0.90
#...
·
3
likes
|
|---|
|
My best friend and imaging buddy meteoritehunterjim and I have close to identical systems. Both are OPT's 10" RC with Optec TCF 2" focuser and the Astro-Physics CCDT67 - 0.67x Reducer. We both recently imaged M13 and we both wound up with this nasty arc running through our images. We're at a loss as to the actual cause and if any of you know what is causing this please let us know. Today I started searching for ways to eliminate the arc and I stumbled upon Adam Block's amazing YouTube called EZ Selective Rejection in PixInsight. Adam does a great job of explaining (as always) so I'm not going to even try to explain the steps. The video is located here --> https://www.youtube.com/watch?v=ZZVJKo47Ctk The arc is not in all of my subs so Adam's Selective Rejection works. The arc was in 141 out of 153 subs. Because I only had 12 subs without the arc the background in the selected area was noisy and I used the selected area GAME script mask and then MLT Linear Noise Reduction to get the two backgrounds to match as closely as I could.  |
1.81
#...
·
1
like
|
|---|
|
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 |
|
0.90
#...
·
1
like
|
|---|
|
Hi Rob, Thank you for the information! I was wondering if M13 is bright enough that it's causing some sort of internal reflection and then also if it was why it was not in all of the subs... Thanks again! Paul |
|
1.81
#...
·
1
like
|
|---|
|
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. |
3.10
#...
·
5
likes
|
|---|
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. |
|
1.81
#...
·
2
likes
|
|---|
| ^^^ 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. |
|
3.10
#...
·
2
likes
|
|---|
To tell you the truth, I think the reflection is light catching the edge of the secondary mirror which makes that big arc on subs. By extending the secondary shroud it totally eliminates the problem.. But man, is it an ugly fix...  |
|
1.81
#...
·
1
like
|
|---|
|
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. |
9.03
#...
·
1
like
|
|---|
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. Amen. |
|
0.90
#...
·
2
likes
|
|---|
Rich Sornborger: 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 |
|
3.10
#...
·
3
likes
|
|---|
|
Please update the thread on a permanent solution to this ugly fix Paul. But I do guarantee your reflection problems will be a thing of the past. I have so many targets that I had to severally crop to get rid of the problem. Rich |
0.00
#...
·
2
likes
|
|---|
| 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. |
 #...
·
1
like
|
|---|
|
I wonder if a little flat matte black paint on the cardboard fix might help a little? I don't think it's ugly, but wonder how much of the primary light cone is lost to the extension? If one could catch such reflections in the first few subs, then an adjustable extension might allow one to dial in the fix, yet minimize light lost. |
|
3.10
#...
·
2
likes
|
|---|
| 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. |
|
0.00
#...
·
1
like
|
|---|
| I will test this with a strip of black paper and a rubber band and piece of tape next night out I hope. Thanks. |
|
3.10
#...
·
1
like
|
|---|
|
You have nothing to lose Jim. Good Luck and enjoy clean subs. Rich |
|
0.90
#... |
|---|
|
Hi Guys! Thank you all so much for giving your input. As Jim and I experiment we'll keep this updated. CS! Paul |
0.90
#...
·
1
like
|
|---|
|
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.  |
|
0.90
#...
·
2
likes
|
|---|
|
Of course, maybe the optics are such that such stray light would not be concentric with the source. I'm not an optical engineer so don't know. If you wanted to pursue it further, you could try a set of images with the centre moved a little bit north, and another set a little bit south (in my image, up-down is north-south), to see if this has an obvious effect on the arc. cheers, Richard |
|
0.00
#...
·
1
like
|
|---|
|
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. |
|
0.90
#...
·
1
like
|
|---|
|
If it switched sides with a flip it suggests it's something in the telescope system itself. Is there any light source (eg a power light on the camera) which might be sneaking in? My own scope has numerous lights on it -- it's like a Christmas tree from the outside (power lights on the various equipment, data lights on the many data-transfer devices on-board, built-in computer screen, environmental sensors which flash for some reason, etc.) I haven't had a problem like yours but if I did, that's where I'd start looking -- by covering them with black tape. cheers, Richard |
|
3.10
#...
·
1
like
|
|---|
Hi Richard, 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 |
|
0.90
#... |
|---|
Richard Francis: Obviously I'm assuming that here you mean "switched sides in the image space" so that as the image inverted the arc stayed on the same side in the scope frame of reference and so switched in the image frame of reference. On the other hand, if you mean that the arc remained on the same side of M13 (north-south) while the scope flipped then my assumption is wrong. |
|
0.00
#... |
|---|
|
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. |
|
0.90
#...
·
1
like
|
|---|
|
Hello Everyone, Jim and I will be experimenting tonight and we'll follow up here. As an update to the processing of M13, I was not able to use Selective Rejection on the R, G, and B as every sub contained the arc. I was able to remove the arc from the RGB masters by using the LinearStarNet script and CloneStamp. Thanks again for everyone's feedback! Paul |
