Flat Frame

[wouterw]

Hello,

Is the attached histogram normal for a flat frame (Canon 1100D) ? Is it an issue that the color channels does not align ? Here is a link to the actual flat frame:

https://photos.google.com/search/_tra_/photo/AF1QipNXZh2tWzsg_PwKEpSlEjelGFk4dZFn1OYsqhRG

Also I assume it's okay to slew the telescope after imaging, before taking the flat frames ?

Thank you.

[PeterCPC]

In BYE there is the ability to take flats by using AV-Flat where you have LIGHTS showing. Flats should be taken with the scope in exactly the same position as the lights were taken as any artifacts in the imaging train could move otherwise.

Peter

[wouterw]

Regarding staying in the exact position as imaging, how do people that use sky flats or their laptop screen manage this without slewing the telescope between imaging and taking flats ?

Thank you.

[PeterCPC]

Speaking for myself I just hold the laptop/panel in front of the scope whilst BYE does it's thing. Remember to put a delay on the first image so you can get into position.

Peter

[Pompey Monkey]

In BYE there is the ability to take flats by using AV-Flat where you have LIGHTS showing. Flats should be taken with the scope in exactly the same position as the lights were taken as any artifacts in the imaging train could move otherwise.

Peter

I think this is a bit of a misinterpretation: The optical train i.e. camera, focuser and, if you have one, the dew shield, should not be moved relative to each other in-between taking the lights and the flats. Pointing the telescope in another direction, e.g. to point to a luminescent panel or twilight sky is perfectly acceptable. Indeed, it is usually required to do so.

The idea of flats is to compensate for the lack of linearity in the optical train. If you find that this is changing significantly as the scope slews, then you have much bigger problems to deal with than taking flats!

[PeterCPC]

It's also to do with any "artifacts" (dust) that get into the imaging train which could move if the telescope moves.
The Flat Frames are used to correct the vignetting and uneven field illumination created by dust or smudges in your optical train so best taken without changing anything but, I agree, that it is possible to take them later.

Peter

[Pompey Monkey]

During an imaging session my scope might start pointing at an elevation of, say, 45^o, go up to near the zenith, and end up back at 45^o ele. When do I take my flats? I simply have to take them later. And let's not go to the filter wheel changing position many times during the night...

Although it's all a bit moot with the weather that we're having at the moment

[Alien 13]

I only ever take flats once then use PS with the master flat to remove any dust related artifacts, the dust bunnies etc are then dealt with by creating an artificial flat from one of the subs.

Alan

[PeterCPC]

I'll just quote from Digital SLR Astrophotography by Michael A Covington:

Flat - an exposure of a blank white surface taken through the same telescope as the astronomical images, with the same camera at the same ISO setting, and preferably on the same occasion so that dust particles will be in the same positions.

But, of course, practicalities will always come into it. I'll just get my coat..

Peter

[Pompey Monkey]

I think with a dedicated CCD set-up that's not taken down at the end of every session, te cahances for new contaminants to get into the imaging train are significantly reduced. That might be why the CCD imagers get away with it*

*Coming form the guy who's only done one half-decent CCD image to date!

[dph1nm]

There is absolutely no need to take flats with the telescope pointing at the same position in the sky - twlight flats are fine.

NigelM

[davedownsouth]

I take tend to take flats at the end of a session by just taking the scope off the mount and laying flat on a table next to the mount.  I put the screen in-front of it, leaning against a chair and after messing about to get the light levels right, just knock off twenty shots.  I could be wrong (wouldn't or be the first or last time ), but I do not think the direction / position of the scope matters at all.  You just need to make sure that the focus is the same (not better or worse) as it was for all of the shots you just took.  If clouds float over, I might do the flats mid-session, but putting the scope back on the mount in the same position can be a bit of a pain.

[ollypenrice]

I have no intention whatever of taking flats after each session! They are long enough as it is... Once ever six months is more than enough. I use permanent setups with sealed filterwheels and nothing gets removed and dust doesn't get in. (I have a clearly written sign on the observatory saying 'No dust.')  Permanent setups without flat-destroying DSLR chip-shakers (which can be disabled) don't need constantly renewed flats. Since everything likely to cause a dust bunny will rotate with the camera assembly it is also very unlikely that rotating the camera in the OTA will affect your flats. It doesn't affect mine. (Poorer quality focusers may be off axis and cause a relative rotation of the vignetting but will it eally matter? Suck it and see.)

Olly

[Pompey Monkey]

I have no intention whatever of taking flats after each session! They are long enough as it is... Once ever six months is more than enough. I use permanent setups with sealed filterwheels and nothing gets removed and dust doesn't get in. (I have a clearly written sign on the observatory saying 'No dust.')  Permanent setups without flat-destroying DSLR chip-shakers (which can be disabled) don't need constantly renewed flats. Since everything likely to cause a dust bunny will rotate with the camera assembly it is also very unlikely that rotating the camera in the OTA will affect your flats. It doesn't affect mine. (Poorer quality focusers may be off axis and cause a relative rotation of the vignetting but will it eally matter? Suck it and see.)

Olly

That's a good point Olly, one that I hadn't thought of myself. Another benefit to having a permanent set-up in an obsy

Now I wonder how much muck I need to get on my secondary before it shows up on the stacks.........

.....Although seeing as I might only get two or three sessions a year with the current weather, I might never need to take flats again!

[ollypenrice]

That's a good point Olly, one that I hadn't thought of myself. Another benefit to having a permanent set-up in an obsy

Now I wonder how much muck I need to get on my secondary before it shows up on the stacks.........

.....Although seeing as I might only get two or three sessions a year with the current weather, I might never need to take flats again!

Awe...

 lly

[PeterCPC]

Unfortunately, for those of us mortals who have to assemble camera to kit each time, life is a bit more complicated. I put up a no dust notice but the blighters don't always comply.

Peter