angryowl

Thanks for the reply Mark I must admit I hadn’t thought of that but now that you mention it, it makes sense. I know I read somewhere that NB filters nowadays use several coatings sometimes more than ten per filter and the idea that one or multiple of these coatings are reacting differently to light emitted from this particular star is indeed interesting. Learn something new everyday...

Hi all, Whilst capturing data over the last clear nights on my latest object, NGC 7822, I noticed a star with an odd profile in the OIII subs but put that down to cosmic rays or some other weird artefact. But after my usual preprocessing, I’m still seeing this strange star in all of the OIII subs and much clearer in the stacked image. The OIII data is a stack of 54 300s subs making for a total of 4.5 hours and similar integration times for Ha and SII. What’s strange is that this only shows up in the OIII data and not the other channels. As can be seen, Ha and SII look completely normal. At this point I’m trying to understand what might cause this behaviour only in one channel. I have thought about things such as collimation, tilt, debris on the corrector plate, tracking/guiding errors, dew, shooting through branches and filter reflections. However, none of these fit as the star in question is roughly in the center of the FOV and there are other stars around it similar in size and brightness (judging from the other two channels) which logically should show the effects as well. These were captured using a Mesu 200, RASA 11”, Atik 414EX and Baader F2 High Speed filters. The subs were dithered and 2X Drizzle integration was used. I also rotate my exposures through the filters every imaging session. No other processing done apart from the usual preprocessing steps and an STF curve. Having a look at the Simbad, 2MASS, Gaia DR2 catalogues reveals the star has designation [D75] 7p (Simbad) and is close in magnitude to the star at the bottom of the images. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=[D75]+7p&submit=SIMBAD+search Can anyone shed some light on what might be happening here? Is this an artefact due to some component in my imaging train or something that I’m missing entirely? As this doesn’t show in any of my other stars in this image, or any other image I’ve taken with this setup before, I’m not concerned about this being an issue with my equipment at this point. I’d simply like to find out what might have caused this out of pure curiosity.

Thanks to all who took the time to reply. I have found a QHY5L-IIC which should be delivered any day now. This has a 1.25" body so should be perfect for my setup.

Hi Nikolas, I appreciate you taking the time to reply, but unfortunately the price is a bit out of my range and what I'm willing to spend on this since it'll only be used for polar alignment using SharpCap.

Yes, that's exactly it. A mate of mine found someone selling a QHY5L-IIC which is perfect and am currently communicating with the seller. If this materialises then I'm sorted.

I see, I've attached an image to my original post of my 60mm modified guidescope to better illustrate where the guidecam imaging chip needs to be.

Thanks for replying, but I updated the Ad with more information. I have seen that DMK21 and I also have a DMK41, but these can not be inserted into a 1.25" focuser enough to reach focus with my guidescope.

Looking for a guide camera (CCD or CMOS) with a 1.25" barrel size that could slide into my guidescope focuser. To reach focus with my guidescope the guidecam imaging chip will need to be positioned inside the 1.25" focuser so the camera will need to be inserted almost all the way inside the focuser hence the need for a 1.25" body. Examples of compatible cameras are QHY5 series, Lodestar guide cameras, ZWO ASI 120 mini. This is my guider and roughly how much the camera needs to be inserted into the focuser to reach focus Any make and model considered. Thanks for looking

When I tried the OAG lent to me by @souls33k3r I did manage to get the spacing right and guiding gave me pinpoint stars in all exposures. So far the OAG is the only thing I tried that actually works and currently I’m in the process of building a DIY one. The reason I’m not going with a commercial one is that the guide camera body and USB cable get in the light path of the scope and cause weird diffraction spikes whereas with a DIY one, I’m hoping to avoid that. Chip moving inside the camera…wow that’s scary! Glad you got that one sorted as I can see it being hard to solve. Seeing as the images in my OTA and guidecam drift the same direction and amount I’m inclined to believe the guidecam/chip is securely attached and not moving relative to the main OTA. At least not by any significant amount anyway. But a really good suggestion, thanks.

Oh yeah, that I will do given the opportunity. Perhaps when Ahmed gets his one finished he'll let me throw it on top for a quick test ?

Managed to get out during a clear night last week and tried guiding with the refractor piggy backed on top of the RASA and also with the 60mm guider clamped onto the bottom dovetail of the OTA. I also cleaned up the residual adhesive on the tripod head and sanded the bottom plate of the Mesu down which resulted in a much larger contact area with the tripod head. The results are not really what I expected, but a lot better than before and guiding is more consistent. That being said, guiding with either the 60mm guider or the refractor still doesn’t produce round stars. The elongation is much smaller than before and looks like guiding is now actually correcting for most of it, but still not completely. Not sure why this is as the elongation direction and amount are now very similar in both guide and main images. The guided subs are now almost usable, but I’d still very much like to get as close to perfectly round stars and just don’t see this happening with the current guider. Therefore I’ve started designing an OAG using a right angle prism and will be using the bare CMOS board from a QHY5L-II-C camera to avoid obstructing the light path with either the camera body or the USB cable. I have most parts and it’s just a matter of finding a way to mount them. I won’t be troubleshooting this any longer as I’ve spent far too many clear nights on this and I now know that an OAG will certainly solve the problem. I really wanted to get to the bottom of this but it just seems like it’s not practical at the moment. Will update this thread once the OAG is complete Some examples of what elongation looks like now

Hi Jakub Welcome to SGL, you’ll find the very helpful and experienced people here will provide invaluable assistance and guidance when dealing with issues such as yours. Don’t yet have a final word on this, but so far it seems like the PTFE pads were only part of the issue and although I can now guide most of the elongation out, it’s definitely still present in most guided subs. In response to your question, I’ve tried anything from 1 second exposures all the way to a minute in most directions in the sky and no elongation whosoever was present in these short exposures. I find that in subs up to 3-5 minutes I usually see no sign of elongation wherever I’m pointed. That being said, the first thing I’d suggest is rotating your camera 90 degrees to see if the direction of elongation changes by 90 degrees. If it doesn’t, that could point to the camera not being completely orthogonal. This would explain seeing elongation during such short exposures and also remember when pointing at the zenith the gravity would no longer be pulling the camera/imaging train to one side to such an extent and would explain why stars come out fine. Apart from this, we’d need to know a lot more before anyone on here can offer any meaningful advice on troubleshooting this. Please let us know how you get on with this as I’m very much interested in what this’ll turn out to be.

That makes a lot of sense now, but didn't know any better two years ago when I built the tripod. Looked at the EQ8 and saw that the tripod they sell had PTFE pads and the mount is rated at 50kg, I thought adding 0.5mm thick pads on mine would be a safe bet. However, I'm thinking the EQ8 might have more contact area with those pads than my Mesu did which would make a difference.

Hehehe, thanks mate. Don't want to get too excited as I said it could be only part of the issue, but further testing should confirm.

Forgot to mention the subs above were taken with the mirror unlocked.