Megawatt

Quick update for those who have been assisting me with this issue: I borrowed an Orion universal field flattener from a friend. The recommended 55mm back spacing was clearly not enough, but that aside, it didn't help much at all. Star shapes are still garbage. The first pic shows a recent sub with the replacement flattener and my newly-repaired ASI2600MC Pro. The second picture is just for comparison. This was shot last year and shows much better, though still far from perfect, star shapes. I'm really not sure what to make of all this. All I can think to do is loosen off my tube rings a bit, and maybe call around to some local places about repair.

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.

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.

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?

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.

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.

I'll try that on the next clear night.

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...

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!