Is this field rotation?

[Gib007]

Last night I went out to image the Crescent Nebula again and decided this time it was the night to go for Alignmaster and get my polar alignment down right. I did the initial EQMod polar alignment, set Dialog Based on EQMod, ran Alignmaster and started using it. I initially chose Vega and Caph as alignment stars and Alignmaster complained the error was massive and that it needed reducing. The error reported was something along the lines of 322°. I thought to myself that it was impossible and it just needed a different pair of stars. Vega and Gamma Cas were fine and I got an error of about 6'. I repeated the process once more with Dubhe and Gamma Cas and got a 3' error. In each iteration, it was an easy process centering the stars in the eyepiece crosshairs.

Being happy with my polar alignment correction (but not going for a third or fourth iteration!), I set Append on sync in EQMod and did a GOTO alignment (I chose Vega, Deneb and Altair - three stars in a triangle surrounding my target) and set off my exposures of the Crescent Nebula. The exposures were in Luminance (with my LPS filter in front), 10 minutes long with 1x1 binning and -10°C CCD temperature. I also have a coma corrector on the camera with appropriate spacers. Today I opened a raw image (no calibration yet!) in PixInsight, did an auto-stretch, saved as JPEG and here it is:

Disappointingly, look at the star shapes (particularly on the top left corner). I can't seem to escape from this - every long exposure image I capture whether I use Alignmaster or not, has these. I'm genuinely at a loss at what to try. Should I just use Alignmaster for a number of extra iterations? The problem I have there is that clearly even if a pair of stars is listed as Best, it may not work as I may get a weirdly calculated error of like 322° like with Vega and Caph. Perhaps it's the pairs of stars I choose? Is there a guideline on this? This problem is really getting me down - I hope you can help! Thank you in advance!

P.S. For reference, I use ST4 AutoGuider port guiding in PHD Guiding and the graph is fantastic - very, very flat on both Dec and RA with very minor corrections being made throughout the exposures. Extreme Dithering is also enabled in Nebulosity with settle at set to <0.2. No guiding issues with this either.

[dph1nm]

Looks like something isn't centred to me. You vignetting pattern is clearly offset to the right.

NIgelM

[Gib007]

Looks like something isn't centred to me. You vignetting pattern is clearly offset to the right.

NIgelM

Yeah, that has always been the case though, ever since I bought the telescope. Collimation is very good though. I check it every single time while setting up before imaging. Even having changed the original focuser to a Moonlite one, the same vignetting is present. How would that affect the rotation though?

[Gib007]

Looks like something isn't centred to me. You vignetting pattern is clearly offset to the right.

NIgelM

I'm thinking this through more now and you may be on to something. I may be doing something fundamentally wrong, such as collimation. Is it correct to say that the vignetting should start from the centre of the image produced? It may not be a coincidence that the star shapes go wrong mostly on the outskirts of the vignetting and that it always seems visible to the left of the image and not so much on the right. My train of thought here is that my optical axis alignment is wrong and my left side of the image is outside this. I was imaging overhead and the site was very dark indeed so I can safely say the vignetting seen in this image is purely from the optical train.

[Gib007]

After I posted the above, I made a flat image with my camera's CCD sensor looking straight down into the focuser. Here's what I got in terms of vignetting:

Clearly, as was suggested by Nigel, the vignetting is way off centre. I think you may have hit the problem on the nail. I re-checked collimation and went through the entire process bit by bit and here is the result after collimation:

Overall it's looking much, much better and the view through my Cheshire Collimation Eyepiece is one of perfect collimation according to all the sources I used to help me go through the process from beginning to end. Fingers crossed this sorts me out next night out!

Thank you again Nigel. I will post back to report.

[Daniel-K]

id say your secondary isnt sitting under the focuser quite right

[Gib007]

id say your secondary isnt sitting under the focuser quite right

Thanks, after thinking about what Nigel posted earlier, I thought I'd give OCD-like collimation a go and my flats are looking very healthy now so I expect my lights to follow suit. Will report after tomorrow night!

[ollypenrice]

You've certainly improved it but it wasn't a game stopper to start with, in my book. I get some pictures taken!

Olly

[Gib007]

You've certainly improved it but it wasn't a game stopper to start with, in my book. I get some pictures taken!

Olly

Will do ASAP! Tomorrow it's my next planned outing. Mind you, I agree it wasn't a game stopper but I'm being perfectionist in trying to get my entire image to come out as it should. The left side of my images were just a mess compared to the rest. The collimation was probably the cause in the end. Proper verdict tomorrow night.

[astrosathya]

Olly and others, just a question.

The coma corrector corrects about 82% of the coma. Therefore, would it only compound the effect?

[Tim]

I'd say your secondary mirror is slightly rotated.

It can look like good collimation at first glance but in reality it isn't. It's one of the trickier things to get right as well as an iterative process. If you need help, PM JasonD, he's got all the pics and tricks you need to get it nailed.

When your collimation is good, your vignetting pattern will be centred on the chip.

Cheers

Tim

[Gib007]

I'd say your secondary mirror is slightly rotated.

It can look like good collimation at first glance but in reality it isn't. It's one of the trickier things to get right as well as an iterative process. If you need help, PM JasonD, he's got all the pics and tricks you need to get it nailed.

When your collimation is good, your vignetting pattern will be centred on the chip.

Cheers

Tim

Thanks, indeed, I've already noticed a massive improvement with the vignetting pattern. I think tonight will show much nicer images as a result of my collimation. I'm going to buy a laser collimator to aid me in collimation as well. I've looked through my telescope with my Cheshire eyepiece again today and it looks perfectly ok. My new flat images also show centric vignetting patterns. I have faith now that it will look better. I will post an image or two resulting from tonight's imaging session.

[Gib007]

Good news and bad news after last night's imaging session. The good news is that the collimation now produces images where the vignetting indeed comes from the centre of the images. The bad news is that it hasn't seemed to cure my star trail rotation issue. For example, this is one raw image of 10 minutes in Hydrogen-Alpha of the Crescent Nebula:

Observe the stars along the four corners. The top-right seems to look ok but the other three corners are pretty bad. I modified the image above to include arrow vectors to show you direction of star trails:

I don't know what to look at next. My Coma Corrector has always worked well and though I've never had 100% flat images (does any Coma Corrector ever completely correct coma on an f/5 and faster?), this is pretty bad.

On setting up, I ensured the mount was perfectly horizontal with the bubble level, balanced it with the imaging equipment on there and then performed the EQMod polar alignment. Once done, I used Alignmaster with the stars Vega and Gamma Cas twice to reduce the error of the polar alignment. Both times this was very successful and the error dropped noticeably after the first iteration. I then just performed a quick GOTO alignment to one star very near the Crescent Nebula (with Nearest Point selected in EQMod for alignment) and voila, slewed to Crescent Nebula, which was perfectly on target and imaged away with 10 minute exposures. PHD Guiding graph was excellent throughout, with minor, minor bobbing over and below the zero line for RA and Dec, which is normal. Any suggestions? Thank you!

[RobH]

Is your guidescope exactly aligned with your imaging scope? Field rotation can occur if it's off, but in my experience it has to be quite a lot off unless your exposures are very long.

Do you get this effect on stars using very short exposures? If so, then it's not field rotation.

Have you tried forgetting the EQMod alignemnt routine and drift aligned instead? You can use PHD for this (calibrate, then disable guide commands. watch for N/S drift only and adjust the mount until you don't get any obvious drift for about 2-3 minutes.), but I'd use your main imaging scope as a 50mm guidescope won't have a long enough FL to show drift quickly unless the PA is way off.

[ollypenrice]

Is your guidescope exactly aligned with your imaging scope? Field rotation can occur if it's off, but in my experience it has to be quite a lot off unless your exposures are very long.

Do you get this effect on stars using very short exposures? If so, then it's not field rotation.

Have you tried forgetting the EQMod alignemnt routine and drift aligned instead? You can use PHD for this (calibrate, then disable guide commands. watch for N/S drift only and adjust the mount until you don't get any obvious drift for about 2-3 minutes.), but I'd use your main imaging scope as a 50mm guidescope won't have a long enough FL to show drift quickly unless the PA is way off.

I agree that Drift is a direct measurement and software-free. Rob, any particular reason for using PHD over the imaging camera in its own software? Artemis Capture can throw up a reticle.

Olly

[RobH]

I agree that Drift is a direct measurement and software-free. Rob, any particular reason for using PHD over the imaging camera in its own software? Artemis Capture can throw up a reticle.

Olly

If you have the graph in PHD active Olly, you can see the trend of the drift very quickly, and within a couple of seconds it's easy to see if any adjustments you've made have improved it or not. For me it's not a problem using the guide cam as I use an OAG, so obviously you're using the main scope.

[ollypenrice]

If you have the graph in PHD active Olly, you can see the trend of the drift very quickly, and within a couple of seconds it's easy to see if any adjustments you've made have improved it or not. For me it's not a problem using the guide cam as I use an OAG, so obviously you're using the main scope.

OK I get the point about the graph. Thanks Rob.

Olly

[Gib007]

I think I may have found out the cause of all my problems. My Moonlite focuser doesn't sit square on the tube. When I mount the secondary mirror completely centric inside the tube (constantly measuring and adjusting the spider vanes), looking through a Cheshire eyepiece shows that the secondary mirror is too far up. I disassembled the whole thing and quickly remembered my initial troubles mounting the Moonlite focuser on to the OTA. The "universal kit" simply does not have the screw holes in the correct places for my Skywatcher Explorer 150PDS OTA. There is no way whatsoever to fit it square on, unfortunately. I will contact Moonlite about this as there is no way to collimate my telescope with this focuser on this "universal kit".

[Gib007]

I just wanted to update this thread. Yesterday a drill four new holes into my OTA so that the Moonlite focuser mounting plate could fit on there square-on. Once that was done, I performed as precise a collimation as I could and by the end of it, it was looking fantastic as per the alignment of everything. Tonight I went out for a quick test run and captured the Crescent Nebula with 1, 2, 4, 6 and 10 minute exposures. I'm happy to report everything looks fantastic now. The source of my problem was basically that the focuser wasn't sitting square-on, on the OTA. Once that was fixed and I could collimate fully, it was sorted out. Now back to serious astrophotography!