33 replies to this topic

[Jeff B]

Being cloudy, cold and snowing, what better time to hole up downstairs in the bunker and do some more DPAC testing (!).  This time with my old AP 178 F9 pre-ED Starfire.

 

Visually, this cope gives really excellent views of solar system objects and an excellent star test as well.  It is a complete hoot for deepsky too.  However, I have noticed that the images of Jupiter and, to a lesser extent, the Moon are a little on the "warm" side.  

 

Will DPAC give me a little insight into this impression?

 

First, the set up. 

 

Ok, stop laughing. 

 

My work room is actually and old cistern under the dining room (driest room in the basement actually) with an excellent, stable, temperature environment and no air currents to speak of to much up images.  A great place for this type of work.

 

Jeff 

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[Jeff B]

So here we go.  Inside focus for blue, green, yellow-ish (has some red in there) and red (I'm getting better at messing with my phone's camera settings, specifically the white balance, which has improved my red images).  Hover the cursor over the image to read their titles.

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[Jeff B]

Now, outside of focus, same colors.

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[Jeff B]

Now at focus in green.  But also one with the shadow on the right and the other with he shadow on the left.

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[stevew]

It looks excellent Jeff, but who would have expected anything different from a classic Astro Physics.. 

[Richard O'Neill]

  I'm a bit surprised to see multiple zones in an AP lens, especially since it looks almost the same as my first mirror, a 4.25", ~ F/12. Overall this lens should deliver decent enough images.

 

 Before AP lovers start casting stones I will say that I've owned four AP triplet refractors and remain delighted with their quality and performance.  

 

Richard

[Jeff B]

Thanks Steve.

 

Right up front I see the lens is overcorrected in blue.  And even a tiny bit it green, and if I stare long enough, a trivial amount in yellow too.  Red is a tad undercorrected.  So, comparing the red and blue images, you see a bit of classic spherochromatism. Also, creeping in to the red image, maybe a little bit of a local turned edge.  There is a bit of roughness and the two circular zones, but remember, this is in double pass so it looks a lot worse than it really is and seems typical for this vintage lens. 

 

So, the sphero content in the blue may explain the warmth.  But not all the way really as I've seen similar amounts of spherical in blue but those lenses did not display the warmth I see in this lens, or, at least, not as much.  

 

One thing I did notice, was that blue had a subtly different focus than green through red, and very hard to measure.  So, I did some white light images.  The "white" LED (thanks again Paul) is really a combination of red, blue, and green LEDs.

 

Jeff 

[Jeff B]

Richard, this is in double pass, so it looks worse than it really is.  Also, while testing in yellow and red, the zones at focus were much less obvious and I could not reliably image them.  Roland has told me a couple of times too that these vintage lenses were corrected for yellow and, indeed, that is the case here.

 

So here are some white LED images.

 

Holly smoke!   Just look at the fringing and blue tint to the in focus image.  Now part of that blue content is the camera system as visually, it's not that blue.  But, yes, the blue is obvious.  

 

I think what's going on is that, in addition to that moderate sphero content in blue, there is indeed a little different focal distance to blue relative to green through the red.  These combined with the camera's predilection for blue, gives the obvious blue fringing and blue tint overlay at focus.  

 

This conclusion is supported by some very early DPAC images I did of this lens, one with an incandescent bulb, which has very little blue content to "pollute" the image and the other, which is over-exposed, with an LED light bulb, all with an early digital camera.     

 

But visually, this lens is exceedingly sharp.

 

Jeff

 

Edit:  BTW, the old white LED light bulb photo is labeled wrong.  It's inside focus.

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Edited by Jeff B, 08 February 2020 - 03:56 PM.

[Tyson M]

Thanks for sharing your analysis on this fine refractor! 

[starman876]

Love the double pass test in all the colors.  When I first had my DPAC all I had was a blue LED.  I figured if it passed in blue the other colors would fall in line. Later when I did start using a green LED the same lenses tested fantastic. Nice straight lines.  Got to love the double pass test.  Tells us real fast what kind of lens we have.  

[Jeff B]

So I redid the inside/outside of focus white LED images but spent some time adjusting the camera settings to get much closer to what I saw through the screen.  These shots are similar to the old LED light bulb shot but not as over-exposed.  Seems my phone camera likes blue.    But the lens's blue error is indeed real.  This makes sense as these older lenses were more visually corrected towards the yellow/red side while letting blue hang out there a bit, which, helps for the planets.  

 

Now I have DPAC images of a 6" F9 pre-ED Starfire of a similar vintage scope.  However, that one went back to Roland about 8-9 years ago for service, during which time he refigured the lens to be more "balanced" with best correction a bit on the blue side of green.  He said that's where he puts best correction today.  He also did that on a subsequent 5" F8 pre-ED Starfire I sent in for service recently, saying he thought he could do better than the original figure....and he did.

 

Jeff

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[CHASLX200]

Looks good.

[Eddgie]

Applause for your taking the time to do these tests and publish the results.

 

A few times over the decades AP has slightly tweaked color, or in some cases, even made slightly different telescopes.

 

In particular, the early 6' f/8 was made in two different versions.  One version was corrected more for visual use, while the other was definitely corrected on the blue side for imaging.  They looked identical, but anyone buying one used should try hard to ensure that the version they are buying is suitable to their needs. 

 

Now this was a very long time ago and the reason he focused on the blue side for the imaging scope was because at the time, film was being used, and films were not sensitive to red (remember the old red dark room lights) so the red blur could be enormous and one would not have to worry about seeing it on the film exposure.

 

Anyway, I have been enjoying seeing your tests.  I am hoping you can find some mass produced MCTs to test. 

[starman876]

Looks like a nice lens with some minor issues.

[vahe]

 

In particular, the early 6' f/8 was made in two different versions.  One version was corrected more for visual use, while the other was definitely corrected on the blue side for imaging.  They looked identical, but anyone buying one used should try hard to ensure that the version they are buying is suitable to their needs. 

 

 

I recall reading Roland's comment on this particular issue, he stated that most 6" f/8 refractors offered back in eighties were in fact visually corrected versions and that very few were made or corrected for film based imaging.

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Vahe

[Eddgie]

I recall reading Roland's comment on this particular issue, he stated that most 6" f/8 refractors offered back in eighties were in fact visually corrected versions and that very few were made or corrected for film based imaging.

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Vahe

Yes, in all the years I have only seen one photographic version of the 6" f/8 for sale.   

 

A buyer though should be aware of this.  It could be possible that one day one comes up for sale and the owner does not know it is the photo version. Now I have no idea of how the photo-version would work for someone wishing to do only visual with it, but I would assume that the difference would be enough to see visually.   

 

Again, I have only seen one of the photographic versions ever for sale.  I had the visual version and while I did not have a DPAC, using Suiter's method with a 33% obstruction, it was the most perfect star test I had ever seen.. It looked like the cover art on Suiter's book.  Super smooth.  The Intes Micro Deluxe Mak Newts (6" and 5") also showed perfection.

 

I know a lot of people dismiss the star test, but when you have done a star test for a perfect telescope, it really burns into your memory.  

 

DPAC Is better, but the analysis is not as easy (or at least not to me).  I can see errors like zones quite easily but quantifying errors shown in DPAC are beyond my ability.  I can see a less than great lens, but I can't tell how much less than great it is...

 

Still, I am hugely enjoying these tests and hope we see many more...

Edited by Eddgie, 11 February 2020 - 05:25 PM.

[fate187]

Isn't DPAC a qualitative test method, not quantitative? I think people also derive that from star tests... I need to practice more. And I need a proper flat for DPAC.

[vahe]

Yes, in all the years I have only seen one photographic version of the 6" f/8 for sale.   

 

A buyer though should be aware of this.  It could be possible that one day one comes up for sale and the owner does not know it is the photo version. Now I have no idea of how the photo-version would work for someone wishing to do only visual with it, but I would assume that the difference would be enough to see visually.   

 

FYI, Astronomy Magazine did a review of AP 6” f/8 back in 1986, it was a fairly detailed two page review, the scope was on loan from Astro-Physics and strangely the one reviewed was the photographic version.

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Here is a clip from that review:

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“The 6” f/8 Christen photoapochromat, while not perfect, displayed considerably less secondary spectrum than the f/15 doublet. This version of the lens, which has minimum focus in the blue, still combines the violet, green, yellow, and orange quite well. Red light, however, focuses further from the lens, forming a slight crimson halo around bright objects. Epsilon Lyrae, for example, was cleanly split, but each star was surrounded by a faint ruddy halo of light……......
A month after I returned the apochromat to Astro-Physics, I saw one of the 6” f/8 visually-corrected triplets at Stellafane. The visual version of the lens is virtually color free with no trace of red halo characteristic of the photo version………....”

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Vahe

[Jeff Morgan]

First, the set up. 

 

Ok, stop laughing. 

 

My work room is actually and old cistern under the dining room (driest room in the basement actually) with an excellent, stable, temperature environment and no air currents to speak of to much up images.  A great place for this type of work.

 

Jeff 

 

Perhaps you can tell me the problem with this DPAC set-up?

 

(story to follow)

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[Wildetelescope]

😂. Made my day!!

Jmd

uote name="Jeff Morgan" post="9978860" timestamp="1581542778"]

Perhaps you can tell me the problem with this DPAC set-up?

(story to follow)[/quote]

[Jeff B]

Perhaps you can tell me the problem with this DPAC set-up?

 

(story to follow)

Silly, the scope is upside down! 

[Jeff Morgan]

Silly, the scope is upside down! 

, yes I did not want to take the dovetail plate off.

 

But look closer ...

[Jeff Morgan]

My friend Gordon wanted to see DPAC in action. He suffered a broken knee while on vacation in England (and has some rather choice words for their medical system). When he got home, surgery was required and he is still working through physical therapy.

 

Instead of setting up on the oak table in the dining room, I thought it would be easier on his knee to set up on the taller kitchen island - forgetting that an hour earlier I had started the dishwasher. Max cycle, high heat, steam sanitize, heated dry. Look at all those green lights on the dishwasher. 

 

I was puzzling over why such a fine performer had bowed patterns at every focus position ... in every color. And then the dishwasher came to a stop between cycles ...

 

Doh! 

 

Should have known better, since in Dan Joyce's mirror shop we used to play a joke on newbies doing focault testing - placing a thumb on their mirror surface when they were not looking.

[peleuba]

I know a lot of people dismiss the star test, but when you have done a star test for a perfect telescope, it really burns into your memory.  

 

DPAC Is better, but the analysis is not as easy (or at least not to me).  I can see errors like zones quite easily but quantifying errors shown in DPAC are beyond my ability.  I can see a less than great lens, but I can't tell how much less than great it is...

 

Still, I am hugely enjoying these tests and hope we see many more...

 

If you search on these 3 words:  "Bench Test of", you will come across 4 (or more) topics I have started that include DPAC analysis of various telescopes.  Jeff has a bunch, too.

 

I also want to point out that both the star test and autocollimation are qualitative and are not a reliable means of giving an exacting wave front rating to an optic - estimates only. 

 

To me - for refractors - the star test, performed outdoors, is an order of magnitude more difficult to interpret then the test results I glean from autocollimation - for two primary reasons:

 

(1) Spherochromatism

 

(2) Atmospheric instability

 

To reliably star test outdoors you must have longish periods of good seeing which is not a common phenomena in my area or most of the U.S. -  Southern CA, Hawaii, Gulf Coast and Florida are three three major exceptions.  To combat the seeing issues, I built a collimator using an LED, a 5 micron precision pin hole and some aspherics which generates parallel light for an artificial star.  Indoor star testing is SO much easier to perform reliably and the results are repeatable - there are no thermal gremlins.  And, the star test views closely match what Aberrator displays.      

 

I am a true believer of testing in double pass.  Employing DPAC has helped me become a better star tester.  For DPAC, try using these two software programs:  Diffract and Ronchi for Windows to estimate the correction similar to what Aberrator does for the star test.

 

I have never once seen a telescope perform poorly in DPAC and be excellent outdoors in the field.  I have seen several telescopes that on a given night had crappy star tests but do well at other times.  Moreover, its not easy for even the best star testers to differentiate high order SA from a turned edge.

 

I have put together two interferometers - a Bath and a variant of the Michelson which itself is a variant of the Twyman-Green.  Each is not currently on my bench.  Rather, right now, they are on the shelf because they are finicky beasts and have been replaced by a DPAC test stand/flat combo and a collimator for star testing.   As I was telling Roland in a private email conversation a few months back, I am just glad when I actually get fringes to analyze regardless of the quality of the wave front .  I have come to the conclusion that testing in DPAC and then using the indoor star test is all I really need.  Interferomtry, while fun to play with can be a time-suck and a frustrating endeavor.

 

A good friend and professional astronomer/optics guru has created a star test device that utilizes a 5 micron precision pinhole, a beam-splitter and some auxiliary optics that, when placed into the focuser of the telescope under test, outputs a point source.  When placed in front of the optical flat, and viewed with an eyepiece through the beam splitter, you are able to star test in double pass.  I have the device and its really cool to use.

 

At the end of the day, while both autocollimation and the star test are qualitative, when combined they are powerful tools that compliment each other.  I perform both tests - almost exclusively indoors and results of both tests always agree.  The star test is best for astigmatism/coma/tilt/pinch/wedge and is OK for spherical.  But in an imported ED refractor, pure SA is often overshadowed by edge issues and spherochromatism.  Have you ever tried to determine whether an edge is turned or if the lens is suffering from high order SA?  In real world star testing to inexperienced eyes, they look nearly identical.  So, what happens if both aberrations are present?  How do you sort it all out when star testing in typical nighttime conditions? (rhetorical)

 

Ed - I really appreciate your dedication to the star test.  I have learned a lot from your years of posts going back to the AstroMart forums especially pertaining to star testing with obstructions.  And, I really enjoyed reading about your restoration of Buffy and your progression to using a Dob.  Fast forward to today you've almost got me convinced to apply for a HELOC and buy night vision gear .

Edited by peleuba, 12 February 2020 - 08:52 PM.

[peleuba]

My friend Gordon wanted to see DPAC in action. He suffered a broken knee while on vacation in England (and has some rather choice words for their medical system). When he got home, surgery was required and he is still working through physical therapy.

 

I know Gordon - he and I share love of Portaballs and TAK TSA120's.  

 

Here's a few VERY early DPAC tests in the kitchen.  Black and white film scanned in so I could post to CN...  

 

Notice the foam blocks and 2x4 wood used as shims under the telescopes...  we all start somewhere.

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Edited by peleuba, 12 February 2020 - 08:53 PM.