This is a bit of a curiosity but I just wondered what the guys with PME's have to say about this issue.
Doing 20 minute subs has led to the discovery that I am getting some field rotation in the deepest parts of the corners on my images. 15 minutes subs are fine but most 20 minute subs show some slight degree of field rotation.
I am according to the Tpoint manual set for around the refracted pole and that is around +72" for my location for the ME.
So is it better to be at the pole? I would have thought so given I am getting some field rotation. I have been doing 20 minute subs for a while; long before I sorted the issues with the pointing, but the PA must have been better.
Just a thought Paul but if the OTA wasnt totally orthoganal to the axis
could it cause this ? I have heard of people having to shim the OTA
a tiny amount to correct for this.
Just a thought Paul but if the OTA wasnt totally orthoganal to the axis
could it cause this ? I have heard of people having to shim the OTA
a tiny amount to correct for this.
Mark
Hmmm actually I don't know. I suppose that could be a factor, but I had not considered this before. I might have to contact QSI. Given that I get good stars at 15 minutes though I just naturally assumed it was PA. Hmmm now that is something to think about.
It would be useful to post an image so that we can ascertain whether it is in fact field rotation, or another issue.
Field rotation over that length of time is most definitely a result of less than perfect polar alignment. Less likely, but theoretically possible, is flexure over a long enough time, and in a direction that mimics the effects of poor polar alignment induced field rotation.
A non-orthogonal optic won't cause field rotation. It's the polar axis - pure and simple - that needs to be aligned with the pole.
Hi Paul, Paul and Mark,
Just out of curiosity.... what would one be looking for in the pictures ( that Paul H has posted) in regards to field rotation?
I am sorry that I have nothing to contribute to fix the problem.
I just want to learn what field rotation looks like.....I cant see any distortion on the main pics, but on the zoomed in pics, I see some just slightly egg shape stars or out of focus stars.
To be honest the aberrations are not worth mentioning.....they are beautiful pics!
I understand what field rotation is and does ( google is thy friend), but cant see it in your pics Paul....to that extent........
I guess when I get () to your level I will look back at this post and cringe.....
Bartman
I really wonder if your problem is field rotation or just registration problems due to field curvature. I know the TSA is well colour corrected but it dose have issues with curvature. I have also found there can be small changes in focus between subs which can make itself evident in the registration of the 3 colours and this would also show up closer in the corners during longer exposures as the stars start to bloat a little with the extended exposure time.
Overall the issues you show here aren't to obvious unless you do a Fred style zoom in.
It is very hard to differentiate between field curvature and field rotation in your shots. To check for minute field rotation the best way is to shoot wide field in Ha so you get tiny stars and pick a guide star right in the corner of your frame and look at the opposing corner. On a 20min subs you're bound to see any rotation for sure. Mike BJ (sculptor) has done the maths on field rotation and he has developed a technique of misaligning the scope by a couple of arc seconds from the pole in a line that is perpendicular to the DEC axis of your scope. He routinely does 3h subs with pinpoint stars in narrowband. I'll have to dig the old email he sent me at the time and post it when I find it.
Just out of curiosity.... what would one be looking for in the pictures ( that Paul H has posted) in regards to field rotation?
What you look for with field rotation is that each corner has stars that are curve in a radius eminating from the center. In this case I have provide just two corners on the left side but each corner has the problem. This small amount is not much but it is there and the problem is that I know it is there.
Quote:
Originally Posted by Hagar
I really wonder if your problem is field rotation or just registration problems due to field curvature. Just a thought.
I don't think it is Doug, well not entirely. There might well be some registration problems, but I don't think these are caused by field curvature. Curvature has that look where stars are pointed toward the center of the image. These are perpendicular and curved from that line. There might be some registration issue worth looking at though.
I noticed that when I put the reducer on this scope this problem became more evident. The reducer may be causing some distortion.
Quote:
Originally Posted by multiweb
To check for minute field rotation the best way is to shoot wide field in Ha so you get tiny stars and pick a guide star right in the corner of your frame and look at the opposing corner. On a 20min subs you're bound to see any rotation for sure.
I will have to give this a shot to know for certain. Like I said, I don't think it is field curvature as the stars don't really match that pattern. However, shooting longs subs in Ha should give me a definitive answer.
Just to add to this one last time, you need to rule out the unknowns. With your RGB composites there is too much going on to be certain. Take 10, 20 ,30, etc minute exposures through clear (or H-a as has been suggested) in order to ascertain whether this is in fact field rotation. Longer trails should give you a better picture of what's going on. If you don't get longer trails, or if they don't appear as you would expect field rotation trails to appear , then the problem lays elsewhere.
At the very least, rule out any effects from field curvature or RGB compositing.
On another note, if you have a large and well modelled TPoint sample, use Protrack. It works extremely well for long exposure tracking by counteracting the effects of flexure over time. Tests we've performed over the years at D21 and D22 at long focal lengths have shown that (provided you have a good model) Protrack works very well indeed.
Good suggestions Paul. My model is only 180 odd points but I need to do a larger model in the moon lit part of the cycle. I will try for 300 then and apply protrack.
One othe thing to note that unless you're guiding around a star in the centre of the field (which would be difficult with my experience of OAG) then the field rotation shouldn't be symmetrical around the centre of your field.
From my experience the field rotation is centred on the guide star.
One othe thing to note that unless you're guiding around a star in the centre of the field (which would be difficult with my experience of OAG) then the field rotation shouldn't be symmetrical around the centre of your field.
From my experience the field rotation is centred on the guide star.
Cheers
Stuart
Yeah that is true. This rotation is nearly symmetrical. Lums taken on the Eastern side of the meridian have this problem more than on the Western side.
My model is only 180 odd points but I need to do a larger model in the moon lit part of the cycle. I will try for 300 then and apply protrack.
In my experience 180 points all over the sky is actually more than sufficient for ProTrack to work well so long as you're using TheSkyX's supermodel feature to create your model.
In my experience 180 points all over the sky is actually more than sufficient for ProTrack to work well so long as you're using TheSkyX's supermodel feature to create your model.
Hi Ernie. I only have the sky6 at present and don't want to pay for the upgrade to skyx.
Yeah that is true. This rotation is nearly symmetrical. Lums taken on the Eastern side of the meridian have this problem more than on the Western side.
OK, so that points to a balance problem more than anything.
As the camera/OAG will not be balanced around the axis of rotation, do you run an instrument rotator?
Could be minor flexing of the imaging train, rotationally. Try with the OAG vertical, so it's not placing any torque on the rotator/nosepiece etc.
Additionally, you should notice field rotation between subs, no matter the length, I routinely see this as my Polar Alignment is rarely exceptional (imaging stuff just a bit too heavy for the G11). This time you would have to single star align your subs, then if there's any field rotation then it will be around the alignment star. Alternatively, once the subs are registered, flick through them, the edges of the frame will rotate.
Finding the causes of this stuff can be painstaking, but you have to be barking up the right tree to find the problem.
OK, so that points to a balance problem more than anything.
I don't think this is really the problem. I have ensured that the PME is well balanced. I am yet to find a part of the sky where the mount moves. Having said that though I will check just to be on the safe side.
Quote:
Originally Posted by rat156
As the camera/OAG will not be balanced around the axis of rotation, do you run an instrument rotator?
The OAG is part of the camera and the camera is not that heavy. I don't have a electronic rotator but I do have the Tak one on board. Worth a try at least.
Quote:
Originally Posted by rat156
Could be minor flexing of the imaging train, rotationally. Try with the OAG vertical, so it's not placing any torque on the rotator/nosepiece etc.
It could be I suppose, especially since I only get this infrequently, then flexure is the most likely cause, however small that might be.
Quote:
Originally Posted by rat156
Additionally, you should notice field rotation between subs, no matter the length.
Just checked that and no rotation seen. The stars stay in exactly the same position over 4 hours. It has raised another idea though.
Quote:
Originally Posted by rat156
Finding the causes of this stuff can be painstaking, but you have to be barking up the right tree to find the problem.
Cheers
Stuart
Yes that is the truth. This might well be a focus related issue. The image that is worse has some subs near the end of the run that are less than sharp and that could be part of the problem too. It is not the spacing of the reducer but if the focus just goes out a little this type of aberation can occur from what I know.
I don't think this is really the problem. I have ensured that the PME is well balanced. I am yet to find a part of the sky where the mount moves. Having said that though I will check just to be on the safe side.
The OAG is part of the camera and the camera is not that heavy. I don't have a electronic rotator but I do have the Tak one on board. Worth a try at least.
I meant that the camera is not balanced. I use a MOAG and Sbig guide camera, there is considerable torque placed on the rotator by this when it's not vertical.
No rotation between subs means that your PA is fine (congrats, I find this one of the hardest things in imaging).
Could still be the Tak rotator flexing, but it's sounding less likely, focus issues are now the leading contender.
Just with the camera, the QSI is pretty small. It is the size of CD is diameter and about 100mm thick I suppose. Probably one of the lightest CCDs on the market with a filter wheel. That's why I thought it was not a problem.
) to your level I will look back at this post and cringe.....
Lums taken on the Eastern side of the meridian have this problem more than on the Western side.
