I have these really odd looking triangular star shapes that I once thought were pinched optics, which after freeing up the mirror to the point its almost floating around in its cell, i can confirm if it is pinched optics, it not the mirror clips. I also dont think its collimation as im doing the same thing i usually do. Im starting to suspect it might be the reducer im using. Ill be testing that theory tonight, but in the mean time.. what do you think the image attached looks like? In the lower left you can see each star splitting into 3.
Gear:
Skywatcher Quattro 10"
Starizona Nexus 0.75x Reducer
ASI2600MM
EQ6R Pro
Also, the vignetting cant be from the filters as they are 50mm Antlia's with an OAG-L, so im guessing thats either unavoidable light falloff or maybe the light cone being restricted by the 2" focuser. Any ideas there also appreciated. Id like to eliminate that if altogether if possible.
Im starting to get why most people just get fracs.. in my own experience they are just so much easier to get good results out of.
Hi Andrew-
are your spacings correct between
the coma corrector reducer and your camera sensor?
I had to use this to adjust mine to get it right:
Baader VariLock 29 T2 Extension Tube - 20-29mm Length
as per here: https://www.flickr.com/photos/247194...in/photostream
How are you collimating the Newt?
An f4 mirror is critical for collimation.
I also have an 10" f4 Newt.
I use a laser in the focuser attached to a 2X Barlow and look
for the reflection of the paper reinforcement on the center of the primary
mirror in the 45 degree window -
I adjust the 3 screws to see the shadow of the paper reinforcement there.
That makes it an easy 2 minute job.
Spacing is set to 55mm as per the Starizona specs. From the image though, it looks a little long so have removed a 0.8mm shim from the imaging train to hopefully address that.
Collimation: I use the Infinity Catseye XLK tools along with a TS Optics Concentric eyepiece to centre the Secondary mirror and all looks perfect. To verify i drop a Farpoint laser (also collimated) into the tube to double check, and its always perfect too.
Im going to try a different Coma Corrector tonight and see if it fixes anything or if the triple/triangle stars still persist.
Spacing is set to 55mm as per the Starizona specs. From the image though, it looks a little long so have removed a 0.8mm shim from the imaging train to hopefully address that.
Collimation: I use the Infinity Catseye XLK tools along with a TS Optics Concentric eyepiece to centre the Secondary mirror and all looks perfect. To verify i drop a Farpoint laser (also collimated) into the tube to double check, and its always perfect too.
Im going to try a different Coma Corrector tonight and see if it fixes anything or if the triple/triangle stars still persist.
Wow - it sounds like you really know what you're doing.
If it doesn't come right after tonight I
would be thinking that it's a dud mirror.
That happened to me once and I swapped it for
a more expensive replacement mirror of another brand
which instantly fixed the problem.
You seem to have more than one issue, based on the provided images.
First you should try to figure out why there is such a big difference in focus between the top and the bottom of the image. You seem to have some sort of tilt problem that can be caused by anything in the optical train, including collimation.
The devices that you mentioned can only guarantee collimation on the optical axis, which is good enough for visual use but say nothing about squareness of the focal plane to the optical axis. I'm afraid there are no easy methods for collimating a Newtonian astrograph. In fact that is the main reason I don't make such instruments.
The only thing I can suggest is that you take systematic test images where you rotate various components of the optical train and try to understand the tilt problem. Once you get symmetrical stars in the corners you can start work on tuning the camera spacing.
That sounds like a good approach. I noticed the tilt also today, confirmed by a trial version of CCD Inspector, and i think i might have found the source (hopefully the only one). The ZWO OAG- L came with one of the tilt adapter grub screws slightly extended, so basically pre-tilt adjusted.. I've returned that to a default state, and I'll try again & check that out in CCD Inspector. Just can't bring myself to set up & try again tonight. I don't think it will fix the triangular/3-star issue though as it was present before I started using an OAG.
I've looked at the coma corrector/reducer, and starting to think that might have the pinched optics. I can't loosen the locking ring to release the pressure on the glass elements as it appears to be glued in place also.
I'll need to test a different coma corrector to see how it compares.
I have these really odd looking triangular star shapes that I once thought were pinched optics, which after freeing up the mirror to the point its almost floating around in its cell, i can confirm if it is pinched optics, it not the mirror clips. I also dont think its collimation as im doing the same thing i usually do. Im starting to suspect it might be the reducer im using. Ill be testing that theory tonight, but in the mean time.. what do you think the image attached looks like? In the lower left you can see each star splitting into 3.
Gear:
Skywatcher Quattro 10"
Starizona Nexus 0.75x Reducer
ASI2600MM
EQ6R Pro
Also, the vignetting cant be from the filters as they are 50mm Antlia's with an OAG-L, so im guessing thats either unavoidable light falloff or maybe the light cone being restricted by the 2" focuser. Any ideas there also appreciated. Id like to eliminate that if altogether if possible.
Im starting to get why most people just get fracs.. in my own experience they are just so much easier to get good results out of.
I feel your pain and frustration
The amount of times Ive read folk having trouble with f4 Newts on various forums you could fill a book
I was advised by a guy at Bintel back in 2016 to go f4 for imaging.I didn’t take his advice and went f5 and so glad I did with all my scopes 6” , 8” , 10”and 12”
A snack to collimate and rarely have any optical issues.
If I can offer any advice in regards to a good quality coma corrector for f3.5 to f5 Newts it’s the TS Optics GPU coma corrector ( recommended by Diego from Sidereal Trading )
There not cheap but the results are excellent, absolutely no coma and stars nice and tight , a quality optical instrument ( I used a Baader Mk3 for 3 years and struggled with it )
Unfortunately in your case A-Z trial and error elimination testing of the optical train is the only solution to find the problem.
Good luck with it and I’m sure you will get to the bottom of the issue in time
Thanks Martin for the solidarity. I think I'm onto something after reading this thread from cloudy nights. Looking for a clear night soon to try the approach of rotating the camera & corrector and seeing if the astigmatism moves or stays fixed, indicating if it's the primary mirror or the corrector. My hunch is that it's the corrector. I've had lots of past success with f4 but i take your point, they're a little more finicky but nothing good collimating tools can't handle. Actually want to check out the SkyWatcher 150P f4 (or f3.45 with it's native corrector). 515mm @ f3.45.. yeah ok, I'm interested. Anyway, will update once the next test happens.
I’m in the tilt camp on this one. Where it’s coming from though you’ll have to spend time finding. Mine was in the focuser. F/4s are fussy. It doesn’t take much tilt to have what Stephan said about focus shift across the image. With the weight of an OAG on the small nose of the MPCC I’d be surprised if you could get it to work consistently. Martin is right about the GPU corrector with how much less boated the stars are though it but just as important for you is the length of the corrector. It’s the full length of the draw tube in the focuser making it much easier to centralize and sit true. Depending on how much money you want to spend refining this setup, I’d start with the corrector to help fix the tilt and I’d also heavily advise a focuser upgrade to handle the weight better.
Thanks Ryan, and I agree, going to take some trial and error. Was thinking about the Starizona Nexus reducer that i used (not an MPCC), its a little on the stubby side compared to a Paracorr Type 2, with the Skywatcher F4 coma corrector being the longest of the three. Ill try with the Skywatcher cc and see how that goes, as you, Stefan, Martin are right, all that weight (ASI2600, OAG-L, 7x2" EFW all hanging on the collar of a stubby Starizona Nexus might cause issues, including tilt, and possibly pinching. It could be that weight, could be the 2 thumbscrews ive tightened around the collar (but probably less likely), or could also be a pinched optic within the Starizona Nexus itself. Given the weight though, and the basic focuser that it is on the Quattro 10, i could def see how that might be the weakest link.
