724 replies to this topic

[Scott in NC]

FS-60Q inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

[Scott in NC]

I think it looks pretty good both ways, although the Q module appears to accentuate the minor edge error (by 1.7x, I'll presume).  Now that I think about it, even though the Q module improves the view (at least for visual observation), reducing chromatic aberration and field curvature, I guess I shouldn't have expected that it would make the scope look better on DPAC testing.  The Q module serves as a combination Barlow / Field Flattener, but there's nothing about its optical properties that I'm aware of that would improve any imperfections inherent to the original optics set.  

[Scott in NC]

What I see here (on both the base FS-60 and the Q) is almost no discernible spherical aberration (maybe a touch of overcorrection at the very most) and a very minimal edge imperfection.

 

I'm starting to wonder if maybe there's something about my technique that I'm doing wrong, as I can't imagine that every apochromatic scope that I own has a less than perfect lens edge.  A lot of what I've demonstrated here on photographs isn't easily apparent on visual inspection through the Ronchi eyepiece.  Maybe some of what the camera is picking up is just diffraction around the edge of the lens cell or dew shield?  I'm using a fixed focal length 50mm f/1.8 lens with my Canon DSLR, and the reason I chose that lens is that it's the only non-zoom camera lens that I own.  Could that be the wrong lens (too fast?) and I need to use a slower lens?  

 

In comparison to astronomy, I know very little about photography, so any of you that are more knowledgeable in this area, please chime in if you think I'm doing it wrong.  I'd hate to inadvertently attribute camera-related aberrations to my scopes.

[Scott in NC]

On my first and only attempt at DPAC, I got an image like this.

I was using a Gerd Neumann 10 L/mm Ronchi Eyepiece on which I taped a green LED over the lower half. I have a 6" round coated flat (unknown flatness). I placed it on table and put the 4" refractor on it and took a quick look. The DPAC image looked very bad, but the refractor (which is homemade using a 25x100 bino lens) star tests ok at 95x. I thought that the reason for the twisted lines was because of improper alignment of the flat with the objective.

 

I've only been doing this for a little over a week, but I've found that if the flat isn't lined up properly with the objective, you won't see any aberrations attributable to that.  It's actually much simpler--the flat won't return the images back through the Ronchi eyepiece, so you won't see anything at all.  I like that, as it eliminates one source of error in the testing process.

[Villa_il_Gioiello]

What I see here (on both the base FS-60 and the Q) is almost no discernible spherical aberration (maybe a touch of overcorrection at the very most) and a very minimal edge imperfection.

 

I'm starting to wonder if maybe there's something about my technique that I'm doing wrong, as I can't imagine that every apochromatic scope that I own has a less than perfect lens edge.  A lot of what I've demonstrated here on photographs isn't easily apparent on visual inspection through the Ronchi eyepiece.  Maybe some of what the camera is picking up is just diffraction around the edge of the lens cell or dew shield?  I'm using a fixed focal length 50mm f/1.8 lens with my Canon DSLR, and the reason I chose that lens is that it's the only non-zoom camera lens that I own.  Could that be the wrong lens (too fast?) and I need to use a slower lens?  

 

In comparison to astronomy, I know very little about photography, so any of you that are more knowledgeable in this area, please chime in if you think I'm doing it wrong.  I'd hate to inadvertently attribute camera-related aberrations to my scopes.

I agree. I don’t see any difference between the two tests.

 

Kinda makes me question the claims of improved optical performance at the eyepiece with the use of these extenders, both from the manufacturer and the end user. How could anyone see an improvement other than it just being in their head? 

[Scott in NC]

 

Kinda makes me question the claims of improved optical performance at the eyepiece with the use of these extenders, both from the manufacturer and the end user. How could anyone see an improvement other than it just being in their head? 

 

Well, the extender definitely improves chromatic aberration and flattens the field, at least for the FS-60Q.  So much that I don't really like using the FS-60 without the Q module in place (not that I've tried it with my current model, but several years ago I briefly owned a base FS-60 and wasn't overly impressed).  It wasn't a bad scope, but had too much CA and field curvature for my liking during visual observation.

 

Those benefits are real, but wouldn't show up on DPAC testing in green light.  CA would show up when DPAC testing in white light, but I don't yet have the capability of doing that.

[Maciek_Cz]

To determine the effect of the extender on the SA correction, measurements should also be made for red and blue.

For now, we can only say that in green they go head to head.

What I'm talking about is in this link: http://www.scopeview...k/TakFS-60Q.htm

[Alan S]

What I see here (on both the base FS-60 and the Q) is almost no discernible spherical aberration (maybe a touch of overcorrection at the very most) and a very minimal edge imperfection.

 

I'm starting to wonder if maybe there's something about my technique that I'm doing wrong, as I can't imagine that every apochromatic scope that I own has a less than perfect lens edge.  A lot of what I've demonstrated here on photographs isn't easily apparent on visual inspection through the Ronchi eyepiece.  Maybe some of what the camera is picking up is just diffraction around the edge of the lens cell or dew shield?  I'm using a fixed focal length 50mm f/1.8 lens with my Canon DSLR, and the reason I chose that lens is that it's the only non-zoom camera lens that I own.  Could that be the wrong lens (too fast?) and I need to use a slower lens?  

 

In comparison to astronomy, I know very little about photography, so any of you that are more knowledgeable in this area, please chime in if you think I'm doing it wrong.  I'd hate to inadvertently attribute camera-related aberrations to my scopes.

It’s a good question certainly about your imaging technique.  Also, I would hazard a guess that edge aberrations are the most common “imperfections” in lenses. I’ve certainly seen a lot of imperfect edges, and not just in your images. When fabricating (polishing) lenses, the edges are the most time consuming portion of the surface.

But again, how close to perfect do our telescopes need to be?  (Perfect, I know )

 

Alan

[m0bius]

Scott - I'm a curious guy. Maybe try slapping an aperture mask on one of the previously tested scopes? If you find similar 'defects' near the reduced diameter edge, then I'd suspect that it's mostly diffraction related.

Edited by m0bius, 26 February 2023 - 04:55 PM.

[Lagrange]

I agree. I don’t see any difference between the two tests.

 

Kinda makes me question the claims of improved optical performance at the eyepiece with the use of these extenders, both from the manufacturer and the end user. How could anyone see an improvement other than it just being in their head? 

You'd have to be practically colourblind to not see the difference at the eyepiece between the FS-60 and the FS-60Q.

 

Correction in green might be very similar but it's the blue and red ends of the spectrum where the improvement is readily apparent. Violet bloat in particular goes from being easily visible in the FS-60 to only just discernible on the most challenging targets in the FS-60Q, and there's a dramatic reduction in false colour either side of focus.

 

The Extender also flattens and corrects the off-axis field, making the FS-60Q suitable for imaging or use as a lightweight telephoto lens during the day.

 

DPAC at different wavelengths or in white light should look very different.

[Lagrange]

What I see here (on both the base FS-60 and the Q) is almost no discernible spherical aberration (maybe a touch of overcorrection at the very most) and a very minimal edge imperfection.

 

I'm starting to wonder if maybe there's something about my technique that I'm doing wrong, as I can't imagine that every apochromatic scope that I own has a less than perfect lens edge.  A lot of what I've demonstrated here on photographs isn't easily apparent on visual inspection through the Ronchi eyepiece.  Maybe some of what the camera is picking up is just diffraction around the edge of the lens cell or dew shield?  I'm using a fixed focal length 50mm f/1.8 lens with my Canon DSLR, and the reason I chose that lens is that it's the only non-zoom camera lens that I own.  Could that be the wrong lens (too fast?) and I need to use a slower lens?  

 

In comparison to astronomy, I know very little about photography, so any of you that are more knowledgeable in this area, please chime in if you think I'm doing it wrong.  I'd hate to inadvertently attribute camera-related aberrations to my scopes.

I don't think you're doing anything wrong.

 

The results for the FS-60 with and without the Extender seem reasonable - having best correction in green light seems to be typical of a modern Tak and the lens shows the very high standard of figure and polish that seems to be the norm from Canon Optron. Testing at other wavelengths or in white light should give very different results and I suspect the FS-60 would perform worst in violet light.

 

I doubt your lens is a problem. The Canon 50mm f/1.8 might be cheap but it's been long renowned for its optical quality and should be much better than any budget zoom. Stopping it down a bit will improve performance over shooting wide open - centre resolution peaks at f/4 and edge resolution peaks at f/5.6.

[Scott in NC]

Scott - I'm a curious guy. Maybe try slapping an aperture mask on one of the previously tested scopes? If you find similar 'defects' near the reduced diameter edge, then I'd suspect that it's mostly diffraction related.

That’s a really great idea, and I’ll definitely give that a try one of these days.

[Scott in NC]

 

DPAC at different wavelengths or in white light should look very different.

That’s definitely on my agenda, once I have the capability of testing in colors other than green.

[davidc135]

Scott - I'm a curious guy. Maybe try slapping an aperture mask on one of the previously tested scopes? If you find similar 'defects' near the reduced diameter edge, then I'd suspect that it's mostly diffraction related.

Great idea.

[Jeff B]

Scott - I'm a curious guy. Maybe try slapping an aperture mask on one of the previously tested scopes? If you find similar 'defects' near the reduced diameter edge, then I'd suspect that it's mostly diffraction related.

The trick is making the mask.  For such small apertures and thin masks, I use sticky backed foam that comes in sheets.  It's a couple of mm thick and you can find it at any hobby store or Wal-tucky.  I use a metal straight edge and sharp box cutter/utility knife to cut strips then gently press them to the front inside wall of the cell (but not up against the glass!).  You can even build up the layers of foam to make incrementally smaller stops.  The stuff is easy to remove with no residue as long as you gently apply the stuff.

 

One layer applied to the FS-60Q should be all that's needed to sort it out as any real edge condition is tiny.

 

Jeff  

[Scott in NC]

Thanks, Jeff.  And while this would be an interesting project, I'm not even sure it's worth my time, as the views through the FS-60Q are so good that I seriously doubt that I'd see any difference afterwards (in DPAC maybe, but not visually through the scope).

[Jeff B]

Thanks, Jeff.  And while this would be an interesting project, I'm not even sure it's worth my time, as the views through the FS-60Q are so good that I seriously doubt that I'd see any difference afterwards (in DPAC maybe, but not visually through the scope).

Yup, just academic really.    

[Scott in NC]

I’ve learned to learn what I can from this testing but not to take the results too seriously.  Or at least not so seriously that I get bent out of shape or try to “chase perfection” in a scope that already provides outstanding views.

[Jeff B]

I’ve learned to learn what I can from this testing but not to take the results too seriously.  Or at least not so seriously that I get bent out of shape or try to “chase perfection” in a scope that already provides outstanding views.

Good luck with that Scott and support groups are available in case you slip.   Helpful hint:  Stay away from the ATM forum, specifically those who push glass.

[Scott in NC]

Now that I've done all of my apos except for the Big Kahuna (the FS-152), for which my 150mm flat isn't large enough to easily test without doing it section by section, I decided to test one of my prized achromats. 

 

This is the scope that started it all, my very first refractor, purchased brand new from Stellarvue in 2004.  It's an SV80/9D, an 80mm f/9.4 doublet achromat.  I've not ever tested it visually side by side with a more modern doublet ED refractor, but I'm pretty sure it will hold its own as far as chromatic correction goes when compared to an 80mm f/7 ED scope made with FK-61 or FPL-51 glass.

 

Here's the scope all set up for testing:

 

[Scott in NC]

Stellarvue SV80/9D inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

[Scott in NC]

Not bad at all in green light.  I see a touch of overcorrection, but then I knew from talking with Vic Maris soon after purchasing the scope in 2004 that these lenses apparently all had slight overcorrection.  The funny thing is that I had absolutely no idea what he was talking about back then.  It sounded good to me--hey, "overcorrection" certainly has to be better than "undercorrection," right?  But seriously, the amount of spherical aberration really isn't bad at all here, IMO. Now I don't have the ability to test using white light yet (but will soon).  When I do that, this achromat will show its true colors (pun intended).

[Scott in NC]

This is a Stellarvue Nighthawk (the original 80mm f/6 doublet achromat version).  I've had it since 2010, and it was probably manufactured somewhere between 2003 and 2006.

 

 

[Scott in NC]

SV Nighthawk inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

 

 

[Scott in NC]

As you can see, it's got a bit more SA (overcorrection) than my SV80/9D, but it's much harder to produce well-corrected achromatic optics at f/6 than f/9.4, especially at this price point.

 

But that's absolutely fine by me.  I purchased this gem in 2010 for a whopping $200 , and that included a nice aluminum case and shipping.  I bought it purely for a trip to the dark skies of the Colorado mountains that summer (Alan S., if you're reading this, you'll recall that you were there too).  I thought for that price I'd buy it, not fret too badly if it didn't hold up well to airline travel, and then assuming that it survived the trip unscathed, plan to resell it for what I paid when the trip was over.  Well, it turned out that I enjoyed using this little scope so much that nearly 13 years later I still have it.  If you look closely at the picture, you'll see that I've even souped it up with a 2" FT focuser.  I don't use this scope a whole lot, but occasionally I'll bring it out for H-alpha solar observing, outfitted with a Lunt 60mm etalon and B1200 blocking filter.