Hi Guys! I hope you can help me with my problem on my Newtonian... Short explanation of my Equipment: - ZWO ASI 294MC Pro - Skywatcher Explorer 250PDS - Skywatcher EQ6-R Pro - ZWO ASI AIR - ZWO EAF I used this combo to photograph C27. The mount was polar aligned, autofocus routine was successfully completed and the first shot (60 seconds) looked like this:  At first I thought it was a problem with the autoguider, but the values looked like this:  Of course after some time, clouds came in and the session was over. i was not able since then, to do some testing. The Scope has been collimated with a Baader Laser Collimator and a cheshire eyepiece. In theory everything looked fine. My Imagetrain is as follows: 294MC (6,5mm) -> Filterwheel (20mm) -> Adapter (11mm) -> Adapter (16,5mm) -> Baader MPCC MIII Does anyone have an idea where i could start? Is it a Focus issue? Tilt in the image train? Tilt on primary (or secondary?) mirror? Thanks in advance! Michael
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It is a problem with guiding, of which you don't say anything specific. Was it AOG or with a guide scope? It looks like either differential flexure or mirror flop.
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andrea tasselli:
It is a problem with guiding, of which you don't say anything specific. Was it AOG or with a guide scope? It looks like either differential flexure or mirror flop. Thanks for your reply! Sorry... My Guiding-Equipment is an ASI 120MM with a 240mm Guidescope. No OAG
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Because it's consistant throughout the frame, that points to flexure. Likely the mirror moving in it's cell. The skywatcher/GSO primary cell design has rather poor side support. Other culprits could be focuser sag or a loose secondary. Either way, something is moving around in the primary scope causing the image to shift while the guidescope keeps on trucking like nothing is amiss. OAGs exist to counter this.
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Michael Wimmer:
Thanks for your reply! Sorry... My Guiding-Equipment is an ASI 120MM with a 240mm Guidescope. No OAG So it is more likely than not mirror flop but other sorrces of differential flexure exist such as poor guider support, slack in the focuser (although this shouldn't really occur for just 60s) and overall poor support between mirror and camera. To try and remove mirror flop you'd need to take the primary cell down and put some low-modulus silicon (black!) around the points of contact between mirror and cell but in a way not to create stress at either low temperatures or high temperatures which is easier said than done, to be fairly honest.
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I have a PDS200 and -as supplied - the mirror clips did need a bit of tightening to get things right. But don't overtighten or you'll get distorted shape stars. It was pretty easy to do and it's been good as gold ever since.
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Honestly, I looks like you bumped the mount or something changed suddenly during the exposure. Could eve be a cable snag I suppose. It's certainly not an optical defect.
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I have a 200PDS and it took me 2.5 years to make it almost perfect (star shape). In the end it was a combination of mirror cell improvements (side support and clips; not too loose, not too tight), focuser alignment (I first had a laser collimator but then I bought the OCAL collimator and ended up with the focuser now leaning roughly 1° towards the primary mirror) and backfocus adjustment (with the coma corrector I'm far away from 55 mm, more like 58 or 59 mm). I did multiple other improvements but these were the ones regarding the star shape.
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Is this your only telescope? try to do a 20-30 second unguided exposure if the stars are round it is not the mount, in the past I had a similar problem it was the mount, just purchased, solved by sending the mount for assistance, otherwise perhaps a completely incorrect collimation, the defect is uniform across the entire frame, so it is not tilt. Also try a bathinov to check the focus, to see if the eaf worked properly.
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Thanks guys! I will check the mirror clips and also try an unguided exposure, if the weather allows it.
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masluigi:
Is this your only telescope? try to do a 20-30 second unguided exposure if the stars are round it is not the mount, in the past I had a similar problem it was the mount, just purchased, solved by sending the mount for assistance, otherwise perhaps a completely incorrect collimation, the defect is uniform across the entire frame, so it is not tilt. Also try a bathinov to check the focus, to see if the eaf worked properly. i have a RC6 with the same Mount/Camera/Filterwheel-Combination. The images are tack sharp:  |
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I cannot escape nothing that the vertical spikes are double, not blury. This suggests that during the exposition, the scope moves from position A to position B and spend most of its time at position A and B, not in between.
Guiding per se is ok, focus is good. This is no tilt or BF or collimation as the pattern is uniform across the field. You can rule out all these possibilities. In your case, the problem is that the FOV center shifts a bit during imaging.
Could be differencial flexure.
Did you dither ? If so, could you check your dither parameters ? Maybe you need to let the scope settle for a few second.
Is the case reproducible ? Under what conditions ?
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in my opinion the 250pds which weighs 12 kg, plus being very long it unbalances the setup and with a very light wind it is unusable, plus the setup which will weigh another 2 kg is too much for that mount, also the focal length of the guide is too low better an oag, it seems like a backlash of the mount try to balance it perfectly I would not go above 11 - 12 kg all included. first change the guide then balance. the rc weighs less than half … and being compact it balances better.
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I saw similar stars on my newt yesterday. It turned out that I had inadvertently changed my sidereal tracking rate, but only on one axis.
Most above causes are more likely - but it’s worth checking as you seem to have elongated stars only in one direction.
I hope you find the reason.
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I have a SW 250PDS which I use for visual. Just recently (just over a week) I thought Id give it a whirl thinking that it was way too long, camera gear way too heavy and cumbersome on my SW AZEQ6 etc. Same horse different jockey compared to an EQ6R. Well I was surprised just how well it tracked. Far better than my AskarFRA500. Im using a ZWO2600 mono, FW, OAG-L, 174mini and televue parracor. I do have a TS Focusor as the original could not handle the parracor and an ethos. Only drama I have had was double spikes like yours. Was out of focus. The stars though were elongated due to the focus unlike the photo above. I now have a mirror mask and the stars are fine. I also thought that short subs would be a go…pushed them to 600sec on the Helix. Mount is fine.
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Hello
Did you try to take photos without refrigeration? to rule out fan vibrations
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Alejandro Moreschi:
Hello
Did you try to take photos without refrigeration? to rule out fan vibrations Not yet, due to the weather conditions. I will give it a try. Thank You
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Hi Guys! yesterday I had the opportunity to test the Newton again: - balanced the tripod - balanced the mount (with counterweight bar extension) - Polar aligned - run autofocus routine (double-checked with bahtinov mask) - calibrated guiding First Test-Object was "Altair" (60s, guided) - the image still shows the same problem:  After that i adjusted one screw at the back of the primary mirror cell i got this: "Markab" (30s, guided)  Next object was "Tarazed" (30s, guided)  What do you guys think? Is it getting better? Thanks!
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This just looks like bad guiding, plain and simple. The trails are short because exposure time was short. I do think your focus is off by just a hair too. What was your guide error during those exposures?
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Hi Tony, for Markab the Guiding-Graph looked like this:  and on Tarazed it looked like this:  Looks actually quite normal.
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I think it was not well collimated
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Given your graphs I still think it's guiding. The RMS numbers match up with your images. Although your RMS numbers are ok you need to look at the excursions on the graph. For Markab you have a 2 arc/sec spike and in general your first guiding graph needs to be smoother. If you showed both graphs at the same scale you'd see how rough your graph for Markab is. I have an EQR-6 pro and I know it's much better than this. I typically get an RMS error of 0.40 to 0.35 arc/sec RMS, I've even seen it dip down to below 0.3 in periods of really good seeing but the main difference is I get a much smoother graph.
Best advice is to look at everything. I don't use the AIR but I understand it uses a variant of PHD2 for guiding. Check your balance and make sure it's reasonable. Go back to setting up your guiding using the default values, do all the usual calibrations. Double check your system for possible movement of the guide scope relative to the imaging scope. Bottom line is at 1200mm, your guiding just has to be smoother. There's not a lot of room for error. I think you can feel confident that It's not tilt or anything to do with the primary shifting around.
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The other thing I forgot to mention is that in your guide graph for Markab, Dec isn't converged. I'd know better with a longer graph but it's hanging out about two arc/sec north from where it should be and the guiding doesn't seem to be able to bring it down to the zero line. It could be that guiding isn't aggressive enough or something mechanical is working against it.
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Thanks Tony,
i will check the settings next time the weather conditions allowing some testing.
Thanks, mate!
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