Help needed

[lhansen (Lars)]

I'm in the process of comissioning a new camera on my RCOS and I'm getting some weird flares on bright stars. These did not appear with my old camera (ST8XME) however the new camera a PL16803 has a much larger chip size. Although the flats for this image werent perfect, I dont believe they have anything to do with the flares. Notice that they are basically concentric with the centre of the FOV.

One theory is that it may be due to reflections within the baffles? I'm not sure. If anyone has experienced (and diagnosed) a similar problem I would be grateful for some advice.

The image is 10 * 20mins @ -30c dark, bias and flat callibrated. Stacked by CCDStack.

Thanks in advance

[Stevec35 (Steve)]

I guess you would have checked all the obvious ones like collimation, camera not being properly square on, etc. Also you may need to adjust the BFD by adding/deleting more extension tubes.

Cheers

Steve

[lhansen (Lars)]

Hi Steve

Collimation is basically perfect (< 1"), the camera is square to the optical axis of the scope as evidenced by CCDInspector. BDF is at 10" which is what the manual says is optimal.

I spent a day getting these basic things right. Internal reflections are possibly a culprit, the tube of the RCOS is flocked but perhaps I need to get a can of dead black spray paint to ensure that there are no reflections from the baffling. What about stray light? The flaring exclusively associated with bright stars which would suggest, at least to me, that it isnt stray light. The instrument bay in which the RCOS is housed is mostly kept dark, but there is some illumination from power supplies and the telescope position sensor (to ensure I dont close the roof on the scope).

All advise is welcome

Thanks

Lars

[gregbradley]

Your secondary shroud is not baffling the larger chip as it was designed for smaller chips. Same thing happened with the CDK17.
What Planewave did was design a different secondary shroud which is wider at the base where it meets the secondary (about 5mm wide) and that angles back to a sharpish edge at the end of the shroud. It may be slightly longer but not sure about that.

However I got the same results by flocking the inside of my original secondary shroud. That will also make it a bit wider anyway.

So flock the inside of your secondary shroud and see if that handles it. It most likely will.

Hopefully your secondary shroud simply unscrews and you can take it off and do it on a bench as it is tricky to do in place. If you do do it in place make sure your scope is horizontal as dropped scissors etc could hit the primary mirror. You wouldn't be able to reach it anyway.

The hard part about flocking the inside of the secondary shroud is to get the flocking to sit flat and not have gaps behind it. That's what happened to me doing it in place. Then I discovered after I finished the shroud simply unscrewed off! So check to see if yours does that first off.

Nice image by the way. That looks very promising indeed.

Greg.

[lhansen (Lars)]

Thanks Greg

That sounds very likely it. I can do a quickie test by using a black cardboard extension to see how the secondary shroud size impacts on the problem. To be honest that was not something I had thought about.

Re image, seeing was not great last night and I ended up with a guide interval of 8 secs. FWHM values around 3.5 average was not that great either, I would normally hope for around 1.9 - 2.4.

Looks like rain or showers here for the next week si I will have plenty of time to fiddle.

Cheers

Lars