JP-S

I've never tried imaging a comet before but I was inspired to give it a go by some of the Leonard pictures out there. I don't regret it and I'm looking forward to doing it again with future comets 😄 These are my first attempts captured on new years eve and new years day using the setup from my signature. 20x45s and 40x20s.

Oh right, yes that's thicker due to the wire from the secondary heater.

Ok, I think the penny's finally dropped for me after your comments. I want to have the secondary mirror round and concentric with the edge of the focuser (this part I was missing) and have the reflection of the primary concentric with the edge of the secondary mirror. If these 2 conditions are met then that's it for secondary mirror positioning, correct? The results of my last session are below. Collimation cap view: I added the concentric circles after the fact using Al's collimation aid. Does that look like a lot of error or is this good enough? It's honestly difficult for me to tell that there's error without the circles and I certainly can't tell by eye when using my collimation cap/dust cover. I'm sure I'm also getting a certain amount of variation from the camera given I'm just holding my phone up to the cap as best I can. I'll give it another crack though if need be. Cheshire/sight tube view: Is there any thing wrong with this view? All the mirror clips are visible and seem to disappear at the same time if I rack the focuser in/out. Mr Spock mentioned I may have bent/twisted vanes. I am a bit concerned as I did go a bit overboard with the tilt adjustments when I first started fiddling with the secondary. Tracing lines suggests the vertical vanes are off: Is this something to worry about? How should I go about remedying it? I haven't actually attempt to square the focuser or adjust the vanes. The manual for this scope (Skywatcher Quattro 8") suggests the secondary is positioned 'full offset' so I wasn't sure if the vanes all need to be the exact same length from the tube to the center screw like the collimation guides suggest. I did a quick measure and it looks like they're 1 or 2mm out from one another.

Well I let loose and starting twiddling all the knobs to see what does what. I made a big mess and after tinkering for a few days this is the best I've been able to get it back to: I'm sure this is much worse than when I started but it's still not clear to me exactly what needs to go where. Can someone look at the image below and explain it to me? I've added coloured circles for reference. Do I want to center the red circle inside the yellow circle? Or the blue circle inside the green? Or are these not the important circumferences? The other thing I'm struggling with is trying to gauge things by eye. My cheshire/sight tube doesn't quite let me see the whole mirror (one of the above pics is through this tube) and when just looking straight down the empty focuser I get a lot of variation with my perspective of the green/blue circles as I can never really be sure my eye position is central. I've drilled out the center of a dust cap so maybe this will help going forward. Thanks.

Thank you Piero! Can you help me understand what part of the visual shows the tilt error of the secondary? Is it because the visibility of the tube where it meets the spider vanes is more present on the left of the photo than on the right? Or does it relate it something in the center of the image in the reflection of the secondary in the primary. This is the part of collimation that confuses me as even in your own photo it's not obvious to me how your primary axial alignment is off. Nonetheless I will attempt to adjust the secondary and maybe things will start to become more clear to me as I see how those adjustments change the visual I'm seeing. To adjust the tilt do I need to loosen the central screw and adjust the secondary by hand or will the loosening of the screw in itself perform a slight adjustment? Or should I instead be using the three thumbscrews also present on the secondary holder? Thanks again for your help. I will have a play tomorrow and see what results I get. Undoubtedly I will be back for more help 😅

I've acquired an 8" F4 Newt for AP as my second scope and I'm still trying to wrap my head around collimation. After watching and reading a bunch of guides and examining various diagrams, it's still not clear to me what things should actually look like when they're good. I think the main thing throwing me off here is the lobey lopsidedness on one side of the reflection of the secondary mirror in the primary as the guides are suggesting that everything should just be concentric circles. Is this the so called secondary offset and is it supposed to look like that? The first attached image is the view down the empty focus tube, the second image is the best view I could manage to capture through my cheshire combo tube tool thingy. All I've done so far is slightly adjust the primary mirror to align the 'hole' in the middle of the reflective element of my cheshire with the inside of the center spot on my primary. This adjustment was done inbetween the taking of the two photos. I haven't touched the secondary at all as the previous owner of this scope said he'd got it into a good position so I figured I'd check before fiddling with it.

This point gets made a lot but if it's true then I wonder why is it that so many people have big SCT's, RC's & various other kinds of long FL scopes for imaging? I understand that it still makes sense for planetary, cameras with large pixels and anyone with extraordinarily good seeing but the proliferation of these scopes seems to exceed the prevalence of those factors.

Enlightening thread - thanks vlaiv. Still, I am curious to see the results.

This is the hard part really as the LED layout is not often in the product description or at least it's often not clear. In this case I actually asked the seller specifically if it was just a strip down one side and they said it was not but that's aliexpress for you. It's also just possible that I got a dodgy unit or maybe it took some damage in transit as it took a look time to arrive and packaging from aliexpress is always a bit dodgy. If anyone else wants to share specific brands and models that have worked for them that would probably be a good idea. I took a punt on this one as I struggled to find references to any model that people found sufficiently uniform for flats or at least any that were also available in my region for reasonable shipping costs. That AGPTEK one looks like it's available from Amazon aus so I'll probably spring for that next. Otherwise I might get a Huion as they're readily available and look like a quality product but are also relatively expensive.

I bought a Chipal A3 drawing pad from aliexpress as the seller told me it's brightness was uniform and it had stepless dimming. I've just received it recently but unfortunately it appears to be not that uniform after all. It looks alright to the eye but in photos I can see a region along the top which is not quite as bright as the rest. I've haven't created any flats yet to test but I don't think I'll ever feel comfortable using it to that end.. It cost me the equivalent of 25 pound so maybe this price point is just too low to expect something up to the task.

No, no filters. I'm in a rural location so it's relatively dark.

This is a single 2m sub from my astromodded 600D. I've opened it with irfanview and just saved it as jpg but I think irfanview does its own stretch so maybe not an ideal candidate for comparision.

Sorry to distract but which model of tracing pad do you use? I just got a newt and I'm looking to pick one up but it's very difficult to gauge which ones provide uniform light.

Interesting point about the dithering, I was wondering how that affects things myself. Is it possible to filter dithering using pecprep I wonder? Otherwise I found this video useful:

Good point! I haven't actually resolved the original issue but have instead identified and resolved a different problem. Still though I am very happy as I feel like my setup is actually usable now and @ 384mm FL, with most objects I can afford to crop the outer regions of the frame where the aberration is most noticeable. Unfortunately I'm not sure how to proceed on the spacing issue short of going to a guide scope. I did check closely for slop in the imaging train around the reducer and everything is tight. I'd be fine going to a guide scope if I had one but I started with an OAG so that means spending more $$$. That said I did just buy an 8" Quattro as my second scope so maybe I can now just mount this ED80T on it and swap the guiding and imaging cameras between them depending on which one I want to image with.