Telescope Reviews: Does focus affect flats? An experiment.

Edit: Please read later post in the thread- Newtonian results are much different!

Does focus affect flats?

I have seen many posts stating that the focus point of flat images must not change from where the focus was for the lights. This never really made sense to me since all the imperfections that you are trying to remove occur in the telescope itself. Most of the dust that is visible is either on the filter, reducer or CCD window- none of which should be affected by changing the focus point of the telescope. Dust on the primary or secondary may be affected slightly by changing the focus of the telescope, but I wonder how much it would even effect that. So I decided to do an experiment.

Equipment: SBIG ST8300, Meade 10" LX50 SCT telescope, Homemade EL Flat Panel
Settings: Bin 2x2, 2 second exposures

Methodology:

21 exposures, dark subtracted and STD Dev 1.75 stack in Nebulosity for 3 imaging runs:

1) Base
2) Control
3) Out of Focus image.

The control was done with no changes to the focus.
For the out of focus image, I racked my SCT focuser all the way in- 27 rotations of the focus knob.

Then I applied the Control and Out of Focus stacks to the base as flats for comparison

Image1: Stacked Base

Image2: Stacked Control

Image3: Stacked Out of Focus

I left the level of stretch the same after applying the flats. This basically shows how much flatter an actual picture would be after applying the flats with both the control and the out of focus flat. Both did an excellent job of removing the dust donuts:

Image4: Base with control applied as flat

Image5: Base with Out of Focus applied as flat

As I was first stretching, I could not see any significant difference between the control and out of focus. However, as the stretch point became very extreme, a slight gradient began to emerge in the out of focus image. I suspect this is due to the primary mirror's significantly different position between the 2 sets of data and the internal reflections being slightly different. Dust donuts were not affected but this slight gradient clearly demonstrates that changing the focus will affect the data in the flats.

Image6: Base/Control and Base/Out of Focus combined in photoshop and extreme stretching applied to compare

Conclusion:

The level of stretching that I applied in order to identify a change between the control and out of focus images was much more than I would ever apply to actual data for an image. My conclusions for this SCT:

1) Dust donuts are not significantly affected by focus.
2) An out of focus flat is much better than doing no flat at all.
3) Focus will affect your flats, so try not to change focus between your imaging and your flats. But don't worry too much about it!

Edited by John Wunderlin (06/11/11 12:42 PM)