Teardrop Star Issues

[tony_cameraguru]

Equipment: EQ6R-Pro; Explore Scientific 80ED Triplet; ZWO ASI120MC guide camera; Canon 250D imaging camera; OVL field flattener. Software: NINA; PHD2 (multistar).

I cannot solve this issue with the stars in my images. It started some weeks ago, and basically I am not able to image at all. Image 1 shows a 10sec exposure on the star Sirius, with the teardrop shaped stars around. Image 2 shows 120sec exposure on NGC7522, same night, same setup, again teardrop stars. On both images you can see the tracking is lower than 1, and on a windy night. Both images here show the exact same type of distortion to the stars. I did try a 240sec exposure and that was the same. Image 3 shows PHD2 on Sirius, changing any of the settings doesn't work. This was after a new calibration was carried out, which was very good (see image 4).

Any ideas please?

Tony

 

[michael8554]

Hi Tony

Doesn't matter how big or small your Total Guide Error is if RA and Dec figures are very different.

Your PHD2 screenshot shows RA = 0.79arcsecs,  Dec = 0.37arcsecs.

So RA error is twice that of Dec .

That means elongated stars.

The elongation in the NINA shots hopefully suggest that the imaging camera is rotated 90 degrees, so that RA is up/down ?

Also the guide rate is only 1.5arcsec/sec due to Guide Speed being set to only 0.10x, as shown in the Review Calibration shot.

Needs to be at least 7.5arcsec/sec.

Michael

[tony_cameraguru]

Thank you Michael. When I did the tracking calibration RA showed as up and down. How do I adjust the guide rate to get that increase in guide speed Michael?

Tony

[michael8554]

Sorry Tony,

If it's not the setting in the mount menus, then I guess it's in NINA, which I know zero about.

Michael

[ollypenrice]

In my opinion this is not a guiding issue. Look at the elongation of the stars. On the left hand side the elongation points down and to the left. On the right hand side it points down and to the right. In the centre it's vertical.

Elongation due to guiding error produces parallel elongation and doesn't usually show much in short subs. I think this will be an optical or mechanical error, the lens not being optically aligned with the tube or either the focuser sagging or the chip being tilted.  Take a sub, rotate the camera 90 degrees and take another. If the tilt is in the chip it will rotate with the camera. If it's in the optics or the focuser it won't.

Olly

[michael8554]

There is a degree of coma, but centre is elongated too, so primary problem is guiding.

RA error is twice that of Dec, which means elongated stars.

Michael

[inFINNity Deck]

I agree with @ollypenrice that this is not a tracking issue. I have taken Tony's second image (the NINA screendump), cut out the image and extended it upwards. I found this point from which radial lines seem to coincide with all star elongations. The stars near the centre of the upper edge are almost round, no significant elongation there, but those near the bottom are terribly elongated. To me this is an indication that the issue must be sought in the imaging train, just as Olly wrote: something is not perpendicular in it.

Nicolàs

[Stuart1971]

18 hours ago, michael8554 said:

Hi Tony

Doesn't matter how big or small your Total Guide Error is if RA and Dec figures are very different.

Your PHD2 screenshot shows RA = 0.79arcsecs,  Dec = 0.37arcsecs.

So RA error is twice that of Dec .

That means elongated stars.

The elongation in the NINA shots hopefully suggest that the imaging camera is rotated 90 degrees, so that RA is up/down ?

Also the guide rate is only 1.5arcsec/sec due to Guide Speed being set to only 0.10x, as shown in the Review Calibration shot.

Needs to be at least 7.5arcsec/sec.

Michael

These settings don’t mean elongated stars at all, I have settings similar and get perfect round stars, I often get R.A. twice the error of DEC and an RMS of 0.9, and with my imaging scale being 1.72”/pixel, it’s absolutely fine….the seeing will probably limit it to 2”/pixel anyway

My last guided session two days ago, I had 0.18” in DEC and 0.45” in RA, so 0.47 RMS overall, and stars were perfectly round…👍🏼

but the guide speed should be increased as 0.1 is too low, try at least 0.4 or 0.5, 0.5 is the standard and what most would use.

 

Edited March 9, 2022 by Stuart1971

[ollypenrice]

1 hour ago, michael8554 said:

There is a degree of coma, but centre is elongated too, so primary problem is guiding.

RA error is twice that of Dec, which means elongated stars.

Michael

See the post above. Nicolàs has clearly demonstrated a systematic error in the image which bears no relation to a guiding error.  How can a guiding error produce a quite different angle of elongation on different parts of the chip? The entire chip is subjected to exactly the same displacement from the guide star centroid when guiding fails to hold the centroid in position.

Nor do I think it is it coma because coma is an off axis aberration producing fan-like extensions which worsen as we look away from the optical axis. We see nothing like that: the stars are best near the centre of the elongated line, not at the side closest to the optical axis.

As Stuart says, it is perfectly possible to have the full resolution of which the system is capable (which is more demanding than having round stars) with an error of zero on one axis and an error of half the  image scale in "/PP on the other. Provided the error lies below about half the image scale it won't show.  Besides, the elongation is so extreme that, if halved and applied to the other axis, it would surely be visible.

Olly

[Stuart1971]

Is it just purely a field flattener spacing issue, or possibly and optical error in the flattener itself…🤔🤔

[Stuart1971]

16 hours ago, tony_cameraguru said:

Thank you Michael. When I did the tracking calibration RA showed as up and down. How do I adjust the guide rate to get that increase in guide speed Michael?

Tony

You alter the guide rate in the mount driver, so either EQMOD or GSS whichever you use, for EQMOD it’s here…circled in red, BUT you also have to change in PHD2 under advanced option to match whatever to set it to in EQMOD

 

[ollypenrice]

1 minute ago, Stuart1971 said:

Is it just purely a field flattener spacing issue, or possibly and optical error in the flattener itself…🤔🤔

Spacing issues usually worsen away from the optical axis. Like all these things, 'usually' applies to an ideal world rather than this one!   What is certainly possible is that there is more than one origin to the problem, though, so it could be spacing plus collimation.

One test which should certainly be performed here is the classic star test. Place a star precisely in the centre,  set an exposure which does not saturate it, then take a number of images of very short duration in focus and outside and inside focus by differing amounts.  The out of focus images should be round. My money says they won't be. The short exposures eliminate tracking error.

Olly

[inFINNity Deck]

In order to check the collimation of the objective one can place a Cheshire in the focuser, cover the objective with its lens-cap, illuminate the Cheshire and examine the reflected image of the Cheshire. If properly collimated a series of concentric reflections can be seen. If not, the reflections will not be concentric.

Nicolàs

[tony_cameraguru]

Hi Everybody

Firstly thank you all for the contribution's you have made to my Teardrop star issue. Last night the tracking was around 0.7 too 0.9, which I left running for some 20mins and subs were no different as before. I also tried two different field flatteners and the result was the same. I tried varying the back-focus and no change, I even changed the 2in to a 1.25in configuration - no change. I recently had the telescope checked and adjusted for collimation, and it has worked fine up until some weeks ago. During focusing last night I saw that out of focus the stars were perfectly round. I will change the guide speed in Ascom as that is what I use, and change it in PHD2 as well. However no clear night tonight so patience is key now🥵.

Tony

[michael8554]

Nicholas's analysis is compelling.

But it makes no sense to me, if RA is deviating twice as much as Dec, that the stars can be round.

Is DSS (or whatever) only stacking the subs with round stars ?

Michael

[Adam J]

5 hours ago, ollypenrice said:

In my opinion this is not a guiding issue. Look at the elongation of the stars. On the left hand side the elongation points down and to the left. On the right hand side it points down and to the right. In the centre it's vertical.

Elongation due to guiding error produces parallel elongation and doesn't usually show much in short subs. I think this will be an optical or mechanical error, the lens not being optically aligned with the tube or either the focuser sagging or the chip being tilted.  Take a sub, rotate the camera 90 degrees and take another. If the tilt is in the chip it will rotate with the camera. If it's in the optics or the focuser it won't.

Olly

I agree its not a guide issue, if anything it looks like poor colimation to me. 

An out of focus star test showing 3-5 rings will confirm. 

If not the primary lens colimation then the draw tube needs to be checked with a laser to confirm that it is parallel to the optical axis of the scope. 

OP it is simply not possible to produce the distortion you have via a guiding problem. 

Adam

Edited March 9, 2022 by Adam J

[gorann]

I agree with the great majority here. There is no way this can be due to guiding. Hopefully it is something sagging in the imaging train. I had an ES ED80 and the focuser is far from the best and can easily be sagging, but there are some screws that can be tightened which could fix it. Does it all feel stiff if you try to move the camera? Hopefully it is not collimation, since this is a refactor it is not something you can fix yourself. However, bad collimation would probably result in fan-shaped stars as @ollypenrice pointed out, so hopefully you can fix it by tightening up the focuser. Could also be that the sensor inside your DSLR camera is not perpendicular to the optical axis (if that issue you need a new camera).

[Stuart1971]

1 hour ago, michael8554 said:

Nicholas's analysis is compelling.

But it makes no sense to me, if RA is deviating twice as much as Dec, that the stars can be round.

Is DSS (or whatever) only stacking the subs with round stars ?

Michael

I’m afraid this statement does, not make much sense to me either….

”if RA is deviating twice as much as DEC, That the stars can be round”

🤔🤔🤔
like many of us have said, yes they can be, mine deviated nearly three times as much as DEC and my stars were pretty darn decent…….see graph here….not a great picture but you get the idea….👍🏼

BTW the stars you see are from the very edge of my FOV, with no flattener, and an APC-S sensor…

 

 

Edited March 9, 2022 by Stuart1971

[gorann]

I think this was already pointed out but here it is again: I just calculated the pixel scale of his set-up, and it  is 1.61"/pixel, so any guiding error in any direction less that 0.8"/pixel could never be seen in the image. So guiding error is totally exluded here.

[ollypenrice]

1 hour ago, michael8554 said:

Nicholas's analysis is compelling.

But it makes no sense to me, if RA is deviating twice as much as Dec, that the stars can be round.

Is DSS (or whatever) only stacking the subs with round stars ?

Michael

But they can be round, as I explained earlier: in order to elongate a star, the guide error has to be more than half the image scale, or thereabouts. If pixels were dimensionless points this would not be true but pixels have a dimension capable of absorbing a small but significant degree of error. This tolerance diminishes as the image scale increases in resolution, which is why you need better guiding to image at finer pixels scales.

I'd also want to think about the shape of the elongation. It is spindle-shaped, ie pointed at each end and thicker in the middle. In many years of imaging I have never seen guide error produce that shape and I'm struggling to think of a guiding error scenario which could produce it.

The killer indicator, though, is that the elongations are not parallel across the chip. That absolutely rules out tracking error.

Olly

Edit: I 'crossed in the post' with Wim who has put a numerical value of 0.8 arcsecs on the RMS below which guiding will have no effect and with Stuart whose experiences match my own.

Edited March 9, 2022 by ollypenrice

[Adam J]

19 minutes ago, gorann said:

Hopefully it is not collimation, since this is a refactor it is not something you can fix yourself. 

It probably is sag, those ES focusers are not great. But I do believe that the ES scopes are colimatable by a skilled user. Its important not to confuse refractor colimation with centring the lens elements. A refractor can be user colimated through multiple methods, normally either a star test using an artificial star or via a Cheshire colimator and a illumination source when the user would adjust until the reflections from each lens element are concentric. I speak from personal experience.

The lens is held in place by three sets of push pull screws and these are the colimation screws, the ones you should never touch are located around the perimeter of the lens cell and are used to centre the optical axis of each lens element. The are also the ones that typically cause pinched optics. 

Adam

 

 

Edited March 9, 2022 by Adam J

[ollypenrice]

5 minutes ago, Adam J said:

It probably is sag, those ES focusers are not great. But I do believe that the ES scopes are colimatable by a skilled user. Its important not to confuse refractor colimation with centring the lens elements. A refractor can be user colimated through multiple methods, normally either a star test using an artificial star or via a Cheshire colimator and a illumination source when the user would adjust until the reflections from each lens element are concentric. I speak from personal experience.

The lens is held in place by three sets of push pull screws and these are the colimation screws, the ones you should never touch are located around the perimeter of the lens cell and are used to centre the optical axis of each lens element. The are also the ones that typically cause pinched optics. 

Adam

 

 

Yes, collimating the entire lens cell as one (which just means making it point light straight down the tube) is something I've done as well and found it very easy using an illuminated ballbearing as a point source. This method was given to me by a TeleVue optician over the phone, not from uncle Mick on Facebook...

😁lly

 

[gorann]

8 minutes ago, ollypenrice said:

Edit: I 'crossed in the post' with Wim who has put a numerical value of 0.8 arcsecs on the RMS below which guiding will have no effect and with Stuart whose experiences match my own.

You keep mixing up us Swedes Olly😁

[ollypenrice]

3 minutes ago, gorann said:

You keep mixing up us Swedes Olly😁

Sorry Wim!!! 👹

Perhaps I should settle for Wimran or Gorim!  (When I was at school I had two inseparable classmates called Unsworth and Drinkwater.  One teacher found their inseparability intolerable and called both of them Unswater. He took this to an extreme when he wrote only one end-of-year report between them - describing Unswater's progress that year.)

Olly

[Stuart1971]

28 minutes ago, Adam J said:

It probably is sag, those ES focusers are not great. But I do believe that the ES scopes are colimatable by a skilled user. Its important not to confuse refractor colimation with centring the lens elements. A refractor can be user colimated through multiple methods, normally either a star test using an artificial star or via a Cheshire colimator and a illumination source when the user would adjust until the reflections from each lens element are concentric. I speak from personal experience.

The lens is held in place by three sets of push pull screws and these are the colimation screws, the ones you should never touch are located around the perimeter of the lens cell and are used to centre the optical axis of each lens element. The are also the ones that typically cause pinched optics. 

Adam

 

 

deleted..
sorry Adam meant to reply to a different post….😂😂😂

Edited March 9, 2022 by Stuart1971