Schmidt-Newtonian Flared Stars

Hi Jay,
The Meade Schmidt Newtonian has a spherical primary and a corrector placed near focus. As such there will be the field aberration coma, that will flair off axis stars, even in the absence of other mechanical issues like a pinched mirror.

Here is a reference https://www.telescope-optics.net/SN.htm https://www.telescope-optics.net/Mak-Newton.htm#approximated

Regards,
Dave

I got pretty good stars from my former 8" Schmidt-Newtonian until I mistakenly took the corrector plate off to clean it. Even though I was careful to reassemble it in the exact same orientation, I thereafter had flared stars. Turns out it's devilishly hard to retighten the screws that hold the corrector plate in place so that pressure on the corrector plate is completely even. Just the slightest over- or under-tightening of the corrector place screws would introduce pinches in the optics. I spent a lot of time trying to re-adjust but I never got it exactly right.

I'm afraid I have to concur with the previous poster. Despite the heavy processing it has all the hallmarks of field coma. Fact is that a SN isn't that much better than a traditional newtonian in terms of field correction (about 1/2 of a similarly sized newtonian). It was merely devised in order to use easy to make good spherical mirrors so you would still need a coma corrector for any wide field imagery. Collimation must be done properly, which is in focus and possibly at high magnification, after a first pass using the traditional out-of-focus techniques. Needless to say, it is best done the old fashioned way; with an EP and Mk.1 eyeballs.

Keith Egger:
I used a coma corrector with my SN8 - a Baader MPCC - although the MPCC had an odd reflection that appeared in my flats and over-corrected the image so I’m not sure I’d recommend that one.

It is got to be a coma corrector (and flattener) designed specifically for the SN, not for Newtonians.

andrea tasselli:

Keith Egger:
I used a coma corrector with my SN8 - a Baader MPCC - although the MPCC had an odd reflection that appeared in my flats and over-corrected the image so I’m not sure I’d recommend that one.

It is got to be a coma corrector (and flattener) designed specifically for the SN, not for Newtonians.

When I had one there was no coma corrector designed for the SN as far as I know. The MPCC worked well for the SN except for the reflection in my flats - the stars were round across the chip of my Atik 383L+ at least.

Thank you @hbastro , @Keith Egger and @andrea tasselli .  I'm sure that in some combination of your advice / suggestions lies the reality.  It is also becoming increasingly clear (unlike the skies) that there is no easy fix - like many of the issues we deal with ....

Last night, I was imaged (Test #3) M101 with the SN10 and was able to WBPP 51x120s Light Frames.  The stars were as good as I have been able to get ... but the reality is:  even at its best (or at least as well as I've been able to tune it), my FRA600 is able to capture better detail. 

I knew going in that this investment (albeit comparatively minor) had a higher risk / lower reward factor.  I was just looking for a scope in the 1000mm range ... and the amount of photons the SN10 captures is impressive.  It's probably time to invest in another refractor ....

Again, THANK YOU ALL,

Jay

Hi, all. I came across this thread while desperately searching for some help with my SN8. I'll try to keep the backstory short. I started with an SN8 a few years ago. The scope seemed to perform fine. Somewhere along the way, I started getting stars like the ones in the image below. I tried everything to fix it. After many failures, I was forced to scrap it entirely as there seemed to be nothing I could do. Maybe somewhat in error, I've always loved this scope. I'll spare you my list of reasons and just say that I ended up buying another SN8 immediately. That was in December. All was fine until this past week.
I've often struggled with collimation. I suspect part of that was operating with a scope that had additional problems. I've loved the new scope so far. I've always wanted to get a shot of the Apollo 11 site with all 3 craters named after the astronauts. The last scope wasn't up to the task. Already resigned to the idea that I would only be able to get the larger Armstrong crater, I finally tried with this one last week. While I was able to get the crater, it just wasn't a good photo. I went back out determined to get collimation down. Finally satisfied (mostly), I turned my scope to the moon. I knew immediately the improvement as, for the first time, I could see the Armstrong crater in the live view without stacking. After stacking, I was excited to see I had all three of the craters. I finally had an Astrobin worthy image!
And that's where the fun stops. The day prior to my failed lunar shot, I imaged Saturn and got what I thought was a pretty good image. It wasn't until the next night with the failed lunar shot that I realized there was a problem. Having fixed that problem, I wanted to try Saturn again. However, it was next to a mostly full moon. So, I decided to wait. This is where it gets weird. I got a fantastic image of the Apollo 11 site on Tuesday night. When I was done, I parked the scope and covered it. That was all I did. I touched absolutely nothing else. On Thursday night, I went back out and uncovered the scope. When I turned on my laptop, I was horrified to see these stars on my screen. They're the exact same stars that caused me to scrap the last SN8. I can't afford to replace this. I don't know what to do. I'm sickened. This scope literally didn't move since the Apollo 11 image. It doesn't make sense and I can't fathom what would cause it. I'm at a complete loss.

Wow ~  I see my last entry here was well over a year ago, and the frustration has well subsided.  I don’t know when it’ll be, but I’m not entirely ready to abandon this project. With my OCAL collimating gizmo, I was able to align the mirrors quickly, so another night in the driveway is calling.  We’ll see ….

Hi Joe, I have a 6" SN and love it. Here are the things I would look at in order:

Pinched optics - Take a look at a star with high magnification both in and out of focus. The airy disk should be round and the rings of light inside and outside of focus should be to. If it passes this test you do not have pinched optics.

Collimation - The SN design seems to be very sensitive in terms of collimation. Don't use a laser unless you are very sure it's properly adjusted. It would be better to again, use a star image at high magnification as your guide here.

Light path intrusion - Anything that enters the light path will cause diffraction spikes or flairs as you call them. Take a good hard look at this. Common offenders are mirror clips and one thing a lot of people don't think of, the bottom of the focuser intruding into the light path. 

Reflections - Anything that's in the tube that reflects light is unwanted. Look at everything, anything the sees light and isn't painted black or covered with flocking, should be. These things can be the hardest to track down because you can pass all the test above and still see weird flairs on long exposure images.