57 replies to this topic

[peleuba]

I received my new AP Stowaway in early October as part of the second run of the FCD100 based 92mm all spherical, air spaced triplet.  Because of some previously scheduled international travel, I’ve not had a lot of time to use and test the OTA.  I have had several quick look sessions mostly looking at the moon, and brighter clusters (M45, Double Cluster etc).  Jupiter and Saturn are too low and have sunk behind a neighbor’s trees.   I did view Neptune and Uranus.  Both were unremarkable but did resolve to planetary disks.  I did not do any critical star testing outside but have done so indoors.  Overall, my impressions are that this is a terrific telescope.

 

Below are some photos of my Autocollimation test setup.  My optical flat is 10” in diameter, 2” thick and made from Zerodur.  It was sourced from Lawrence Livermore National Laboratory.  In the center 6 inches or so, it approaches 1/30 wave.  The great thing about testing in double pass autocollimation is a flat this precise is NOT a requirement.  You can have a optical flat several waves from being flat and  its still be good enough for testing in double pass.  This is so as the errors in the flat only contribute fractionally to the error of the telescope under test.  One of the benefits of an accurate flat is that I am able to use it with an interferometer.  In interferometry all errors present in the test path are additive.

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Edited by peleuba, 16 November 2019 - 04:04 PM.

[peleuba]

Below are bench images taken, analyzed and processed Friday evening.  I tested in the three primary wavelengths of Red, Green, Blue.  Light travels in straight lines, but it does so as a wave.  Meaning the different colors have different wavelengths – some shorter, some longer.  It’s this wave nature of light that causes the different colors to come to focus at differing points.    Typically, you correct a lens as best as possible in green.  It’s the color that our eyes are most sensitive to.  The blue and red are usually corrected, but not as perfect as green is.  Blue and red will (usually) be balanced with opposite signs of correction – one will be overcorrected while the other under corrected a similar amount.   This lens shows this very well.

 

For the images below:

 

 

Left = Inside of Focus

Center = At Focus (Null)

Right = Outside of Focus

 

 

Unfortunately due to file size constraints I had to convert these from .RAW to .JPG formats.  Meaning they are now highly compressed and some nuances have been lost.  But I think you get the idea. 

 

Thanks for looking.

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Edited by peleuba, 16 November 2019 - 04:05 PM.

[gezak22]

Can the results be quantified?

[peleuba]

Can the results be quantified?

 

Depends on what you mean.

[gezak22]

Assume that I am trying to separate the good from the very good, can you extract a figure of merit from these plots so that it can be compared to the same figure of merit taken from a different refractor?

 

I know it's easy to tell the difference between a fast achro and a premium triplet in raw DPAC images. But I've always struggled to tell the difference between an AP, Tak, TEC when looking at raw DPAC images, and data reduction would perhaps help.

[peleuba]

Assume that I am trying to separate the good from the very good, can you extract a figure of merit from these plots so that it can be compared to the same figure of merit taken from a different refractor?

 

I know it's easy to tell the difference between a fast achro and a premium triplet in raw DPAC images. But I've always struggled to tell the difference between an AP, Tak, TEC when looking at raw DPAC images, and data reduction would perhaps help.

 

The easiest way to tell the diff between brands is to look at the label.  I am not being sarcastic as the test bench is agnostic when it comes to specific brand manufacturers.  There is no tell-tale sign that says "I'm a TAK or "I'm a TeleVue" etc.  The test bench strips away the fanboy status of the brands and evaluates the performance of the optic.  Typically ED doublets will be great in green and further out in red and blue then this triplet.

 

There is no way to unequivocally assign wavefront ratings to the lenses and mirrors tested in DPAC.  However, you can estimate - with some accuracy - the spherical correction, the smoothness etc.  Only an interferometer combined with some type of data reduction fringe analysis software can accurately spit-out PtV, RMS and Strehl.

 

DPAC is quite useful in conjunction with the star test and can absolutely separate the good from the bad.  In many ways for the amateur its superior to interferometry as you can see exactly whats going on with the lens while under test.

Edited by peleuba, 16 November 2019 - 04:41 PM.

[Erik Bakker]

Thanks for taking the effort to do this and show the results of your scope here Paul 

[gezak22]

The easiest way to tell the diff between brands is to look at the label.  I am not being sarcastic as the test bench is agnostic when it comes to specific brand manufacturers.  There is no tell-tale sign that says "I'm a TAK or "I'm a TeleVue" etc.  The test bench strips away the fanboy status of the brands and evaluates the performance of the optic.

As always, poor phrasing on my end

 

But imagine we were testing band pass filters, and people posted the transmission curves of said filters. If I wanted to determine which one is worthy of my money, the people who collected the raw data would make my life easier if rather than just showing the raw data they also posted the peak transmission, the center wavelength, and the FWHM. With these numbers in hand, and given sufficiently many samples, it is straightforward for me to tell where to spend my money. Similarly, reducing DPAC images to a handful of numbers would also help. Sure, a star test is helpful, but that assumes I have already spent my money on the scope.

[peleuba]

As always, poor phrasing on my end

 

 

 

 

With these numbers in hand, and given sufficiently many samples, it is straightforward for me to tell where to spend my money. Similarly, reducing DPAC images to a handful of numbers would also help. Sure, a star test is helpful, but that assumes I have already spent my money on the scope.

 

OK - I understand.  There is no way to to take the images and assign a specific number or wave front rating.  An interferometer can, but not an autocollimation test.  

[peleuba]

Left = Inside of Focus

Center = At Focus (Null)

Right = Outside of Focus

 

 

I should also point out that my camera does not take pictures in red light very well.  When viewing the test in person, red looks better to the eye then it does in the photo.  My LED is creating a hot spot in both red and to a lesser extent in blue that the camera cannot compensate for.  That's why there is brighter/dimmer areas.

 

The overall correction of this lens is as follows:

 

Green - nearly perfect spherical correction.  Really, Really good.

Blue - very slightly overcorrected

Red - Slightly undercorrected

 

Also, don't forget that all errors seen are DOUBLED.  This is a very sensitive double pass test. Meaning they appear twice as bad as whats actually in the glass.

 

The slight circular polishing artifact seen in the null image in green and red (but not blue) is probably only 1/10 wave tall/deep.

 

This is a very good lens.

Edited by peleuba, 16 November 2019 - 05:34 PM.

[Jeff B]

"This is a very good lens."

 

Paul, it's rare but I have to disagree.

 

It's an excellent lens.

 

Jeff

[moshen]

Superb bench test images. Those 'knife-edge' images are tough to capture and you got a lot of detail there. Looks essentially perfectly corrected in green. This looks nearly identical to my DPAC of my Stowaway.

[rockstarbill]

Wow Roland. Just wow. Out-did himself on this product. Phenomenal results! 

[Wildetelescope]

Can the results be quantified?

It seems to me at the very least you should be able to measure the degree of curaviture of the interference lines for each color.  Has anyone looked at the math that describes the DPAC process?  Maybe a better question is whether or not there is math to describe how you form the DPAC image?  At the very least one should be able to describe a semi-quantitative “degree of distortion”  by measuring how much the lines deviant from parallel, no?  You have a single wavelength, a diffraction grid of known spacing, and a known distance that the light has traveled from source to sensor.  Seems to me you should be able to get more from the test than visual inspection of the image.  Maybe not the usual parameters, like strehl, etc, but some quantitative measure that could be compared with an ideal case or a standard.  Just curious. 

 

Jmd 

[peleuba]

Seems to me you should be able to get more from the test than visual inspection of the image.  Maybe not the usual parameters, like strehl, etc, but some quantitative measure that could be compared with an ideal case or a standard.  Just curious. 

 

Jmd 

I understand what you are you are saying.  I’m at a lacrosse tournament all day so don’t have a lot of time to respond or think about this.   But will do so later.

[Wildetelescope]

I received my new AP Stowaway in early October as part of the second run of the FCD100 based 92mm all spherical, air spaced triplet.  Because of some previously scheduled international travel, I’ve not had a lot of time to use and test the OTA.  I have had several quick look sessions mostly looking at the moon, and brighter clusters (M45, Double Cluster etc).  Jupiter and Saturn are too low and have sunk behind a neighbor’s trees.   I did view Neptune and Uranus.  Both were unremarkable but did resolve to planetary disks.  I did not do any critical star testing outside but have done so indoors.  Overall, my impressions are that this is a terrific telescope.

 

Below are some photos of my Autocollimation test setup.  My optical flat is 10” in diameter, 2” thick and made from Zerodur.  It was sourced from Lawrence Livermore National Laboratory.  In the center 6 inches or so, it approaches 1/30 wave.  The great thing about testing in double pass autocollimation is a flat this precise is NOT a requirement.  You can have a optical flat several waves from being flat and  its still be good enough for testing in double pass.  This is so as the errors in the flat only contribute fractionally to the error of the telescope under test.  One of the benefits of an accurate flat is that I am able to use it with an interferometer.  In interferometry all errors present in the test path are additive.

What did the optical flat cost?

 

jmd 

[Wildetelescope]

I understand what you are you are saying.  I’m at a lacrosse tournament all day so don’t have a lot of time to respond or think about this.   But will do so later.

No worries!  I find the topic interesting and am just curious.:-).  Have fun at the tournament!!!

 

jmd 

[Jeff B]

Paul, I have to say I really enjoy your straight forward, clear, no nonsense explanations in your postings.   Refreshing and well done Sensei Paul. 

 

Jeff

 

BTW, I've found testing in my basement to be ideal.  My work room is a large old cistern (only dry part of the basement really) and the air is very uniform and steady.  No air currents at all!

 

Jeff

[Bill Barlow]

How does this AP 92 lens compare to the AT92 lens or others in the 90mm range you tested? I guess the real test is how the in focus images look.

 

Bill

Edited by Bill Barlow, 17 November 2019 - 11:01 AM.

[Jeff B]

Assume that I am trying to separate the good from the very good, can you extract a figure of merit from these plots so that it can be compared to the same figure of merit taken from a different refractor?

 

I know it's easy to tell the difference between a fast achro and a premium triplet in raw DPAC images. But I've always struggled to tell the difference between an AP, Tak, TEC when looking at raw DPAC images, and data reduction would perhaps help.

Well, actually, I don't find it easy to see the differences in DPAC really, at least at discrete colors.  I've found that a well made fast achromat is just that, well made and tests really well in DPAC.  In fact, the blue and the red images may actually test a bit better in DPAC than those of a fast APO of similar aperture and focal ratio, particularly a doublet.   

 

But look at all that purple in the achromat!!

 

That's because for the achromat the focus positions of the blue and the red are far removed from that of green so the CA is way dominated by that defocus "error" ( called longitudinal chromatic aberration) instead of spherochromatism (variation of spherical aberration with color).  For the modern APO, however, the red, green and blue focuses can be made to be basically fall right on top of each other so what you have left are the various spherical aberration errors at those colors that dominate the "CA", not the focus "errors" and the "SA" may actually be considerable and a bit complex.    

 

However, if you really want to see the differences between APOs and achromats in DPAC, simply use a white light source.  The fast achromat will be a virtual kaleidoscope compared to the APO due to the color focus errors.

 

Jeff

[Jeff B]

How does this AP 92 lens compare to the AT92 lens?  Did you test that one too?  I guess the real test is how the in focus images look.

 

Bill

Actually Paul, you have or had a CFF92 right?

 

Jeff

[SandyHouTex]

I should also point out that my camera does not take pictures in red light very well.  When viewing the test in person, red looks better to the eye then it does in the photo.  My LED is creating a hot spot in both red and to a lesser extent in blue that the camera cannot compensate for.  That's why there is brighter/dimmer areas.

 

The overall correction of this lens is as follows:

 

Green - nearly perfect spherical correction.  Really, Really good.

Blue - very slightly overcorrected

Red - Slightly undercorrected

 

Also, don't forget that all errors seen are DOUBLED.  This is a very sensitive double pass test. Meaning they appear twice as bad as whats actually in the glass.

 

The slight circular polishing artifact seen in the null image in green and red (but not blue) is probably only 1/10 wave tall/deep.

 

This is a very good lens.

Actually that 1/10th wave polishing artifact is concerning to me.  It escapes me how that could have been left in there.

[Jeff B]

Actually that 1/10th wave polishing artifact is concerning to me.  It escapes me how that could have been left in there.

Seriously?  What about it do you find concerning? 

 

I see such artifacts like this all the time in DPAC, some even "worse", from other high end vendors.  They do not visibly affect image quality at all as they are typically in the center, cover little area and the glass does very little "work" on the light in the center compared to the outer portions of the lens.

 

Jeff

[peleuba]

Actually Paul, you have or had a CFF92 right?

 

 

 

Hi Jeff - Yes, I had a CFF92 and sold it   It was one of the finest small aperture telescopes I have ever owned.  It was an aspherized, oil spaced lens at F/6.  Mine has a Strehl of .997.  I sold it once I placed the order for the Stowaway.  CFF is a great company with great customer service.  I never once felt like I was dealing with an overseas (from USA perspective) entity.  The only downside is that its heavy for it aperture.    The CFF

 

 

How does this AP 92 lens compare to the AT92 lens or others in the 90mm range you tested? I guess the real test is how the in focus images look.

 

Hi Bill,

 

The AT92 is probably the best small aperture APO that is available off of the shelf.  I have owned 2 samples both with very high Strehls.  One used visually, the other for an optical-bench project.  These were also my first telescopes made in China that I have ever owned.  I was absolutely astounded at how good the lenses are and how terrific the focuser is.  In the dark you would have a difficult time telling the diff between focuser stock on the AT92 and a FT 2.5" R/P.  I recently told Mike Bieler how impressive these AT92's are.  Its certainly a tremendous bargain at its current price point.  I was really impressed and am currently contemplating an AT152 but I need to better understand the quality in the current production run.

 

The AT92 and the Stowaway share a lot in common from a design standpoint.  Both are air-spaced, spherical, FCD100 based lenses.  The Stowaway has better color correction at F/6.65.  The AT92 is F/5.5.

 

Anyway, if you can't get the Stow, get the AT92.

[Bill Barlow]

Thanks for the feedback, Paul.  I signed up for the Stowaway the next morning on 4/18 at 7:28 am.  Keeping my fingers crossed.

 

Bill