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Odd star shapes...


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I'm a bit puzzled by the shape of the stars I've recently been getting & what to do to improve them. I'm using a SW200dps & ZWO ASI2600MC-PRO with a 50mm dia converted finder scope/ZWO120MM for guiding, which just sits in the finderscope shoe. I also have a Baader MK III coma corrector.

Last Wednesday night I got 43 x 3min subs on NGC3938 & then a further 15 on the Draco Triplet. Guiding for the most part seemed ok (*), but the stars have small tails which look like tadpoles. In PHD I'm using multi-star guiding, but it's possible that the chosen guide star was outside of the main image frame. The scope has recently been collimated. The shorter 10s subs I used whilst plate-solving look like they don't have the same problem, or at least not to the same extent.

Any suggestions/help much appreciated!

Cheers
Ivor

(*) In the attached log, Section 4 + 5 was on NGC3938, with a pause while I stopped guiding & reselected the star. Section 6 was on the Draco Triplet.

distortion.jpg

ngc3938_v1.jpg

NGC598-1_2_5_v1.jpg

PHD2_GuideLog_2022-06-01_225109.txt

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The tadpoles look a bit like tube currents inside the tube when the scope is not cooled to ambient, but in that case the tadpole looking effect would be to the same direction for all stars but in your image they are sort of not. Some stars have the tadpole going the other way which is a bit confusing and so maybe not tube currents, but that's my guess anyway.

29 minutes ago, Aramcheck said:

The scope has recently been collimated.

From personal experience: Unless recently means before each imaging session, one should expect that the scope is out of collimation even if ever so slightly. Especially true if you transport the scope at all, or even take it off the mount and put it away (so in all cases other than permanent observatory).

45 minutes ago, Aramcheck said:

distortion.jpg

The cross looking stars in your bottom right here look like miscollimation for sure. Maybe collimate the scope again, preferably through the coma corrector with a laser and check that it really is collimated?

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Guiding in session 4 was poor, 5 and 6  was good.

Your imaging image scale is 0.78arcsec/pixel.

Guiding on the last two sessions (5 and 6) had errors below that value:    (0.5, 0.36 arcsecs,   and   0.36, 0.32 arcsecs)

So images from the second part of the NGC3938 run,  and the whole of the Triplet run, should be good.

That means any star elongation is probably due to Differential Flexture, not uncommon with wonky converted finders.

In Session 4  RA didn't seem to respond to PHD2 commands.

One period had an RA error of 2arcsec, and PHD2 was unable to correct it between dithers.

Dec had a number of 10arcsec excursions, due to external sources, cable snags or wind ?

RA was 1.53, Dec was 0.60 arcsecs, which would have caused visible RA elongation in the first NGC 3938 images.

So examine the subs from the 3 sessions and decide the causes of the elongations.

Michael

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Posted (edited)
19 hours ago, Aramcheck said:

The scope has recently been collimated

Hi

Flex is very common on low end reflectors.

To be able to analyse the guiding and star shapes, we'd recommend getting the mechanical integrity of the telescope as good as possible.

We've not yet witnessed an out of the box pds which holds collimation.

To guide a 1000mm focus reflector, a modified finder telescope in the supplied shoe will almost certainly be introducing avoidable error.

For collimation to hold at all tube angles (not just the one at which you performed the collimation) the tube needs stiffening: spread the tube rings on a 50cm Losmandy dovetail. The top of the rings then need joining with a rigid metal profile and the guide telescope bolted directly to it.

The SW primary mirror springs need replacing with six 1.6mm wire versions: three replacements and the other three as passive versions over the locking screws. Leave the latter loose.

To prevent lateral movement of the primary, it needs to sit in the cell on neutral silicone sealant to coincide with the SW cork. Lose the mirror clips in the process.

Even better, both the modifications and an oag. You may even get away with just the latter.

Cheers and HTH 

 

 

 

Edited by alacant
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Posted (edited)

Thanks for the comments/suggestions. @ONIKKINEN The scope is kept in a storage area which is near ambient & is only moved a small distance. I re-checked collimation yesterday but I don't have a collimated laser, so could only do so with a cheshire (& obviously without the coma corrector). I took a few pictures with the scope at different orientations, roughly mimicking the positions during the last imaging session, but there could be some focuser slop once the 2600 camera is attached. Why should tube currents create the elongation in one direction?

@michael8554 The session 4 guiding had a four big blips in RA whilst imaging & a drift in Dec at one point. The figures reported in the PHD2 log are calculated on the whole period with these outliers & if they're removed from the calc then the RA/Dec rms is approx 0.44 / 0.41 arcsecs, respectively. Between session 4 & 5 there was only a 9 second gap, when I re-selected the main guide-star, so there shouldn't have been a big difference, aside from the 'blips'. Thanks for the tip on the RMS vs imaging scale & flexure!

@alacant Thanks - I know you're a strong advocate of replacing the mirror clips with neutral silicone sealant, but I've not been able to find any video or pictures of the process (other than the completed result). The only video I've found of someone removing a SW primary:- https://youtu.be/7fIkC9GukrM?t=1073 has the cork attached to a paper disc. I think mine has the cork attached directly to the metal ring. Do you put the put the sealant onto/around the cork & at the outer edge? One old thread advocated using a coin as a spacer, but presumably if the Cork is left in situ then there is no need to do so? (Also should the sealant be Low or High modulus?)

I'll have a look into the Losmandy & a second bar on the top for additional rigidity. I presume it's the longer length that helps? Also I think I need to either go down the OAG route or a top-mounted guider. The converted finderscope was all I could afford originally. Odd that only this dataset seems to have the problem though. With Pixinsight I've blinked through a few, including all the recent sessions since the last collimation, and it's only this one that is showing this particular issue. On this session I did have the camera at a different angle compared to what I usually do & I suspect that the guidescope wasn't aligned properly with the main scope.

Thanks again for the help!

Ivor

 

cheshire_test.jpg

guide.xlsx

Edited by Aramcheck
Low/High modulus query...
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1 hour ago, Aramcheck said:

Thanks for the comments/suggestions. @ONIKKINEN The scope is kept in a storage area which is near ambient & is only moved a small distance. I re-checked collimation yesterday but I don't have a collimated laser, so could only do so with a cheshire (& obviously without the coma corrector). I took a few pictures with the scope at different orientations, roughly mimicking the positions during the last imaging session, but there could be some focuser slop once the 2600 camera is attached. Why should tube currents create the elongation in one direction?

currents2.PNG

Warm air rises and exits from the top of an open telescope such as a newtonian so there is a preferred way for the smearing rather than a random motion.

I have this issue almost every time since i dont keep the scope in an ambient location. I delete most of the worst ones but i found an example of what it looks like when there are still a bit of tube currents left. Far from the worst but its still somewhat clear.

tubecurrent1.JPG.165567fafab88e21cdd5ef58e62a762f.JPGtubecurrent2.JPG.ed5aa2ba819d364b459e9059052294d6.JPG

First image is from early in the night and second is later when the currents have settled. Notice the bottom part of the star is smearing outwards and that would be the axis that coincides with the up-down axis of the scope at that situation.

My coma corrector, a Maxfield 0.95 one, introduces tilt/collimation issues of its own so i found that i needed to collimate through the coma corrector and in the orientation that i would want to image in, so a different collimation for 0 degree and 90 degree camera orientations for example, and different for no coma corrector at all. Dont know how common this is but id say there's a good chance your MPCC has something similar. You cant know for sure unless you have a way to collimate with the corrector in place which i dont think can be done without a laser as the image is all out of focus visually.

Tilt of the focuser is very possible, id say almost likely. But that is apart of the umbrella term that is collimation, if you cant have a reproducible camera/collimation tool position you just cant have reproducible collimation. The scope is collimated when the primary mirror points to the center of the camera and all 4 corners are on the focal plane, so if the focuser is sloppy the scope is just out of collimation. Try to tape a weight onto your cheshire and see if the collimation holds. If it doesn't, it definitely wont with the camera either.

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Thanks @ONIKKINEN , now I understand why some folks have fans attached to their primary! I'll have a go with hanging a weight onto the cheshire (or my poorly collimated cheap laser) - good suggestion!

Cheers
Ivor

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On 04/06/2022 at 15:13, alacant said:

The SW primary mirror springs need replacing with six 1.6mm wire versions: three replacements and the other three as passive versions over the locking screws. Leave the latter loose.

 

 

 

Alacant, do you have details on the springs needed, strength, length etc? 

Thanks

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5 hours ago, edarter said:

springs

This company. The same length and diameter as your existing springs, but in 1.6mm wire.

Place your order dimensions at the checkout.

HTH

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Posted (edited)
On 05/06/2022 at 13:33, Aramcheck said:

sealant onto/around the cork

The 200pds mercifully does not have a cardboard insert. Instead, it has cork pads placed to coincide with the circumference of the mirror; unfortunately, far from the optimal support points:(

Wherever you see cork, put a blob of silicone to extend over the cork with sufficient to spread to either side of the cork and up the side of the cell. Do not push the mirror into the cell; leave it to settle under gravity on a level surface. Keep a blob of silicone separately but under the same conditions to guage when it has polymerised to the extent  that it returns to its original shape when deformed. Then refit to the tube.

HTH

Edited by alacant
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