18 replies to this topic

[lucam]

I put my AP 92mm f6.65 Stowaway through a DPAC test and the results are, not surprisingly, very nice. The test was performed with a 10lpm (254 lpi) Edmund Optics Ronchi grating. The curvature of the bands is twice as large as with the more conventional 5lpm gratings, providing higher sensitivity for well corrected optics.

 

The RGB and white LED montage:

 

 

I used Maciej's AOS v.0.8.1 software to estimate wavefront correction in red, green, and blue light. In green correction is essentially perfect - even with the 10lpm grating no curvature in the bands is detected. In red, my best estimate is wavefront error PV 0.070 waves (1/14th), corresponding to 0.984 Strehl ratio. The worst correction is in blue with a wavefront PV error estimated at 0.102 (1/10th) wave and a corresponding Strehl of 0.967. These are rough estimates and should not be taken literally.

 

There may be just a hint of a turned edge but it's hard to tell because of the multiple reflections so close to the edge of the optics. The at focus images are smooth with no obvious polishing marks or roughness.

 

The OTA puts up spectacular views at all powers and in use it has always been seeing limited. The test confirms the exceptional quality of the optics.

Edited by lucam, 13 April 2025 - 04:48 PM.

[ngc7319_20]

Why are the blue images (and to lesser extent the red images) dark at the bottom?  Does this imply anything?

[lucam]

Why are the blue images (and to lesser extent the red images) dark at the bottom?  Does this imply anything?

I have a slight light leak in the blue LED eyepiece, which causes the bright reflection in the upper 1/3 of the image. I am working to fix it but i had the same issue with the FSQ106 test I posted a few days ago. It has nothing to do with the optics.

 

--Luca

[Bomber Bob]

Thanks! for doing the DPAC & posting your results!

 

Trust... but, Verify.  That's a part of the pay-off for building these rigs & testing lenses.

[Bill Barlow]

Your test and I have seen another for the 92 Stowaway confirms it is a near perfect lens.  What number Stowaway do you own?

 

Bill

[lucam]

Your test and I have seen another for the 92 Stowaway confirms it is a near perfect lens.  What number Stowaway do you own?

 

Bill

Hi Bill,

 

My Stowaway is serial number 546.

 

--Luca

[fate187]

I put my AP 92mm f6.65 Stowaway through a DPAC test and the results are, not surprisingly, very nice. The test was performed with a 10lpm (254 lpi) Edmund Optics Ronchi grating. The curvature of the bands is twice as large as with the more conventional 5lpm gratings, providing higher sensitivity for well corrected optics.

 

The RGB and white LED montage:

 

AP92F665_montage.jpg

 

I used Maciej's AOS v.0.8.1 software to estimate wavefront correction in red, green, and blue light. In green correction is essentially perfect - even with the 10lpm grating no curvature in the bands is detected. In red, my best estimate is wavefront error PV 0.070 waves (1/14th), corresponding to 0.984 Strehl ratio. The worst correction is in blue with a wavefront PV error estimated at 0.102 (1/10th) wave and a corresponding Strehl of 0.967. These are rough estimates and should not be taken literally.

 

There may be just a hint of a turned edge but it's hard to tell because of the multiple reflections so close to the edge of the optics. The at focus images are smooth with no obvious polishing marks or roughness.

 

The OTA puts up spectacular views at all powers and in use it has always been seeing limited. The test confirms the exceptional quality of the optics.

Nice lens, and well executed test! As for the multiple reflections close to focus: try a ronchi with lower density grating or a just a knife edge: this will give you a sharper result and result in a well resolved image at focus. I use 66LPI at focus, and 133LPI outside/inside (the latter is what the forum usually uses..) 

[RichA]

Based on the polychromatic image, very good control of colour error.

[peleuba]

I put my AP 92mm f6.65 Stowaway through a DPAC test and the results are, not surprisingly, very nice. The test was performed with a 10lpm (254 lpi) Edmund Optics Ronchi grating. 

 

Luca - very nice.   It would be interesting to see your DPAC testing set up.  

[lucam]

Nice lens, and well executed test! As for the multiple reflections close to focus: try a ronchi with lower density grating or a just a knife edge: this will give you a sharper result and result in a well resolved image at focus. I use 66LPI at focus, and 133LPI outside/inside (the latter is what the forum usually uses..) 

That's good advice and after your comment I did a bit of searching and found lengthy discussions on the trade-offs between sensitivity and diffraction edge artifacts as a function of Ronchi spatial frequency. I have 133LPI rulings printed on transparency - I have used those for visual inspection of Ronchi tests but obviously they are not as clean and precise as the chrome on glass ruling. Do you have a good source for low frequency Ronchi rulings? Perhaps at low enough frequency, printed screens would work well. 

 

--Luca

[lucam]

Luca - very nice.   It would be interesting to see your DPAC testing set up.  

 

I mostly based my setup on what you, Jeff B, and others have generously shared with the forum. My setup includes:

 

- Moshen's eyepieces, modified to accept 2x2in glass Edmund Optics Ronchi ruling, with R, G, B, and white LEDs

- A 12.5in aluminized flat on a 3D printed holder

- A wood fixture with a Losmandy saddle plate to hold the telescope securely and aid alignment

- I use a Glatter laser and TuBLUG to align the focuser axis with the mirror

- Pictures are taken with a Canon 5D Mk II with Rokinon 135mm lens focused at infinity.

 

Here are a few pictures (ignore the white nylon gear on the lens, that's irrelevant for the DPAC setup):

 

Edited by lucam, 14 April 2025 - 09:52 AM.

[lucam]

Detail of the modified eyepiece:

 

 

Attached Thumbnails

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[Enyo]

That's good advice and after your comment I did a bit of searching and found lengthy discussions on the trade-offs between sensitivity and diffraction edge artifacts as a function of Ronchi spatial frequency. I have 133LPI rulings printed on transparency - I have used those for visual inspection of Ronchi tests but obviously they are not as clean and precise as the chrome on glass ruling. Do you have a good source for low frequency Ronchi rulings? Perhaps at low enough frequency, printed screens would work well. 

 

--Luca

See https://www.appliedi...onchi-gratings/

[fate187]

That's good advice and after your comment I did a bit of searching and found lengthy discussions on the trade-offs between sensitivity and diffraction edge artifacts as a function of Ronchi spatial frequency. I have 133LPI rulings printed on transparency - I have used those for visual inspection of Ronchi tests but obviously they are not as clean and precise as the chrome on glass ruling. Do you have a good source for low frequency Ronchi rulings? Perhaps at low enough frequency, printed screens would work well. 

 

--Luca

No sorry. Mine go back to ronchigratings.com or so, back several years :|

[Drothgeb]

That's good advice and after your comment I did a bit of searching and found lengthy discussions on the trade-offs between sensitivity and diffraction edge artifacts as a function of Ronchi spatial frequency. I have 133LPI rulings printed on transparency - I have used those for visual inspection of Ronchi tests but obviously they are not as clean and precise as the chrome on glass ruling. Do you have a good source for low frequency Ronchi rulings? Perhaps at low enough frequency, printed screens would work well. 

 

--Luca

Applied Image has a good selection of rulings at a reasonable price.

 

I’m using 5 lpm rulings that I made using B&W film. They are not perfect, but work fairly well. 

 

 

Edited by Drothgeb, 14 April 2025 - 06:31 PM.

[lucam]

Applied Image has a good selection of rulings at a reasonable price.

 

I’m using 5 lpm rulings that I made using B&W film. They are not perfect, but work fairly well. 

 

_IMG8994.jpeg
 

I saw the Applied Image chrome on glass rulings but they are 2.5in on the side and it would require making new eyepieces, which I am trying to avoid. Their rulings on photographic paper are available in lower resolution and presumably can be mounted in 35mm slide mounts.

 

Can I ask how you made yours on B&W film?

 

Thanks!

 

Luca

[Drothgeb]

I saw the Applied Image chrome on glass rulings but they are 2.5in on the side and it would require making new eyepieces, which I am trying to avoid. Their rulings on photographic paper are available in lower resolution and presumably can be mounted in 35mm slide mounts.

 

Can I ask how you made yours on B&W film?

 

Thanks!

 

Luca

I used 1/8” black vinyl pin stripping tape on white poster board. Then photographed it at the proper distance. The biggest issue is that the lines are dark gray, not completely black. So I’m assuming my results aren’t quite as sharp as they could be. I made my own because I wanted to experiment with different ways of making an eyepiece, and I needed a number of pieces.
 

It turns out that a 2” hole saw will cut out an exact size piece of 1/8” pvc to fit in a m48 to m42 reducer. And a m48 extension will hold it in place perfectly. I mount a piece of film to each disc with tape. Since the end of the led is sanded flush to the pvc, and the film is tight against it, I get no glare or reflections.

 

[peleuba]

I mostly based my setup on what you, Jeff B, and others have generously shared with the forum. My setup includes:

 

Superb.

 

I appreciate the shout out, but I learn from seeing other setups, including yours!    

 

For most all of this, I studied under Professor Trial and Professor Error - both emeritus members of the faculty at the School of Hard Knocks (SoHK).  I see a few other classmates here on CN.    In all seriousness, I am mostly self taught on this stuff with some help early on from a professional HST astronomer who had the great fortune of managing the group of technicians maintaining the WFPC and WFPC-2 instruments.  And, I am forever grateful for the time he spent with me for more then a decade and many, many testing sessions answering what must have been thousands of questions who's answers were sometimes as obvious as the nose on my faces.

  

Its excellent to see so many CN members wanting to test their scopes.  No longer are we left to wonder if its the seeing or the optics.

Edited by peleuba, 16 April 2025 - 08:43 AM.

[peleuba]

My Stowaway is serial number 546.

 

My own - #228 can be seen HERE for comparison.  Its an equally nice lens.