53 replies to this topic

[Scott in NC]

A fellow CN'er requested that I perform DPAC testing of my Takahashi FS-60Q, both in it's native f/5.9 form (the base FS-60 fluorite doublet) as well as with the 1.7x Q module (which makes it a FS-60Q f/10 fluorite quadruplet).

 

First a picture of the DPAC rig set up with the base FS-60 (f/5.9, without the Q module attached):

 

[Scott in NC]

FS-60 f/5.9 in green light

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

[Scott in NC]

FS-60 f/5.9 in red light

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

FS-60 f/5.9 in blue light

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

FS-60 f/5.9 in white light

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

Next a picture of the DPAC rig set up with the FS-60Q (f/10, with the Q module attached):

 

[Scott in NC]

FS-60Q f/10 in green light

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

[Scott in NC]

FS-60Q f/10 in red light

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

FS-60Q f/10 in blue light

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

FS-60Q f/10 in white light

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Erik Bakker]

Wow.

[Scott in NC]

For the base FS-60 f/5.9 fluorite doublet, I see pretty good spherical correction overall, with minimal overcorrection in green and blue and mild undercorrection in red.  There's a hint of a very shallow central zone, which I can see best in the at-focus white light image. The white light images show pretty decent control of chromatic aberration for a doublet this fast.

[Scott in NC]

For the FS-60Q f/10 quadruplet, I can't tell that adding the 1.7x Q module changes the spherical correction much, if any at all, but it does improve the CA slightly, as the white light image appears to have less purple fringing.  Interestingly, the Q module adds a slight edge aberration (turned down).

 

Edit: Rereading this a few weeks later, I see that I inadvertently wrote the direction of the turned edge backwards, and I’ve corrected it for accuracy.

Edited by Scott in NC, 23 April 2023 - 09:06 AM.

[Scott in NC]

Overall, I'm very happy with this scope, its main limitation being that it's only got 60mm of aperture.  I find it just perfect for traveling, especially when my family and I take our usual summer beach trip, when this scope doubles as a sea-side spotter as well as a nighttime stargazing scope.  I also have a Baader solar film filter so I can use it for white-light viewing. And before anyone asks me, no, it's not for sale!

[Scott in NC]

I need to move on to another project now, but if Maciek wants to analyze any of the data above with his AOS program, I'd be grateful.  Otherwise it may have to wait for another day. 

[Scott in NC]

Some of the images above aren’t perfectly circular and look more oblong. That’s related to how I was holding the camera, and not due to any optical defects from the scope.  One day I might get a tripod for the image capturing part of this testing, but it’s just so much easier to hand-hold the camera up to the Ronchi eyepiece.

[hyia]

I'm not the one who asked for this to be done, but I am glad to see it.  If you followed the TV76 thread, the following came came up, and this is perhaps the better place to address it.  Namely, how do the DPAC results you observe with the FS-60 reconcile with the strehl graph for the FS-60 given at this link:

 

http://scopeviews.co.uk/TakFS-60Q.htm

 

The graph would seem to suggest that the strehl in red is very low relative to the other colors.

[Scott in NC]

Before even looking at the graph that you linked to, I can tell you that my test shows more spherical aberration in red than in green or blue, so would expect a corresponding drop in Strehl there.

[CHASLX200]

Looks good to me. Should do 300x on the moon easy.

[Scott in NC]

Curiously, that graph seems to indicate a Strehl value below 0.10 in red.  There’s absolutely no way it’s that bad, unless perhaps they calculated the Strehl so far in the red that it’s very close to infrared.  At least at the wavelength that my red LED puts out, well within the visual spectrum, the Strehl value is certainly within the diffraction-limited range (i.e., no worse than 1/4 wave).

[Scott in NC]

Looks good to me. Should do 300x on the moon easy.

Sorry, but no.  No matter how well figured the optics may be, this is still only a 60mm scope.

[CHASLX200]

Sorry, but no.  No matter how well figured the optics may be, this is still only a 60mm scope.

I always push super good optics to crazy powers. The image will be dim but will still look good.

[Lagrange]

Curiously, that graph seems to indicate a Strehl value below 10% in red.  There’s absolutely no way it’s that bad, unless perhaps they calculated the Strehl so far in the red that it’s very close to infrared.  At least at the wavelength that my red LED puts out, well within the visual spectrum, the Strehl value is certainly within the diffraction-limited range (i.e., no worse than 1/4 wave).

Do you refocus the scope for each colour?

 

Here's the page on Wolfgang Rohr's site where the FS-60C Strehl curve came from. I don't know much German so I used Google translate to get some idea of what it said, and it looks like the Strehl figures were calculated from the focus offset. Presumably if testing was done at the best focus for each colour, the Strehl would be much higher.

 

This comparison was also interesting - colour correction with and without a prism for the TOA-130 and FS-60C as well as comparing suitability of different combinations in day and night conditions. I notice he used a 2" prism with the FS-60C which I suspect was too big since Tak recommend the use of their 1.25" prism so they may have designed the little scope to work best visually with that small amount of extra glass.

[Scott in NC]

Do you refocus the scope for each colour?

 

I suppose I do.  The reason I say "I suppose" is that it's difficult to give a definite yes/no answer.  I typically start by testing in green, first inside-focus, then at-focus, then outside-focus.  Then I switch to a different Ronchi eyepiece with a red LED, and repeat the process, first testing inside-focus, then at-focus, then outside-focus.  And then I test in blue, and then white.  So yes, I'm sure the focal point changes, but because I have to take 3 separate images with each color, then repeat the process, I really couldn't tell you how much the focal point changes between colors.  I hope that makes sense.

[Scott in NC]

 

Here's the page on Wolfgang Rohr's site where the FS-60C Strehl curve came from. I don't know much German so I used Google translate to get some idea of what it said, and it looks like the Strehl figures were calculated from the focus offset. Presumably if testing was done at the best focus for each colour, the Strehl would be much higher.

 

This is where it gets confusing to me.  Wouldn't one always test at the point of best focus for each wavelength of light? Otherwise, how would one know at which point to test?