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Poor star shapes with William Optics triplet


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Hi all, first post here. 

I'm going to show you a couple of images with poor star shapes. I was going to write a long preamble but decided to just get opinions on the stars themselves without all the backstory.

These were taken with a William Optics GT81 (older model) and 0.8x reducer/flattener (non-adjustable). Cameras used are ASI2600MC Pro and Canon T5i. The stars are sort of almond-shaped but asymmetrical, with little spikes on two sides. 

The star shapes are not affected at all by exposure time. 3", 30", 300" yields about the same results, so it is not a tracking/guiding issue. Removing the flattener seemed to have a bigger impact, but I still see the little spikes. I suspect pinched optics but my stars don't look like other examples I've seen online. Maybe I'm not good at troubleshooting, but I've been struggling with this for years and feel about ready to give up on this thing. I don't believe decent-looking stars should be so hard to achieve. 

The first image is a recent sub with the 2600MC. Second is an old sub with the T5i. Third is with the 2600MC without the flattener. Mount is an HEQ5 Pro with a StarShoot autoguider and 50mm guidescope. 

Visually, the scope performs well and star tests look okay, but dodgy seeing makes it hard to get a really good look.

Any help would be appreciated!

 

star sample saturday west.png

Double cluster_LIGHT_60s_1600iso_+21c_20200911-23h55m12s424ms.jpg

Light_30 second test_30sec_Bin1_-5.4C_gain100_2024-03-07_203121_frame0001.png

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The spacing needs a little tweak, from initial glance the sensor looks a little fraction too far away from the flattener.

Also, as it's a triplet do you allow it to cool sufficiently long enough, or if using dew heater bands are they near the triplet cell position as lenses not acclimatised can also cause pinched optics.

I assume your guide calibration, autoguiding and PA are all fine.

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51 minutes ago, Elp said:

The spacing needs a little tweak, from initial glance the sensor looks a little fraction too far away from the flattener.

Also, as it's a triplet do you allow it to cool sufficiently long enough, or if using dew heater bands are they near the triplet cell position as lenses not acclimatised can also cause pinched optics.

I assume your guide calibration, autoguiding and PA are all fine.

Thanks for the reply. I've lost track of how many times I've tweaked the back spacing. I think my inability to get round stars in the centre of the frame makes it much harder to tell where I'm at with the spacing. 

I think if I had issues with guiding or polar alignment, I would notice my issues getting worse over longer exposures but this is not the case. 

For comparison, below is one of the few images I took with my old ED100. Unguided with no flattener, and the stars are most round. With a flattener and good guiding, the stars would be perfect. This is what I expected from my triplet. I just feel like I'm going in circles with this scope. 

I also attached another recent image where the stars in the centre of the frame are spiky-looking. Again, makes me think pinched optics. I normally use a dew heater around the objective but in this case I forgot it. 

I wonder if the flattener might be the culprit. Maybe I can borrow a replacement somewhere and find out...

NGC6503.jpg

M97 Waterloo.png

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I think you can also defocus a very bright star so you can see the airy disk and take images either side of focus, this may help diagnose.

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5 minutes ago, Elp said:

I think you can also defocus a very bright star so you can see the airy disk and take images either side of focus, this may help diagnose.

I'll try that on the next clear night.

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8 hours ago, Megawatt said:

Thanks for the reply. I've lost track of how many times I've tweaked the back spacing. I think my inability to get round stars in the centre of the frame makes it much harder to tell where I'm at with the spacing. 

I think if I had issues with guiding or polar alignment, I would notice my issues getting worse over longer exposures but this is not the case. 

For comparison, below is one of the few images I took with my old ED100. Unguided with no flattener, and the stars are most round. With a flattener and good guiding, the stars would be perfect. This is what I expected from my triplet. I just feel like I'm going in circles with this scope. 

I also attached another recent image where the stars in the centre of the frame are spiky-looking. Again, makes me think pinched optics. I normally use a dew heater around the objective but in this case I forgot it. 

I wonder if the flattener might be the culprit. Maybe I can borrow a replacement somewhere and find out...

NGC6503.jpg

M97 Waterloo.png

If the stars are problematic in the centre of the image, then it’s not really the flattener spacing, unless you are miles out, and I don’t think you are.

In the first image there seems to be a little tilt which would contribute to your star shapes, and in the T5i image there is some astigmatism in the bottom right corner, as you can see the elongated star with a line through, almost like a bloated cross…but again tilt could magnify this issue…

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Have you tried without the reducer? You will have coma around the edges but should all point towards the centre and stars should be round in centre. If they are anything but round then the issue is with the scope itself.

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You could try pointing close to the zenith focusing a star at the centre of the frame then do the same in all four corners. Assuming you have a remote stepper focuser make notes of the focuser position count for each, lets say it's 400 in the centre...

If all corners are the same higher or lower number it's backfocus...

With varying higher numbers in each corner it's a combination of backfocus & tilt, same applies to all lower numbers...

If when following the routine above you can't get round stars by adjusting focus in each corner it's likely you have an opitical issue!

I suffered a strange problem with a dual imaging setup, first exposure on a new target on one scope would sometimes have elongated stars... recently found the scopes threaded front lens cell was slightly loose!

 

Peter

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On 12/05/2024 at 12:26, david_taurus83 said:

Have you tried without the reducer? You will have coma around the edges but should all point towards the centre and stars should be round in centre. If they are anything but round then the issue is with the scope itself.

I did some testing without the reducer and it seemed to improve, but then my shots with the flattener were also not bad that night. The problem is inconsistent, which doesn't make things any easier. 

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On 12/05/2024 at 12:44, Earl said:

is something overtightened somewhere?

I think so. Now it's just a matter of figuring which "something" and "somewhere". I think there are 6 sets of 3 screws around the lens cell, and I'm hesitant to start mucking about with them without guidance.

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15 hours ago, whipdry said:

You could try pointing close to the zenith focusing a star at the centre of the frame then do the same in all four corners. Assuming you have a remote stepper focuser make notes of the focuser position count for each, lets say it's 400 in the centre...

If all corners are the same higher or lower number it's backfocus...

With varying higher numbers in each corner it's a combination of backfocus & tilt, same applies to all lower numbers...

If when following the routine above you can't get round stars by adjusting focus in each corner it's likely you have an opitical issue!

I suffered a strange problem with a dual imaging setup, first exposure on a new target on one scope would sometimes have elongated stars... recently found the scopes threaded front lens cell was slightly loose!

 

Peter

I use a manual focuser. But interesting point. I don't doubt that I've got some back spacing and/or tilt issues, but I believe they are making the real problem harder to diagnose, and vice versa. 

I sometimes take pictures with my Dob and that can give bad stars, but it is perfectly explicable: poor focus, or bad tracking, or coma from the F/5 mirror. I have little doubt that if my Dob was mounted an an EQ8 or whatever, I would have no issues with the stars. 

With my refractor, the problem is inexplicable to me. Focus, tracking, and polar alignment are good, as far as I can tell. The star shapes aren't just elongated, as you would expect from coma or bad tracking. They are asymmetrical and slightly pointed, like flying saucers, and they stay consistent between different exposures but not necessarily night to night. 

How did you figure out your optical issue?

 

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Looking again at the first 3 images you posted, I think it’s a bad flattener, as the last of the 3 images was without this flattener and the centre stars look very good, unlike the other images, yes the outside are elongated towards the middle but that would be expected, I would try another flattener iff possible 

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12 hours ago, Megawatt said:

I think so. Now it's just a matter of figuring which "something" and "somewhere". I think there are 6 sets of 3 screws around the lens cell, and I'm hesitant to start mucking about with them without guidance.

that would be pintched optics and look differnt, Im refering to everthing past the focuser of the scope. Sorry for not specifiying in better detail originally

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17 hours ago, Stuart1971 said:

Looking again at the first 3 images you posted, I think it’s a bad flattener, as the last of the 3 images was without this flattener and the centre stars look very good, unlike the other images, yes the outside are elongated towards the middle but that would be expected, I would try another flattener iff possible 

You're right, the image without the flattener looks a lot more normal. Would be nice if the flattener is the culprit. The only thing which makes me question this theory is this shot taken the same night with the flattener in place, which also has mostly normal-looking stars (though the corners are still quite messy).

I don't often use filters, though I did in this case. I'll try to borrow a different flattener somewhere and see if it helps at all. 

Light_flaming star_180sec_Bin1_-4.9C_gain300_2024-03-07_224830_frame0006.jpg

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6 hours ago, Megawatt said:

You're right, the image without the flattener looks a lot more normal. Would be nice if the flattener is the culprit. The only thing which makes me question this theory is this shot taken the same night with the flattener in place, which also has mostly normal-looking stars (though the corners are still quite messy).

I don't often use filters, though I did in this case. I'll try to borrow a different flattener somewhere and see if it helps at all. 

Light_flaming star_180sec_Bin1_-4.9C_gain300_2024-03-07_224830_frame0006.jpg

Hmmm, yes this image is a lot better, in fact better all round really, so this tells me then that the culprit is something that can be altered between images and nights, while using the exact same kit, so for me that is focus and pinching of the optics, the stars on some pictures do look pinched as they are all across the images, this image all the stars are the same, and slightly misshapen,  and again pinching slightly could cause this, now you can get this from pinched main optics, OR pinched optics of the flattener…which may be the case…so a dew band around the flattener is worth a try next time out.

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Triplets are much more prone to curvature then doublets, I agree with @Stuart1971 if the stars are not round in the centre of the image it minuses the issue of spacing regarding the flattener.

Did you need to have a dew band on while using the scope while imaging? what was the temperature outside?

It does look like a case of pinched optics. Another thing to check which is often ignored, is the retaining ring in the flattener , the one which rests on the optics. Is it screwed on ok, not loose or anything? 

You could just try imaging without the flattener to see how the centre stars look.

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13 minutes ago, Backyardscope said:

Triplets are much more prone to curvature then doublets, I agree with @Stuart1971 if the stars are not round in the centre of the image it minuses the issue of spacing regarding the flattener.

Did you need to have a dew band on while using the scope while imaging? what was the temperature outside?

It does look like a case of pinched optics. Another thing to check which is often ignored, is the retaining ring in the flattener , the one which rests on the optics. Is it screwed on ok, not loose or anything? 

You could just try imaging without the flattener to see how the centre stars look.

He has posted this image at the start of the thread without the flattener, and the centre stars do look ok, although it’s a bit too compressed to see to well…

image.png

Edited by Stuart1971
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Apologies, yes looking at the image on a big screen, the stars look ok'ish, on the brightest stars you see the reflection of the spacers. If you zoom into the brightest star, you can actually the star being round and there is a ever slight pinching.

Edited by Backyardscope
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9 hours ago, Backyardscope said:

Triplets are much more prone to curvature then doublets, I agree with @Stuart1971 if the stars are not round in the centre of the image it minuses the issue of spacing regarding the flattener.

Did you need to have a dew band on while using the scope while imaging? what was the temperature outside?

It does look like a case of pinched optics. Another thing to check which is often ignored, is the retaining ring in the flattener , the one which rests on the optics. Is it screwed on ok, not loose or anything? 

You could just try imaging without the flattener to see how the centre stars look.

First I just want to say thanks to you guys for helping me out with this issue. Makes me feel a little less crazy when people actually attempt to understand the problem before tossing out solutions.

It seems like there is a consensus that some pinching is happening somewhere. I always use the dew heater since 9 out of 10 nights, dew or frost will form at some point. Though last time, I forgot to bring it and I didn't have issues with condensation, but the stars looked horrible as usual. 

The very best I've been able to manage with this setup is "ok-ish" stars, as you put it. And for what I paid, it is not acceptable. But I bought it secondhand so there's no dealer I can appeal to. I'll start saving for a new scope if I have to, but better if I can get this one working properly. Incidentally, it works wonderfully for visual use, which also makes me wonder about the flattener...

I see the retaining ring you mentioned but I haven't figured out how to loosen/tighten it without risking scratches on the lens. Likewise, I have no idea how to address pinching if it the telescope objective. I've seen others loosen the grub screws on the lens cell of their William Optic doublets, but I'm pretty hesitant to start messing with my triplet until I know exactly what I'm doing. 

I'll do more testing without the flattener and see if I can get consistent results. 

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Ideally before you invest in another telescope. All is not lost and certainly cheaper then buying another.

You need to find a astro retailer/optical engineer who has an optical bench, they would have the expertise  to correct the pinching on the lens and correct any astigimitism which maybe apparent. In newtonians this fix you can do yourself, unless there is something physically wrong with mirrors.

Re the flattener, I always try to obtain the matching reducer/flattener for the scope.

When removing the retaining ring of the flattener there is a slot on each side of the ring. You need something like a metal plate which rests inside both slits (but clear of touching the optical elements) of the ring and twist, but remember the lenses inside the flattener fit inside a certain orientation, so take care when assembling or disassembling if you take this route?

I would tackle fixing the scope 1st before dealing with the flattener issue.

Typically Far Eastern telescopes are manufactured in assembly lines where temperatures are usually warmer then colder climates which when they are used. The screws inside the lenses are usually screwed in tighter. They may check the lenses on some sort of optical jig, as part of QC.

Funny enough, the only manufacturer I have experienced which I have not experienced pinched optics is Sky-Watcher. I am sure there are some examples, but only small percentage. 

Once the telescope is corrected, try without flattener again. Then deal with the latter.

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