So do you think it unlikely to yield any difference with details in areas with nebulosity, faint shadows and highlights or overall clarity?
you often read reports of visual observers noticing improved contrast, pinpoint stars etc with higher quality mirrors, I can't help but think it would be similar for imaging especially since the eye is far less sensitive to subtle variations in detail and if they're detectable by the eye, it might be more pronounced with a ccd? just guessing here.
I'm not sure of the details, but wasn't the initial flaw with the hubble mirror relatively minor but yielded dramatically inferior results? I guess the scale of that mirror is very different from what we're talking about here and could've been the contributing factor?
With imaging it comes down to field correction, spot size and seeing. A well made Riccardi-Honders delivers 5 micron stars across around a 70mm flat field. Most other systems have spot sizes 2x larger on axis, and
degrade to *way more* off axis due to field curvature, astigmatism etc.
Other factors such as thermal and mechanical stability also come into play, and literally shift focus as an exposure is being taken. Exacting focusers and thermally stable materials usually come at a cost. My experience so far has been, you get what you pay for.
A well made Riccardi-Honders delivers 5 micron stars across around a 70mm flat field.
Peter, this kind of performance may be possible on a ray trace program but the reality of diffraction theory says its impossible. For a perfect F4 system of any design the diameter of the Airy disc to the first mimima of the airy pattern will be about 6 micron and then the 50% central obstruction will throw light into the first and second diffraction rings making them very bright , probably doubling the best blur spot diameter to twice that in the presence of seeing.
Peter, this kind of performance may be possible on a ray trace program but the reality of diffraction theory says its impossible. For a perfect F4 system of any design the diameter of the Airy disc to the first mimima of the airy pattern will be about 6 micron and then the 50% central obstruction will throw light into the first and second diffraction rings making them very bright , probably doubling the best blur spot diameter to twice that in the presence of seeing.
Not really. Airy disk size and spot size are two different paramaters. I think you knew that
The physical size of the airy disk is purely a function of the F-ratio. 4.7 microns for a F3.8 system. With 305mm of aperture you'd be looking at 0.84 arc sec of sky.
That is not the same as the spot size.
Spot size (and shape) depends on all sorts of variables, but in a nutshell, mirror/lens quality & optical design.
You only need to look at the off axis "sea-gulls" delivered by most camera lenses to see what happens when a design is not corrected well.
The Honders design covers a 3 degree field with essentially perfect color correction from 400 to 1,000 nanometers (UV to IR). The telescope is fully corrected for spherical, coma, astigmatism, field curvature and distortion, longitudinal and lateral chromatic aberration.
Peter, Yes I know the difference, I was referring to the quoting of what I imagined were spot sizes from geometric ray tracing ( just talking about on axis to simplify things) . Assuming you had pixels to oversample , the true spot size ( allowing for diffraction effects) achieved in an image would be somewhat larger than 5 micron due to the extra energy thrown out of the airy disc by the 50% central obstruction.
See two simulations I have done with Aberrator. When you add in the blurring effects of seeing the true blur spot diameter of the real instrument is going to be somewhat larger than the one calculated by geometric ray tracing. If the first dark minima is at 4.8 microns, the blur spot will be closer to 10 micron in a time exposure.
Unless you are using very small pixels the obstruction probably makes no difference to the quality of the images.
..... just talking about on axis to simplify things......
That's the rub. Agreed, On axis, most 'scopes can perform very well.
35mm off axis is where many systems simply don't cope....as sensor sizes have grown, the merits, or shortcomings of various designs have become more obvious.
I asked RCOS once what the expected gain was from the Ion Milled optics which are to all practical purposes perfect. I was told the ion milling removed micro scratches from the polishing process and the gain was less light scatter.
Much like the gain from fluorite over FPL53. Minor but noticeable.
Less light scatter. This could mean slight improvements in contrast.
Greg.
Quote:
Originally Posted by alistairsam
Hi Clive,
So do you think it unlikely to yield any difference with details in areas with nebulosity, faint shadows and highlights or overall clarity?
you often read reports of visual observers noticing improved contrast, pinpoint stars etc with higher quality mirrors, I can't help but think it would be similar for imaging especially since the eye is far less sensitive to subtle variations in detail and if they're detectable by the eye, it might be more pronounced with a ccd? just guessing here.
I'm not sure of the details, but wasn't the initial flaw with the hubble mirror relatively minor but yielded dramatically inferior results? I guess the scale of that mirror is very different from what we're talking about here and could've been the contributing factor?
My guess is that sky background darkness is a function baffling and F# alone and that smooth optics will mean that precious energy isn't scattered from diffuse objects . However I'm not sure what the relationship between spherical aberration and contrast of diffuse objects. I'd say it is much less important a consideration than optical smoothness for imaging faint diffuse objects. Users who report seeing `darker' sky background in premium reflecting optics I think are having themselves on. Perhaps the coatings are cleaner.
No i'm not just a happy snapper... Though I am majorly ham strung by uni!
I remember watching Top gear one night and they where testing the bugatti Veyron, the new one or the "suped" up version had close to 40hp at the rear wheels but only went a extra 4 or 5km a hour some piddly amount. At 300+ air becomes thick soup! Wind is my major source of loading where P=0.5*1.2kg/m3*wind speed^2*10^-3 = kN/m^2 so yes I understand that at low end it ramps up quickly but at the high end alot more effort is required.
I think presently I have almost reached the quality limits of a mass produced mirror. Its apparent when sitting side by side with other astrophotographers with similar styles of equipment but differing degrees of quality!
No i'm not just a happy snapper... Though I am majorly ham strung by uni!
I remember watching Top gear one night and they where testing the bugatti Veyron, the new one or the "suped" up version had close to 40hp at the rear wheels but only went a extra 4 or 5km a hour some piddly amount. At 300+ air becomes thick soup! Wind is my major source of loading where P=0.5*1.2kg/m3*wind speed^2*10^-3 = kN/m^2 so yes I understand that at low end it ramps up quickly but at the high end alot more effort is required.
I think presently I have almost reached the quality limits of a mass produced mirror. Its apparent when sitting side by side with other astrophotographers with similar styles of equipment but differing degrees of quality!
That's why you shouldn't be systematically ironing out the deficiencies in your mass-produced and self-tweaked system Brendan. The lure of additional quality images is about to COST big time
Well put Peter, the proof is in the pudding, and that m104 is certainly proof, the detail in the edge is outstanding. I hope to also prove the point with my overpriced RCOS optics (albeit pissy 10") when I install my rig at siding spring shortly right amongst the big toys, that'll make for some interesting comparisements
Quote:
Originally Posted by Peter Ward
Shades of gray.
The last 13% of any engineered system seeking "perfection" starts to cost serious $ with not a whole lot of performance return.
Hi-Fi's, cars, etc. all fall in that sort of category.
On a good night (i.e great seeing) average optics will satisfy most punters.
Excellent optics will return good images most nights (in short , it takes crappy seeing to unsettle them)
But, on a really good night (i.e superb seeing) excellent optics will also give you images as good as mother nature allows....
....A place you can never get to if the imaging system is already hamstrung by rough, poorly corrected or short-cut designed optics.
Question is: do you want to pay to capture those rare, albeit sublime, moments??
Proof?? the M104 image I took some years ago with great optics in superb seeing. The link is here
Proof?? the M104 image I took some years ago with great optics in superb seeing. The link is here
I'm sorry I'm not familiar enough with your equipment list, Peter. What was the FL, aperture, f ratio and design of the system that produced that truly impressive image? How does the resolution in that image compare with the theroretical capabilities of the system? What was the sensor/camera used?
regards,
Andrew.
I'm sorry I'm not familiar enough with your equipment list, Peter. What was the FL, aperture, f ratio and design of the system that produced that truly impressive image? How does the resolution in that image compare with the theroretical capabilities of the system? What was the sensor/camera used?
regards,
Andrew.
Err...it's all there on the Web page. Just scroll down
What I don't know is: how the actual FWHM's compare to theory. The optical systems figure is however within 4% of perfection.
Yep .....but you neglected to mention the fact: you got them for a song! (Fred actually only threatened to sing...so I gave him the scope to avoid any more Tinnitus than I already have )
That's why you shouldn't be systematically ironing out the deficiencies in your mass-produced and self-tweaked system Brendan. The lure of additional quality images is about to COST big time
Err...it's all there on the Web page. Just scroll down
What I don't know is: how the actual FWHM's compare to theory. The optical systems figure is however within 4% of perfection.
Thanks - I got a bit caught up in the image. Still - it's only really proof that you got a great image through your system: now someone with a similar aperture but inferior optics needs to step up to that mark.
regards,
Andrew.
Here is one of Rohr's optical tests. This is a superb mirror in all respects , now check out the resolution it is capable of in the 3.3u artificial stars test.In there is also the star resolution test of a GSO mirror and a few others. It clearly shows fine resolution is better with an extremely good mirror. http://translate.googleusercontent.c...M0Kw#post30967