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Craig
Vendor (Stark Labs)
Reged: 09/16/03
Posts: 448
Loc: Irvine, CA
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Mak-Newts reduce the coma found in Newts, but don't eliminate it. We can see for things like an f/4 system (http://silverstar.pccenter.ru/fov.htm) that the corners of a 1 degree field (half degree radius), a Mak-Newt has a spot size of about 33 microns whereas a Newt has a spot size of about 123 microns. Basically, the Schmidt-Newt cut the spot size in half and the Mak cut this in half again. If memory serves this is about the level of coma one would expect in an f/8 Newt here. (Please correct me if I'm wrong on any of this).
Given that this is better than, your f/4-f/6 Newts, typically available coma-correctors for Newts would over-correct and not give good results I would expect. What solutions are there for faster Mak Newts? I know ITE describes a number of the faster Mak Newts there as "modified". Any idea what's been modified? If one wanted to cut out the rest of this coma, how would one go about doing it?
Craig
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Stark Labs Astrophotography software
Borg 101 ED f/4, CPC 1100 XLT, and too many cameras to mention
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 1723
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I am just guessing here... The biggest "Modifications" are the faster speed and the bigger secondary. Prior to these new optomized Mak Newts, the biggerst limitation of the available instruments was that they started to experience field illumination issues with eyepieces with large field stops, and I would imagine that would also show up in images. The simple solution then is to make the secondary larger and closer to the primary.
My guess would be that the relative coma would be somewhat worse simply because the system is faster, but the blur circle would be somewhat smaller at the focal plane, so these would perhaps cancel each other out.
Visually, the compromise would be on contrast due to the larger central obstruction required to illuminate a large chip.
Just guessing as to the "Why" but the biggest differences are the speed and the size of the secondary as compared to the older Mak Newts...
I do know that I had a MN56 and it could not fully illuminate the field of a 22mm Nagler, which has a field stop of about 31mm. It did appear to fully illuminate the field of my 17mm Nagler (Field stop is about 24mm). Many of the new chips are much larger than this.
Regards.
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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)
The night sky is my mistress. She seduces me away from all other lovers.
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bratislav
super member
Reged: 09/07/06
Posts: 127
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Quote:
Mak-Newts reduce the coma found in Newts, but don't eliminate it. We can see for things like an f/4 system (http://silverstar.pccenter.ru/fov.htm) that the corners of a 1 degree field (half degree radius), a Mak-Newt has a spot size of about 33 microns whereas a Newt has a spot size of about 123 microns. Basically, the Schmidt-Newt cut the spot size in half and the Mak cut this in half again. If memory serves this is about the level of coma one would expect in an f/8 Newt here. (Please correct me if I'm wrong on any of this).
Given that this is better than, your f/4-f/6 Newts, typically available coma-correctors for Newts would over-correct and not give good results I would expect. What solutions are there for faster Mak Newts? I know ITE describes a number of the faster Mak Newts there as "modified". Any idea what's been modified? If one wanted to cut out the rest of this coma, how would one go about doing it?
Craig
Craig,
you can design a Mak-Newt to be completely coma free (and flat field to boot) by making the primary oblate and meniscus more negative (stronger). This was done by TEC a while ago commercially (but failed for whatever reason - I think people just did not realized how good these systems were). Not sure what ITE calls "modified", their "optics-speak" is a bit weird and often doesn't make any sense to me.
You can certainly make a dedicated coma corrector for a Mak-Newt. It would take minutes in Zemax to design and many months to make it.
Bratislav
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