Tuesday, February 25, 2014

Chasing Internal Reflections

Propus Flare is Giving Grief

I've been working on the Monkey Head nebula for a little while and have noticed a consistent challenge of flare from Propus casting a gradient and reflection rings across the field.

I first noticed this issue at Calstar.  I figured then that it would be less of an issue once I gathered more subs.

Note that during the Calstar example as seen above, the only filter used was the B+W 486 UV/IR. Note the orange flare across the frame from the lower left upwards. Also notice the two rings at the corner.

A few more sessions later and I'm still seeing it.

I thought that it might be coming from the 52mm filter that's at the front of the Feathertouch focuser, so I recently removed that filter and covered the threads with flocking paper.  I'd gone back and forth on this exercise in the past, not sure if it was an issue or not.  As it stands now, I'd prefer to have a minimal amount of glass in place as possible.

I've started using the IDAS LPS D1 filter as it helps isolate the Ha vs Sulfur emissions and seems to do a better job of eliminating the near IR from the full spectrum camera.  Back at Montebello again and with 13 subs stacked, the flare is just as strong.

Note how the flare location follows the orientation of the sky vs the orientation of the camera.  The Calstar and MBOSP pictures are flipped and the flare changes as well.  This indicates that the issue is in the scope vs the camera.

Looking at the CCDWare website, their donut calculator suggests that the location of the flare - if the math is the same - would be about 98mm in front of the sensor.  This suggests that the flare is inside the flattener.  I can only think of one location that would meet this dimension.

The challenge now is how to flock that small part to control reflection.  Or do I dare paint it?  Or do I send it back to SV for them to paint/flock it for me?

Follow up

I contacted SV and had a brief email exchange with Vic.  His comment was that the retaining ring on the flattener should be loose enough to remove.  The spacer shouldn't be touched.

Well, I tried to move the ring and it's pretty well stuck.  Probably from paint or adhesive from the old experiments with flocking paper.  Used some points to try to get purchase on the ring and those didn't help, either.

So at this time, I'm going to leave it as is and just deal with the fact that there will be flare.  Maybe I can extend the dew shield to eliminate the problem areas.