22 replies to this topic

[Joe G]

So here are the results from a DPAC test of my son's Lunt/APM F7.9 152 ED FPL 51 doublet.  I bought this used for a decent price and compared it to my SW carbon Fiber "Esprit" triplet.  My son lived far away and has moved back closer so I have got a chance to test it.

 

At first this was a frustrating refractor.  The collimation seemed off.  I tried in vain to re-collimate it.  Finally figured out the retaining ring at the back was loose.  Problem solved.

 

Here is the scope under testing.

 

 

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[Joe G]

And the green results:

 

 

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[Joe G]

Red:

 

 

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[Joe G]

Blue:

 

 

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[Joe G]

And white:

 

 

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[Scott in NC]

Spherical correction looks pretty good, and chromatic control looks good too, and about what one would expect for a large, medium focal ratio FPL-51 doublet. I don’t know what it means when the ends of the Ronchi bands are wider than the middle sections, but don’t think that that’s related to the scope’s optics. Perhaps blooming of the sensor related to overexposure?

[Joe G]

Scott I don't know.  I am always amazed that the view through the viewfinder doesn't always match up to the recorded image.

 

These images are the best I could do. 

 

I was surprised the scope tested this well.

 

I wrote a comparison on CNs years ago about the comparison between this scope and my SW triplet.  Couldn't find it.  The triplet was better for sure.  But they are both nice scopes.

 

Peleuba (Paul) tested one of these and the ronchis look worse with a decent turned edge.

 

https://www.cloudyni...-the-apm-152ed/

[Scott in NC]

Joe, I think what we’re seeing is likely an artifact from the camera. I forgot what type of camera you use, but I had to experiment with a few camera lenses before I settled on using 50mm f/1.8 and 100mm f/2.8 lenses with a DSLR. Jeff B. has done some excellent work using an older cellphone camera, but my iPhone 10 gives distorted images, with widening of the edges of the Ronchi bands that look similar to your images. Were you using a cellphone camera?

[Joe G]

No, a Sigma FP with a 45mm lens.  I will have to look at the visual only images again to check that "widening" of the bands.  Silly but I looked visually but didn't make a mental note if that was there visually or photographically.

 

When I rack the focuser in and out the bands look good.  I was surprised the white light images didn't show more color.

 

I ordered one of those Askar 185s.  So larger aperture.  But a triplet.  I hope it tests out okay. 

 

It is crazy what is going on with larger refractors in the last couple of years.  Lots of choices and the quality seems to be okay.

[Scott in NC]

Thanks for contributing to our knowledge base, Joe. I’ll add a link to this thread in the repository.

[Ben the Ignorant]

Can you describe your testing gear?

[nicknacknock]

Thanks for contributing to our knowledge base, Joe. I’ll add a link to this thread in the repository.

Beat you to it!

[PhotogTom]

Not bad, not bad at all. Not sure what I was expecting, but this appears to be pretty doggone good.

[peleuba]

Joe, I think what we’re seeing is likely an artifact from the camera. I forgot what type of camera you use, but I had to experiment with a few camera lenses before I settled on using 50mm f/1.8 and 100mm f/2.8 lenses with a DSLR. Jeff B. has done some excellent work using an older cellphone camera, but my iPhone 10 gives distorted images, with widening of the edges of the Ronchi bands that look similar to your images. Were you using a cellphone camera?

 

The bands spread out near the ends for two reasons...   (1) As you said, maybe some sensor bloom.   (2) I believe there is also a zone approximately where I drew the red ellipse that is causing the bands to expand.  Its tough to see as the photos are low resolution, but I think I see it.

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Edited by peleuba, 27 December 2023 - 10:29 AM.

[peleuba]

Not bad, not bad at all. Not sure what I was expecting, but this appears to be pretty doggone good.

 

This was my exact thoughts...   I think this is the same lens as the APM152.   Joe's sample appears to better then my APM152 which is below and is a hot mess.

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Edited by peleuba, 27 December 2023 - 10:32 AM.

[Scott in NC]

Beat you to it!

Ah, now I see…you categorized it under APM and I put it under Lunt.  I’ll delete the Lunt link.

[DAVIDG]

 The glass looks like it has a number of stria or "seeds" in it which are the white dots. Dirt stops light  so it shows  as black areas. Stria is non-uniformity in the glass causing a difference in refractive index and indicates the use of poor grade optical glass. 

   Here is a picture Byners lens made in the 1860's that shows the problem as well. Back then the process to make uniform refractive index in Flint type glass had not be perfected.

 

                 Happy New Year,

 

                     - Dave 

 

[Scott in NC]

The white dots and lines on Joe’s images represent imperfections in his flat rather than imperfections in the objective of the refractor that he tested.

[Joe G]

I switched camera lenses and this looks better.  The widening of the bands near the perimeter seems to be an artifact.  My flat has a lot of micro scratches.  I think some of the surface issues might be due to this.

 

Here is white:

 

 

 

 

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[Joe G]

And green:

 

 

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[Scott in NC]

Joe, that looks much better. I figured it was probably a camera lens artifact, as the iPhone images that I took when I first started out with DPAC testing looked very similar to the images you posted near the beginning of this thread. Nice job with the redo!

[stevew]

It looks like a well corrected lens.

I can't see anyone being unhappy with the performance.

 

Thanks for sharing.

[lwbehney]

So here are the results from a DPAC test of my son's Lunt/APM F7.9 152 ED FPL 51 doublet.  I bought this used for a decent price and compared it to my SW carbon Fiber "Esprit" triplet.  My son lived far away and has moved back closer so I have got a chance to test it.

 

At first this was a frustrating refractor.  The collimation seemed off.  I tried in vain to re-collimate it.  Finally figured out the retaining ring at the back was loose.  Problem solved.

 

 

Thanks for this report. I briefly owned a Lunt 152 some years ago and it was such a remarkable improvement over my 8" SCT in every respect, particularly its overall contrast advantage. How does the Esprit triplet compare to the Lunt visually, now that you have properly restored its collimation?