14 replies to this topic

[Joe G]

Another of my friend's scopes.  The FK-61 ED doublet. 

 

Amazing value for the money.

 

 

 

 

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[Joe G]

Green:

 

 

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[Joe G]

Red:

 

 

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[Joe G]

Blue:

 

 

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[Joe G]

White:

 

 

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[CHASLX200]

I called mine a best bang for the buck scope when i got mine 4 or 5 years ago. Did 400x fine on the moon.

[Scott in NC]

The surface might be a little rougher than ideal (although I still can’t tell whether those aberrations are real, or are related to your photographic technique), but control of spherical aberration looks pretty good in green, especially for a scope this inexpensive.  As with the last scope that you presented, I really do think I see a bit more SA in red than what your simulated Ronchigram in AOS indicates, but it’s still not bad in red, or even blue for that matter.

[Elusivephotons]

This is very interesting.  With such a high result in RED could it be that this scope was 'nulled' in red?

[Scott in NC]

The white light images illustrate very well how FK-61 plus whatever mating element is used here doesn’t control CA quite as well as a FCD-100, FPL-53, or fluorite doublet, but still way better than an achromat.

[PhotogTom]

Not too bad at all. Pretty nice, in fact.

[Scott in NC]

This is very interesting.  With such a high result in RED could it be that this scope was 'nulled' in red?

Maybe.  However, as I mentioned above, I don’t think that the calculated Strehl in red is correct.  With undercorrection in red, and overcorrection in green and blue, that seems to indicate that the null position is somewhere between red and green (maybe closer to green though? I really don’t know).

[Joe G]

I don't think the red AOS above indicates it was figured in red.

 

Tweaking things in AOS can be very subtle. 

 

Scott is likely right about the red.  So here is another attempt giving a red strehl of 0.92.  IMHO the left image now looks more bowed than the actual ronchi image.

 

Likely the "truth" is somewhere in between.

 

My take on this DPAC analysis is that you can get a "reasonable" test of whether the optics are really good, good, okay, or bad.

 

If a strehl of 0.80 is 1/4th wave, and supposedly diffraction limited, then this scope seems to be in the "really good" to "good" category.

 

Matching these ronchigrams in AOS can make your head spin at times.  I'd caution on a false sense of precision.  Certainly not to three decimal places.

 

 

 

 

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[Scott in NC]

Yeah, now I think that the AOS image looks worse than your real one, lol.  It's hard to get the simulation just right, but regardless, I think your lens looks to be pretty well figured.

 

As much as I like the AOS program and really applaud Makiec's efforts with this, I've stopped playing with it for a while. Right now I'm trying to spend more time concentrating on interpreting what I actually see with my eyes.  But when he finally gets the updated version (accounting for zones, edge issues, etc.) ready for prime-time, then I'm really looking forward to giving it a try again.

[Maciek_Cz]

Hi guys,

If we try, we can make a pretty good match.
Joe, take a look at the picture below. AOS requires practice.

In addition, above 1/6 lambda, the adjustment of parameters is increasingly difficult,

but by doing it slowly and patiently, it is possible to select unambiguous parameters.

 

Maciek

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[Wildetelescope]

I am not a DPAC expert by any means, but these look quite good for a scope of this design.   I suspect through the eyepiece that most folks would find the views very pleasing.   On really bright objects, there will be a small amount of color, that a user may or may not notice depending on their sensitivity to such things, and their experience.  Looks like an excellent scope for folks who are just starting out.  For photography, it will show some color, but that could be minimized(not eliminated) with a uV/Ir cut filter, or one of the double band filters.   Not as good as a doublet with better glass or a triplet(for imaging) but you could learn a lot with this scope.  My 2 cents.  

 

Cheers!

 

JMD