Flat question

[NoelC]

Since Carole raised this I have started looking more closely at my flats (i.e. looking at them at all).
Although they appear to be working when applied to the image, they look quite worrying:-

(http://www.astrobin.com/311148/)
This is made on my 8" GSO RC, using a Revelation 0.75 flattener/reducer shooting with an Atik One 6.0 0.3S aiming for mid range exposure the image stats report Median: 28473.0.  This is a flat master produced from 22 flats using a light  box (which has been rotated during the flat run) and calibrated and obviously stretched to show the artifacts.  The subs show similar patternation although less distinct.  The other flats (Ha & L) are similar but not as distinct.  I have had problems with the OAG causing a gradient on my flats in the past; but this does not look like that.  Has anyone seen this before? Should I worry?

Collimation of the RC is by standard Cheshire; it never needs adjusting and the out of focus stars show no offsets. If this were an artifact of collimation I would suspect the primary to be a long way out, but that is not evident on inspection.

[Carole]

The dust bunnies are a good sign that the flat is exposed to the right brightness, but I am concerned about the circular brighter lit part of the flat which I have never seen before and I don't think should be there or what is causing it.

I wonder whether it is to do with your light source varying during the run of flats which you say were rotated at one point.  I also have no experience of an off axis guider.

See what the more technical people have to say.
Also not sure whether the dust should appear as rings. 

Since you stated in a message to me that you have yet to capture an image, perhaps your camera was not focussed when taking the flats.

Carole

[MarkS]

1) The dust bunnies are ring shaped because each one is an image of the shape of the open aperture of the scope - a circular opening with the secondary mirror obstruction.
2) The very large ring is probably the open aperture of the scope being imaged by the flattener/reducer.  The same thing happens with the SCT design.  You can check this by placing your outstretched hand at the tube opening as you take the flat (between the light source and the scope).  If the shadow of your outstretched hand appears in this very large ring then it confirms the issue.  If you remove the flattener/reducer then this huge ring disappears.
3) The huge ring is off-centre because optical chain is out of collimation.  Probably the secondary needs adjustment or maybe you have a case of a badly sagging focuser.

Mark

[JohnP]

Mark you are like the Benedict Cumberbatch/ Sherlock Holmes of the telescope/ astrophotography world..... 

[ApophisAstros]

Mark you are like the Benedict Cumberbatch/ Sherlock Holmes of the telescope/ astrophotography world..... 

or "all seeing eye..."
Roger

[Carole]

Thanks for your words of wisdom Mark, so looks like properly collimated Noel's flats are OK.  I have no experience of doing flats with an SCT.

Carole

[NoelC]

Thank you very much Mark
I put the Cheshire back on it; and there is a slight offset - will go back over the collimation.

However; the focusser does appear to be reasonably solid.

Noel

[NoelC]

Mark; following your comments I investigated the collimation stability and in light of some others experience with this telescope I stripped it right down.
The primary mirror was quite loose (turned on it's axis with slight finger pressure) and the secondary was held in place with no torque at all on the retaining ring.  I carefully increased the holding pressure to negate mirror movement.
Also discovered my Cheshire cross wires were out of center (giving rise to substantial primary error). So I lined up the secondary using the centering ring, the primary is fairly obvious by eye as secondary and primary light shrouds are same diameter.  Hopefully this has fixed the problem, will let you know how it goes.

[Carole]

Nice scope Noel.  I just checked the collimation of my Newtonian yesterday after it being removed from the mount and sitting in a box for about 10 months after my knee fracture last year.  Was expecting it to need re-tweeking, but seems perfectly fine.  Like you I also use a cheshire.

Hope this sorts your flats out, but on the whole I think your flats were fine.

Carole