Flat Focus point


Wonder if the Flat wizard could somehow be configured to look at the light frames average focuser position by filter and put the focuser at that position for the flats for that filter.

My light frames last week had quite a spread in focuser position (from temperature changes) and I would like to re-set the focuser to close to the light frames position used.


Off topic.

In my experience focus position doesn’t affect flats, I mean unless you are really really off it doesn’t matter. At least in my experience.




You generally want your flats to be in focus but as Jose mentioned it just needs to be close.

Averaging focus points between your lights won’t really get you in focus. When are you taking flats? If they’re at the end of the sequence then you should be in pretty decent focus. Of its the next day, and significantly warmer, you may he a ways out of focus.



I am under the impression that I want the focus point for the flat to be the same as for the lights. I realize there is some tolerance, so the dust balls are the same siize as your lights. I use a flat panel directly on top of my dew shield (Gerd Neuman E-panel).

Over the course of a night my focus point may start at 280 and move to 232 over the course of 8 hours. I think this is pretty extreme and in some seasons the differences are even larger. I suspect this may have an impact on my flat quality.

So, in my example, if it moves by 50 steps over the 8 hours through 4 filters at ten twelve minute exposures each. (assuming purely linear change for my example)

  • The first filter may be from 280 to 268 as the focus point
  • The second from 268 to 256
  • The third from 256 to 244
  • And the last from 244 to 232.
    The next morning when I do flats the focuser will be sitting at 232. If I do all my flats at that point, then the flats for the first filter will be 42 steps away from the average focus point for that filter which is 274. Each succesive filter will have flats that are “closer” to the original light focus point.

My suggestion is to capture the focus point data (this may not be possible) for each light frame filter set, average the focus position and set that as the focus position for all that filter’s flats.

I think an alternative is to set up the flats and lights to rotate in such a way that you shoot a flat after each light. But, I don’t have an automated way of putting my flat panel to the scope for that method. And, especially for Narrowband flats, that takes away dark time for imaging. I would like to be able to complete events in sequence, then do my flats based on the actual sequence focuser points used.



I may be missing something really fundemental here but when shooting a target, your focuser is set for an object at infinity. The EL panel is only 18" away so it is massively out of focus no matter where you position your focuser. Even if shooting sky flats, there is no focusing on a clear sky, so the same situation exists.

The projection of a dust mot is based on the distance between the filter and the CCD chip, which is always the same unless you have your focuser between the filter wheel and the CCD chip.

I’m hard pressed to see how the position of the focuser makes any difference in the quality or the usability of the flats produced.



I think it depends on where the dust is. If it’s on something whose distance from the CCD doesn’t change as the focus is changed then I guess that changes in focus make very little difference.
But if the distance between the dust mote or vignetting and the CCD oes change when the focus is changed then it may make more of a difference.

I think that most visible dust motes come from the CCD cover or the filter wheel so if you imaging train is CCD - filter - focuser you shoud be OK.

This is all very speculative, that indicates my level of confidence in this speculation.



I can’t speak for everyone, but dust on my sensor is easy to control, as well as my filters, to some degree. It is the small dust on the inner glass of the main objective of my refractor that is causing me concern. Or small dust on the outer objective for that matter.

I get that dust past the focuser should not matter, but I have 4 air/glass surface before the focuser. The Main Objective and the Field Flattener/Focal Reducer have two surfaces each before the focuser. Each of these can have things that Flats can help.

Interestingly enough there is a current topic on the importance of focus position relative to lights going on in CN forums right now.


A dust mot on the objective lens or corrector plate will be so far out of focus that the “donut” it produces is probably bigger than your CCD chip and will not show up on a light frame. The donut of a dust mot on the reducer will likely show up on the CCD chip and will be removed by the flat. However, the reducer is probably at a fixed distance from the CCD chip, so moving the focuser will not affect the size or position of a reducer dust mot donut. So, again I can’t see how the focuser position is relevant to the quality of a flat. If the CN folks come up with something that can be supported by the “physics” of their explanation, please share.