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.

6 hours ago, The Lazy Astronomer said:

The next downside is that when you 'over-sample' like this, you negatively impact your signal to noise ratio (SNR), by spreading light out over more pixels than is necessary to record the finest detail your sky conditions allow. You could improve the SNR by binning the image in software during processing, but this has the effect of reducing image resolution (to basically the same as if you imaged with your 130pds), so at this point, you might as well save yourself £500 and use that instead!

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.

8 minutes ago, bottletopburly said:

Check how it’s illuminated I bought one originally but was only lit one edge which looking at it may be what you have try finding one that’s illuminated two edges 

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.

Follow up:

So I've resolved this issue and had one of those 'I'm such an idiot' moments. The problem is that I've been relying on a bahtinov mask to focus and somehow I've been using it incorrectly without it ever occurring to me that that process could be going wrong.

I recently switched to NINA from APT and after getting frustrated by it's bahtinov aid, which never seemed to be able to identify the spikes properly, I decided to drop the bahtinov and try focusing manually just going off the HFR measurement it's image analysis provides. As soon as I did this I very quickly saw the HFR dropping a lot with large progressive turns of my focuser knobs and eventually halved it. Compare the carina sub below to the one I put in the first post..

I still have no idea what I was doing wrong with the baht though.. It always looked good by eye then I would fine tune with the bahtinov aid tool in APT without any issue. Is it possible that the mask just isn't a match with the scope or am I likely to be doing something wrong?

Thanks for the feedback! That gives me some confidence.

Wow a 12", what sort of issues do you get with that?

The scope I'm looking at is the GSO f/4 so also 1m. It'll be in the obsy so I'm not too worried about it being a sail at least.

Hi all. I'm considering getting a 15kg 10" imaging Newt but I'm wondering if my EQ6-R can handle the load OK. I've seen people run C11's and RASA11's on this mount with good results so I know it can handle the weight but I'm more worried about moment arm and those tubes are shorter. Does anyone have any exp imaging with a 10"+ Newt on this mount or know if others have had success with this size OTA?

I'm eager to see how this setup performs. I'm planning on getting a GSO 10" F4 but I'm quite concerned about the load. I know the EQ6R can carry even heavier loads but it's the moment arm that makes me uneasy about pulling the trigger.

I use a relatively small payload on my EQ6R but from my own investigations I've found several instances of others guiding very well with payloads of ~23kg on this mount. It seems the tripod is a major limiting factor so getting the mount onto a pier is key, as well as moment arm as has already been mentioned.

My first mount was a skyview pro and had no end of problems with stiction which made it basically unusable for AP even with my very light payload (CF refractor + OAG). I wasted a lot of time as I was so new it took me a while just to work out what the problem even was before realising it was something well beyond my ability to fix anyway.  Eventually I got an EQ6R and everything basically just worked.

To some extent I was probably just unlucky that my particular mount had the problems it did. Most of these mount that roll out of the factory probably would've been fine but the downside can be so huge if you get one of the bad ones that it's not worth the risk I'd say.

Interesting. It is consistent with being too far away from the sensor then. Would anyone care to guess roughly how far out I am based on the images? Are we talking 1mm or something more like 0.1mm?

If the spacing is too far then I may be in a bit of a bind as I'm not using any spacers currently and don't think I could reduce it.. The reducer requires 55mm of backfocus and with the 44mm of the 600d + 10.5mm of the OAG + canon ring adapter (not exactly sure how thick it is, maybe 0.5mm-1mm), I'm not sure how to make any reduction without turfing my OAG. What do people normally do in this situation? I'll check it tomorrow during the day just to make sure there isnt any slop or slippage in the train somewhere as well.

25 minutes ago, CraigT82 said:

Certainly looks like field curvature.Is the reducer also a flattener? 

This is the reducer. It's labelled only as a reducer not a reducer/flattener. It certainly produces a much flatter image than I had without it but maybe it's still not specced to do the job completely.

10 minutes ago, jambouk said:

Flat field and maybe some field rotation, how good if your polar alignment? 

Under 1' last time I checked and I'm on a pier. Next time I'm out I'll check to make sure though. How can field rotation be identified? Is the shape of the stars in the corner the giveaway? I thought that was due to the flatness but now that I think about it those stars would be stretched across the diagonal of the image if that was the case wouldn't they? Instead they appear to be stretched perpendicular to the diagonal.

Well thanks for the advice, seems like the reducer could be an issue. I'll look into getting an actual flattener and see how it goes. Is it possible this could be due to spacing of the reducer (I'm sure I have the correct spacing but I know this can be finicky), or even due to a slight tilt of the camera? The thin t-ring I use does have a heavy load on it due to the dslr+cooler combo. It feels tight but I wouldnt be surprised if there's a slight sag.

I get quite noticeably reduced sharpness away from the center of my subs and while I know this is to be expected to varying degrees with most optical systems, I'm just wondering if this looks about right for my setup and if there's anyway I can improve it.

My imaging train is an Orion ED80T-CF (apo triplet) -> Orion 0.8x Reducer -> Orion TOAG -> Thin t-ring -> Modded (LPF2 removed) Canon 600D w/ cooling mod.

I've attached a few random subs to illustrate. They're unprocessed apart from resizing and whatever opening them in irfanview does which I think is just a stretch. Theyre all 180s guided exposures except the seagull which is 300s.

I was wondering if it could be something to do with my reducer? I bought it for the flattening effect as my stars were extremely eggy originally. It's helped tremendously in that regard but you can still see some eggyness in the corners. It was also labelled only as a reducer rather than a reducer/flattener. Could this also be related to the camera being modded and different wavelengths focusing at different points? I dont use any cut filter but I figure even if that was the case the effect would still be consistent across the frame. Seeing probably isn't so good where I am as it's sea level and not far from the coast but again I'd except this effect to be consistent across the frame.

Otherwise I'm at a loss unless this is simply a limitation of the scope itself. If that's the case then that's slightly disappointing but will be good to know. I'm already considering moving on to a new scope so that I have something with a bit more reach anyway. Thanks in advance.

One more thing to add is that Gary Honis' testing found the noise levels of the 600d to be slightly lower than the 550d: http://dslrmodifications.com/T3iReview/T3iReview.html

It's a shame that they removed the movie crop mode - is it possible to get this mode back via magic lantern?