Apologies for my questions, but this was is really causing me problems.
Whenever I image with the L-Extreme filter, my images come out clean. Practically no nasty gradients. However, as soon as I image galaxies unfiltered. I am getting a weird light pollution pattern - almost identical in both images. I applied a dynamic background extraction (Pixinsight) on the two images below and then an autostretch. The result is terrifying. Please help! Integration times are 1h for the frame galaxy and 5h for the Sombrero galaxy (subs were 300” each. Too long?). I should also add that there is a street lamp not too far off. Could that be the issue? Any way to easily fix this in processing?
If you are getting consistent gradients, then I personally suspect the flat light panel if you are using one. The one thing about Galaxies is there is a lot fewer photons available to capture.
I found that using my Flat panel using an EL switch-mode power panel light, created interference patterns. Currently, now I am simply taking images of the dawn or dusk blue sky directly. Improvement is better for me. I still have an interference issue with the camera itself but that is an earth-loop issue that creates bars across the image.
Sean and Malcom, you guys are amazing. I know what the problem is. I took my flats before the imaging session and must have left the filter on for the flats. I later realized that I had forgotten to remove it, but by then I had forgotten about the flats and it never even came into my mind that the flats were inaccurate calibration frames. The inaccurate calibration was made based on the L-Extreme's pretzel looking pattern on the sensor. I just re-processed without the flats and got the image below.
Any ideas on how to get rid of the reflection though? I think it's from a nearby street lamp. Other than the obvious of imaging further away...?
I think you mean Malcolm & Sean (or mswhin63 & AnakChan ).
Ah I see, so you had the filter on for your flats, but you took the filter off for your lights - resulting in flats calibration not appropriately matching for your lights. Good find.
Whilst that pretzel will still exist in the lights, as you've taken Sombrero without any filters (i.e. broadband), that pretzel is effectively "hidden".
You're taking this with a Newt. Sure you're not having any light leaks in the focuser/camera region? Anyway you could shield that part away from the street light? If its truly the streetlight, I can't think of anything else but to reposition your scope (which sounds like a limiting factor for you).
There seems to be a bunch of weirdness with your Sombrero light sub there. I'm not certain if it's solely due to the streetlamp.
Thank you Sean for your reply. Yes I meant to thank Malcom. I’ve edited the reply.
I have been suspicious that I might have focuser leaks for some time. Any way of testing this?
In terms of the other weirdness, please let me know what you mean and if you have any ideas as to what could cause it. I was told I might need to paint some exposed bolts in the OTA as they might be causing me some problems - it at least cover them with black tape.
Do you think that street light was making it into the top end of your newt and hitting the secondary Stéphane? I'm not sure what your setup looks like, but a light baffle might do the trick if you're not already using one (or make sure that your existing light baffle is long enough / positioned correctly if it's a partial baffle). Likewise for the bottom end of your newt - you might need to seal it from off axis light entering from behind the primary.
Thank you Sean for your reply. Yes I meant to thank Malcom. I’ve edited the reply.
I have been suspicious that I might have focuser leaks for some time. Any way of testing this?
In terms of the other weirdness, please let me know what you mean and if you have any ideas as to what could cause it. I was told I might need to paint some exposed bolts in the OTA as they might be causing me some problems - it at least cover them with black tape.
Thanks again,
Stéphane
I've seen a beer can holder mod done to shield unwanted light around the focuser/adapter/photographic rain before. U could use any old thing really. I usually put on masking tapes around where adapters join just to be sure (e.g. the ZWO tilt adapter that's screwed onto the OAG).
Aside from the pretzel, what do your flats look like? Any reflection, etc. coming in? That'll at least tell you if it's coming in from the front of the tube, bottom, or focuser.
Dean, I think I will need to find something to block unwanted light. I heard a shower cap does the trick under the OTA. I will use some dark tape on other areas. Thanks for the suggestions.
Sean, I think masking tape will be good for most parts. That’s a great idea. I will need to scratch my head though for the part of the focuser which moves in and out. I obviously can’t tape that. A beer can holder or anything I can slide on will hopefully do the trick though. It’s amazing what I am learning on this forum.
As for the flats, I can’t really see anything on them that rings alarm bells. Either way, I will take on board these suggestions about light proofing the OTA. Thanks so much.
Put the cap on your scope with your camera in live view. Then just shine a torch at the parts in question if the camera lights up, then you have light leaks.
Dean, I think I will need to find something to block unwanted light. I heard a shower cap does the trick under the OTA. I will use some dark tape on other areas. Thanks for the suggestions.
A shower cap may interfere with natural ventilation for your primary. You could consider cutting a black donut of 3 ply with outer diameter a push fit into the bottom of your OTA, but with plenty of free room in the middle. Spray paint it matte black on the inside. It should block off axis light entering the rear of your OTA but not interfere too much with ventilation. You could make one out of cardboard for starters to see if it makes a difference.
You could try making a light baffle for the top end out of black cardboard (black side in) to see if it makes a difference too, and use something more hardy if it helps. It needs to be long enough to stop off axis light making it onto your secondary. If you manage to keep it light, it shouldn't upset your balance too much.
These are the fun parts of customising your rig for best performance - especially for backyard observing.
Dean, I will also look into your black doughnut suggestion, although I don't quite understand where it would be blocking light. True, the shower cap was suggested for taking flats only, not for imaging. I was looking at the rear end of my OTA and I can't see where light would be getting in. Not the ventilation area I presume? But around that area? I will need to try David's suggestion.
I am beginning to realize the importance of no off-axis light coming in and customizing my rig! Up until this point, I have mostly been using an L-Extreme filter which I think was blocking most of the unwanted light. But when imaging galaxies with no filter, these issues are becoming much more noticeable. It's deeply appreciated to have such high quality suggestions from you all.
David, brilliant. That I can easily do! I will try that tonight. Thanks so much.
Put the cap on your scope with your camera in live view. Then just shine a torch at the parts in question if the camera lights up, then you have light leaks.
David, I just did this. No light gets through around the focuser area, nor does it come through around the extension tubes or camera area. However, the screen lights up when I shine the torch at the rear end of the OTA. I covered up the fan area only, the result was the same. I covered up all around the fan (but not the fan area itself) and the screen went pitch black.
Now I understand Dean's comment about a doughnut shape obstruction. The area around the fan is letting in light, but the fan area itself is not letting in light, hence the hole in the middle which allows ventilation but does not let light in.
Am I understanding this all correctly? Thanks again for all the help!
Now I understand Dean's comment about a doughnut shape obstruction. The area around the fan is letting in light, but the fan area itself is not letting in light, hence the hole in the middle which allows ventilation but does not let light in.
Am I understanding this all correctly? Thanks again for all the help!
Hi Stéphane,
you're on the right track now mate - that's great news. You can make a rear light baffle out of just about anything - even the plastic that's used on binders (so long as it's black) if you can work out how to attach it, or flock lined plastic if you have any (would also make a great baffle for the front end too), or humble 3 ply painted black!
Dean, I’ve made a temporary black cardboard baffle where I have cut out a square hole in the middle for the fan obstruction. It slots in the the rear of the OTA beautifully. Hopefully no more light leaks from that area. I do have some silver bolts exposed inside the OTA which I heard might also cause some reflections and halos. I will spray paint them black one day but for now will see if the rear cardboard baffle will make a difference when imagining unfiltered.
Thanks again for your help - and yes, customization is rather fun!
).
