the light is white, ofc.

[Alveprinsen]

So thats a no-go for the "using a flashlight on the t-shirt" idea?

[dph1nm]

This 'you must have exactly the same optical train' isn't as necessary as people make out. If you can put the camera back on the telescope at roughly the same focus and roughly the same angle then the flats will do a pretty good job. Yes, if you want perfection you should do it before you take the camera off, but personally I think with DSLRs it is a waste of time.

NigelM

[Black Knight]

For flats I simply stretch a white t-shirt over the front of the scope (attached with a rubber band), point the scope at my laptop running Notepad (maximized), switch the DSLR into AV mode, and shoot away. Works for me. Mostly anyway

If you use something like APT it's even simpler. Simply create a "Flats" plan with a count of 20, then execute. Re-execute the plan if you want more than 20 flats. Takes a couple of minutes to run and couldn't really be much simpler.

[stem1989]

thanks for opening the post as Im new to astrophotography also and the information here was very helpful!

[themos]

So thats a no-go for the "using a flashlight on the t-shirt" idea?

Depends... when was it last washed?

[RikM]

For flats I simply stretch a white t-shirt over the front of the scope (attached with a rubber band), point the scope at my laptop running Notepad (maximized), switch the DSLR into AV mode, and shoot away. Works for me. Mostly anyway

If you use something like APT it's even simpler. Simply create a "Flats" plan with a count of 20, then execute. Re-execute the plan if you want more than 20 flats. Takes a couple of minutes to run and couldn't really be much simpler.

I use APT to do this as well. I am lucky that I bought an EL panel to use as a light source. It's a little one meant for checking x-rays and it was very cheap. But the white page on a laptop screen, or t-shirt against the sky works was well. I never had any luck just shooting against a cloudy white sky, the t-shirt diffuser seems to help quite a bit.

You should be able to display the histogram on your camera screen. It is a little graph showing the number of pixels at each brightness level. For a flat, there should be a rounded peak somewhere between the left hand side and the middle.

I can't find a very good picture, but something like this only more to the left.

[Black Knight]

Depends... when was it last washed?

Hehehe ...

[Black Knight]

I use APT to do this as well. I am lucky that I bought an EL panel to use as a light source. It's a little one meant for checking x-rays and it was very cheap. But the white page on a laptop screen, or t-shirt against the sky works was well. I never had any luck just shooting against a cloudy white sky, the t-shirt diffuser seems to help quite a bit.

Interesting piece of re-purposing . I spare no expense to take my flats. Hence the use of a rubber band!

[AstroAndy]

My 2 cents, Point in the general vicinity of the part of sky you observe( at twilight, dawn or dusk), then take approx. 1/250s of exposure for 1600ISO, abt. 1/180s for 800ISO. If I don't want to wait for sky conditions, the old wrinkle-free white t-shirt tight around the objective, a moderatelystrong flashlight held to the objective, check the cam view for even illumination, if need be move the flashlight (or the flashlight function on the smartphone)closer or further and center it, and take your exposures.

[Pete Presland]

I use APT to do this as well. I am lucky that I bought an EL panel to use as a light source. It's a little one meant for checking x-rays and it was very cheap. But the white page on a laptop screen, or t-shirt against the sky works was well. I never had any luck just shooting against a cloudy white sky, the t-shirt diffuser seems to help quite a bit.

You should be able to display the histogram on your camera screen. It is a little graph showing the number of pixels at each brightness level. For a flat, there should be a rounded peak somewhere between the left hand side and the middle.

I can't find a very good picture, but something like this only more to the left.

another vote for this method, normally you will have the laptop out there so it's ideal to do while you are packing up