Baskevo wrote: ↑Tue Oct 08, 2019 8:20 am
Hey guys, so I know Spectre has a recent post similar to this about weird stars, but I didn't know if it is the same problem as my stars look a little worse than his, so I didn't want to hijack his post... But I'm having a similar issue with a field flattener.
So I finally figured out my flats, but I now have a new issue
I imaged the other night for the first time with my new Explore Scientific 2" field flattener and my Orion Moon & Skyglow filter, and my stars look super weird... It looks worse with the new FF and filter. Granted, I can absolutely get longer exposures and more detail from objects, but the stars look pretty strange... I'm attaching the FF and filter with my Canon t7i DSLR to my Explore Scientific ED80 triplet APO refractor.
It kind of looks like chromatic aberration? I'm not sure though... I thought maybe at first it was the processing, so I tried different processes in DSS, then I even learned how to stack in PI, and I keep getting the same results on two different targets from the same night.
Here are the images:
They are both 270 second exposures of around 30 stacked images (Andromeda is a few less frames), at 800ISO. These are the stacked images, Andromeda stacked in PI and NGC 6992 stacked in DSS.
Screen Shot 2019-10-08 at 12.34.06 AM.pngScreen Shot 2019-10-08 at 1.07.54 AM.png
I tried to research it, and I read it might be the spacing for the field flattener? I couldn't find anything concrete though... It looks like CA doesn't it? If you guys have any suggestions on how I can fix it, either in my imaging train or in processing, I would greatly appreciate it...
Note: Sorry if I am doubling on posts (with spectre) and if this is in the wrong place on the forums.
Due to an earlier post on this forum about reducers/flattners and their use especially in SCTs I hauled out my Meade 10-inch
f/10 UHC
Coma-free
SCT and the proprietary "Meade focal reducer and field flattner" a couple of evenings ago and I've come to several conclusions about what may be actually happening as far as optical distortions/shortcomings are concerned. I shot three
DSOs with and without the Meade reducer/flattner and each series of captures yielded pretty much the same result so I'm only posting what I found with M27 Dumbbell Nebula - the "reduced image" at .63x, the reduced image scaled up to match the image size of photo C which is a prime focus shot of M27 at it's normal
f/10 2500mm focal length.
I had posted elsewhere on the astrophotography deep-sky forum my off-the-top-of-the-head thoughts on the careful matching of optical components and the basic principle that the more glass introduced to an optical train, the more precisely these elements have to be engineered, manufactured and assembled if they are to truly "correct" the final image. This is opposed to sorta correcting for one optical defect but as the result of an inadequate or imperfect optical solution, other defects are introduced. Sometimes, however, in the case of say, my fixed 200mm
f/2.8 Canon EOS EF-L telephoto, it has ELEVEN optical elements in 7 groups. Now I don't pretend to know what the 1990s Japanese optical engineers/theoreticians and technicians were thinking when they engineered this (frankly) awesome terrestrial telephoto lens - which qualifies as an apochromatic mini-telescope in my view - but my first best guess is they were trying to correct for
CA, astigmatism (which "fast" telephotos can have),
coma, barrel/pincushion, spherical aberration, and trying to get the best corrected, widest, cleanest image at the focal plane that would cover the entire
CCD/
CMOS chip.
To the point. After using several brands of relatively inexpensive, moderate quality reducer/flattners from Meade, Celestron and Antares over the last 12 months, I don't think the problem of
coma and other optical defects in the extended field is the result of the reducers themselves but rather the reducer/flattner module is REVEALING defects which would normally not be viewed or imaged because of the original "field stop" built into a given telescope or eyepieces which would normally be used with their telescope. For example, if you remove the field stop in an eyepiece, you get a far large
AFOV but the image in the extended view is often heavily compromised by any number of distracting optical defects. Optical designers and manufacturers can only correct a given optical train so much, be it a telescope, eyepiece, binoculars, or telephoto lens, and stay within a certain price range. As a result there are certain quality "standards" which get established for a given price point. I suppose it's possible to correct an eyepiece, for example, to deliver certain levels of optical quality across, say, 68 degrees of
AFOV and deliver this eyepiece for under $100 or $150. But to design and manufacturer an eyepiece that delivers 85 or even 100 degrees of quality image would then cost $500. And how many of these more expensive eyepieces would an optical company sell compared to a more pedestrian thogh adequate but much lower priced optical item? And that's probably the main reason why field stops are designed into optical systems, they "cut off" the extreme outer portions of the field of view which are essentially unusable but I suppose if enough customers are willing to pay five times the cost, the optical engineers could design and optical system what would deliver 25% to 30% larger
AFOV. Of course I'm probably mostly preaching to the choir here given the high knowledge level of most visual astronomers and astrophotographers at
TSS.
Now I may be wrong about these relatively inexpensive flattner/reducers (and I'm not even sure flattner/reducers costing five times as much would be all that much better), but I now believe it's not that these reducer/flattner optical hardware are necessarily creating more
coma or more spherical aberration but rather when they by design "reduce" the normal focal field of say an
f/10
SCT down to an smaller scale of image at
f/6.3. In so doing they bring into play/view the very outer portions of the
f/10 image which is normally excluded or rendered unviewable by an internal telescope field stop or the field stop of the eyepiece your using OR the edges of the
CCD/
CMOS chips (the size of the chip essentially determines the "field stop") in your particular
DSLR or astro-camera and that portion of the "reduced" image that you now see is generally never going to be very good because optical designers and manufacturers can't possibly correct the entire image plane (at whatever focal length) because it would be very difficult and prohibitively expensive. I suppose theoretically such a correction is possible, but the telescope or whatever optical device, would probably cost $100K! That's when a government becomes the main purchaser for such extreme "high-end" optical devices. But if funds are unlimited and you're committed to turning your
f/10
SCT into a fully-correct
f/6.3
SCT with a .63X reducer, there's probably some clever optician out there who could grind a custom reducer module to correct for the optical defects of your particular telescope - but for a very steep price. Much like getting a person getting glasses or contact lens custom designed for their eye's particular visual defects, but at 20 times the cost!
If you compare photo B (which is merely an enlargement of the .63X reduced image of photo A) and compare it with photo C which was taken at prime focus of a 10-inch f10
SCT on the same night but 2 hours apart due to other experiments I was engaged in, you'll note there is no extra
coma in photo B. The slight elongation of the stars are probably due to the effects of a very mild 5 mph variable wind - sorry about that. In essence, I believe the Meade
SCT coma-free telescope lineup was optimized to deliver an industry standard portion of the larger focal plane image that would indeed be
coma free, that's why one paid more for this "option." But that part of the image which lay beyond the eyepiece field stop or the size of the imaging
CCD/
CMOS chip will generally not be corrected to any significant degree because under normal viewing or imaging circumstances throwing more difficult optical corrections and money at the outer "unused portion" of the focal plane image simply doesn't make any sense … until the minds-that-be invented the focal reducer!
These photos weren't optimized for relative luminance levels or other other image quality considerations, but rather they are strictly generated to document whether focal reducers actually add
coma or can correct
coma in a given
OTA. I conclude they do neither but rather they in fact allow you to access portions of the focal plane image not ordinarily viewable in a particular telescope's native image scale as it relates to its actual focal length, objective size and
f/value. Also note the heavy vignetting which I did not correct for in either photo A or photo B. This light "fall off" is normal in any optical system as one gets closer to the edge of the native full image produced by a given main objective optical element. And we see evidence of this "vignetting" with our eyes or by our camera because more of the smaller scale can pass through the fieldstop of our eyepiece or more of this vignetted reduced image can now fit on the
CCD/
CMOS chip.
Last. In extreme blowups of Photo B, I don't see
coma as having been made worse by the Meade reducer/flattner unit and I certainly don't see it acting as a "
coma corrector" either, which it was never designed to do, btw. However I think I do detect a hint of astigmatism which has affected the quality of the star images to a slight degree. But I could be wrong and what I'm detecting under high magnifications can be attributed to other factors like transient vibration, wind, I didn't hold my tongue in my mouth the right way, whatever. And it does appear maybe the tiniest bit of
CA has been added to image B, too, which wouldn't be surprising since we're dealing with a fairly inexpensive piece of glass with these entry level reducer/flattners (not to be confused with
coma correctors and the such).
