Arc of light fixed by Adam Block's EZ Selective Rejection - But why the arc? :-)

a lot of times stuff like this is caused by a bright star right outside the FOV of the sensor, but obviously still within the FOV of the telescope. it's worth checking in a planetarium program if there are any bright stars just off to the right there.

rob

if you were dithering then that can account for the presence of the arc in only some of the exposures. you'd have to be on the hairy edge of the pointing that causes the problem. sometimes a bright star can also manifest as a lone diffraction spike at the edge of the frame, emerging from nowhere. but usually these spikes look like a natural gas flame or something and are not sharp like a normal spike caused by the secondary vane.

i don't know if i've ever seen an internal reflection in my RC that i thought was due to the target itself, but i suppose it is possible. i do have internal reflections that seem to be caused by the background LP which don't flatten out. i've always assumed this was due to my panel not being able to illuminate the sensor the same way the sky does.

^^^ well, that is true, i've heard that extending the primary baffle as well can help with light leaks, but i never tried it on my RC10.

i believe it. i've just had a lot of arcs caused by stars outside the field but these GSO RCs are very temperamental and i would not be surprised in this least if this solved the OP's problem.

interesting about the CCDT67 because i am using one too. certainly that widens the field and so more of the mirrors surfaces must participate in illuminating the sensor.

Rich Sornborger:
I have no idea how to fix your present processing dilemma, but I do know how to fix that nasty reflection given I've been fighting it ever since I installed the CCDT67 reducer.  Just take a look at my attached pic of a very un-elegant way of fixing the problem.  I wish there was a commercial fix for this but this piece of cardboard (cut from a case of beer carton) works perfectly.  Just extend your secondary shroud by an inch (25mm) and all will be golden.

Thank you very much for sharing this information.  We've only had this happen with M13 but the scopes are fairly new and I've only done about 8 targets. My imaging buddy Jim is a wiz at building things and if he comes up with anything better I'll be sure to share it here.  Thank you again!  Paul

I am Paul's imaging buddy Jim and of course his best friend. I have looked at Stellarium and the only two stars that are near are 7th magnitude approximately which should not be bright enough I think to cause these reflections. I considered a lengthened secondary shield which I can 3D print for us. I can make the print file available here too if after some experiments with paper the arc disappears. The real mystery is why it has only happened on images when the scope is pointed to the north eastern portion of the sky from our location and not on every sub. This has me still thinking that it is some weird light in our area. When the arc disappears from our images briefly it takes about three frames. It disappears  section after section on the subs and reappears section by section in three frame until it is again a complete arc as seen in Paul's images above. Thank you to all who have posted suggestions it helps and is encouraging for us to know that we do not have something unsolvable in our scopes.

Alan, I've seen no infringement of the light cone on my subs.  I'm guessing I could extend the cardboard shield further if necessary.

You have nothing to lose Jim.  Good Luck and enjoy clean subs.

Rich

I tried manually matching your image with the arc to a recent wider-field image of mine (https://www.astrobin.com/vk05y7/) in PhotoShop (oops -- shouldn't mention the PS-word in this group :-) In this composite, your image is at 50% (ish) transparency.

There is a brighter star near the arc, but it's not concentric. So while it seems that the arc is scattered light from some source, the origin isn't clear.

Hi Richard,
The arc seems to switch sides in my images after a meridian flip.  We are going to experiment tomorrow night with a black paper strip on the secondary baffle tube. This is a weird thing because I covered my whole telescope with black paper months ago to try and get rid of the bands of light . It did not help so the light is not in our local environment and must be something off axis out in the sky. Thank you for you input it is appreciated by Paul and I.

Hi Richard,
The arc seems to switch sides in my images after a meridian flip.  We are going to experiment tomorrow night with a black paper strip on the secondary baffle tube. This is a weird thing because I covered my whole telescope with black paper months ago to try and get rid of the bands of light . It did not help so the light is not in our local environment and must be something off axis out in the sky. Thank you for you input it is appreciated by Paul and I.

And that's exactly what my arcs did too Jim.  They would flip with the meridian flip.  I have an Astrozap Shroud that I tried and it did nothing whatsoever as to the arcs.  I'm confident the paper strip will solve your problem.

Rich

Rich and Richard, From all I can tell the arc is some kind of reflection in the scope. It stays in the same relationship to the scope mechanism and to the image on the sensor. It is in the same place in relationship to M13 for example, and there is only one arc which is sorted out when the subs are spun and stacked by WBPP.
I have tape out all the lights on the scope. mount, Eagle computer and the only one remaining is the small blink security camera that I use to watch the operation and monitor the inside of the dome. I have considered that as a source but Paul has a different set up and also has the same arc. The arc shows when the scope is facing away from my blink camera on the floor of the observatory. I have thought it might be the blink camera but it is IR which should not show except maybe as overlap in the Ha perhaps. I will report to you on Thursday how the experiments went with the black paper baffle extension tests we are doing tomorrow (Wednesday). Thanks again.