35 replies to this topic

[fate187]

Hello fellow DPAC addicted,

 

as some have noticed I was finally able to aquire two flats for DPAC testing of my scopes. I have been looking for flats, but usually they were extremely expensive or downright not available. I tried a flimsy (thin ~5mm) mirror from Edmund optics that was just garbage for the purpose of DPAC. For my smallest scopes (Tak100 and LZOS123) I was able to use my Baader front ERF with 100mm free aperture. Together with Ronchi gratings from ronchiscreens I made some very early attempts towards this testing technique. Now, even with those new large flats (8" with 20mm thickness and 13" with 25mm thickness) I am far away from a professional set up, but it is still good enough for the proper evaluation of the scopes and just learning how this technique works. I use a headlight with white LED to shine through the screen and focuser to the lens, bouncing back on the mirror and then observe near prime focus the ronchi pattern. I mostly used a 200lpi ronchi screen until otherwise noted. Photos were taken with an iPhone 8 camera, mostly hand held. Irfan view was used to separate the color channels to R, G and B, respectively. This is very handy, because you can make one photo and get all (most) of the information in one shot. This reminds me, that white LEDs have large blue spectral output, so blue may be emphasized and your results may vary.

 

Ideally, I would like to use this first post as a landing post and add new measurements in due time. I started "proper" evaluation (with photos) some days ago. Some results made me scratch my head. I contacted Jeff B and peleuba (aka Paul), who helped me a great deal understanding what was going on and how to interpret the results. In the past, I read a lot of their posts regarding DPAC and tried to scrutinize the information out of their measurements and explanations.

 

Following the aquisition of the flats, I started to test all my scopes with the 8" flat, followed by the same procedure with the 13" flat. Some measurements were repeated to be sure first results were correct. I am a physicist by profession and things have to be done correct and in proper order, right ?

With the 13" flat I probed its surface with the Tak100 so to say, in order to locate defects on the mirror. Defects may give rise to false results when testing larger lenses. So I mapped the flat (thanks Jeff B for the heads up) and only one defect showed up, which I marked on the back side. This showed up in the Tak100 as streaks:

Frist the back of the flat:

And the null image with the defects just barely visible going from 2 to 8 o'clock position.

The visibility of surface features or defects largely depends on the position of the ronchi relative to the focal point. This is something I learned during the process. With the 200lpi ronchi you only have to slightly tap the screen to move it and make visbility good/worse. You have to play around with it to get the best result.

 

Lets start with an introduction to the scopes:

• Tak100DC: a fluorite doublet. A marvelous grab'n go scope. I used this lens during summer holiday and enjoyed the best views on Mars I ever had! Under pristine black skies I was able to see nebulae and other deep sky objects, that I can't remember having seen this way with my 12.5" dobson some 20years ago, when I had the dob as a kid.
   
• LZOS123: A very fast F/6 triplet system. This was my second scope, that I aquired after starting a new with this hobby 7-8 years ago. My first reentry scope was the SW 80ED Evostar. I use this triplet almost exclusively for imaging. Most of my pics on astrobin have been made with this scope. It is a gem, that will not go in any near future. The previous owner once called it "a perfect lens". Strehl according to LZOS 98.6%.
   
• LZOS130: This is the slightly larger brother (or sister) of the 123, but at f/9 its a planetary performer. This scope was aquired during the wait for the CFF185 and not having a TEC140 (which I also own previously, but foolishly sold). The lens comes in a Kruppax tube with 2.5" FT forcuser. I have to admit, that I have not used this scope a lot in the past. Due to its long moment arm, the iOptron AZ-Mount is wobbling quite a lot.  And the shorter Tak und even the TEC140 are easier to use with this mount. I now have the ability to use this scope more with my AZEQ6 though!
   
• TEC140ED: I had a TEC once. And sold it . I was able to buy another one in pristine condition and I will not part with it anymore. I regard this scope to being the most versatile among my scopes. With the TEC field flattener you can photograph very large fields (needs camera), get a 0.72 reducer like the one from AP and its faster than the LZOS123. And it is a good planetary scope, being used a lot with the bino during this year Jovian season.
   
• CFF185: "One scope to rule them all" . Not quite. Apos get large and heavy very fast. And I have to admit, that I have not used this lens as much as I would have liked to. Only very recently I moved, so I now have the space for proper set up and observation. Funnily, I bought this telescope for AP. Catalin from CFF asked me how they should tune the lens. During that time I said 80 AP , 20 visual. Today it would be vice versa, because I started to admire the planets and visual observation in general. I do live in a large city, so visual observation is not easy. But with double stars, planets, cluster and even star testing you take what you can.

I am planning to evalute the scopes with different accessory, that I routinely use. I really want to understand the influence of additional optical parts on the overall performance. That being the Gutekunst ADC, Zeiss Apo bino + Baader GPC, Baader BBHS prism/mirror diagonal. Additonally, the TEC field flattener will probably be interesting, because it is said to shift color correction towards blue, due to sensor sensitivity. I will add those measurements, when they are done.

 

Please add you thoughts and comments. Also notice, if there is something done wrong.

 

Let me start with the Tak100 results. The images obtained were nearly identical with the 8" and 13" flat. Hover over the images to see what is what.

Red:

Green:

Edited by fate187, 29 October 2020 - 10:22 AM.

[fate187]

I continue with results on the Tak100 in

blue:

And finally the null images obtained with the grating being at focus. I read, that proper null is usually done with a knife's edge. But I have yet to built such a set up.

Null RGB

Red

Green

Blue:

 

With the help of peleuba and Jeff B, I learned, that this is a great lens, with a very smooth surface (no features showing up in the null image) and a very very good correction in green, but also in red in my opinion. This is due the lines being almost straight inside and outside of focus. Slight overcorrection in blue. You may also notice, that the lines are slightly shifted in blue with respect to red or green. It is easiest seen with the most right/left bars in the test image. This hints towards focal shift:

Red inside vs blue inside

Notice how the bars on the edge are more visible for the blue than the red.

 

The DPAC results are manifested under the sky during star test or double star observation. Easy super high magnification with my Vixen HR line eyepieces from 3.4mm down to 1.4mm possible .

 

More images/scopes to follow.

Edited by fate187, 29 October 2020 - 02:55 PM.

[donadani]

Hi Michael,

 

I knew that lens is great first time I looked though it at a startest... looked through many but this one was really good - no DPAC needed to know - errrr - can I have it back please!

 

Curious about the other lenses

 

cs

Chris

[fate187]

Yes Chris, good lens. No Chris, you can't have it back, sorry .

 

Before I continue with the LZOS123 f/6 there is one term I would like to explain in this thread: Over- or undercorrected lens. I hope I did not mix that up. It essentially means, that for a certain color the focal point is shifted towards the lens (undercorrected) or is further away from the lens (overcorrected).

See this thread from peleuba explaining the terms: https://www.cloudyni...ges/?p=10038184

Spherical correction is usually optimized for green, which means green should have straight bars inside and outside focus. For the "other" colors, the point of focus will be shifted towards the lens or further away from it. The Ronchi bars observed in DPAC will show deformation depending on the position with respect to focus and whether the lens is over- or undercorrected. For undercorrection and being inside focus, the outer bars are bend concave (diverging from the center).

 

Back to LZOS123 f/6:

Red:

Green:

Blue:

For completion the white light shot:

 

And the null images:

RGB:

R/G/B:

 

The surface is similarly smooth as the Tak100. However, you will notice that this system is undercorrected in red and overcorrected in blue. In green the bars are very straight, with a slight hint of undercorrection.

[Haydon]

I'm looking forward to seeing the results on your TEC.  

[Scott in NC]

I'm looking forward to seeing the results on your TEC.  

Same here!

[fate187]

People are always so TEC focused these days . I wanted to continue with the LZOS130 f/9, but let me post results on the TEC instead first.

 

TEC140ED:

Red:

Green:

Blue:

RGB:

 

Null:

RGB first

R/G/B:

My on the TEC: overcorrected, but with very straight bars in red. Please be my guest on numbers regarding correction in terms of waves...

The surface is very smooth with a small bump at 4 o'clock position.

 

Again the TEC performs very well visually. I used it alot during this years planetary season with the Zeiss apo bino and Gutekunst ADC. At this moment it is fitted with the dedicated field flattener. The Tak is a little bit envy . But stars are super tight across the (smallish Atik 460) frame. The field flattener also shifting correction towards blue, due to the sensitivity of modern cameras. I will test this combination later and post results here.

 

This was the TEC during DPACing:

 

Thanks for looking

Michael

[peleuba]

I am so happy to see another member (other then Jeff and I) post these test results.  

 

In the past, I've taken some heat here on CN (both in PM's and in the forums) for being overly critical/argumentative with regard to telescope performance.  Its also been said I prefer to test a telescope on a bench rather then use it under the night sky.

 

Fair enough.  Though, rest assured I like using my scopes.

 

There are compelling reasons for amateurs to learn how to test and then post their test results.  It will confirm (or refute) the quality of the optics and it helps others who are interested in  learning about optics, test methods, wave nature of light etc.  There is absolutely no downside to testing a telescope then posting about it.  No matter the brand, cost, or design of the telescope in question.    

 

Most of the tests you see on the Refractor Forum showing alternating light/dark bands - sometimes in different colors - are simple Ronchi tests performed in Autocollimation.  These tests are often referred to as DPAC (Double Pass Autocollimation) which I always find funny because its redundant.  Any optical test performed in autocollimation is, by definition, performed in double pass... 

 

Typically the test is performed using a Ronchi grating at focus.  The Ronchi test, over the years has been maligned as not being a sensitive enough test to be reliable.   And its true -  testing a telescope at night using a Ronchi eyepiece looking at a defocused star is grossly insensitive - especially in the typical seeing that most of us live under.  However, in autocollimation (double pass) indoors on a bench, this test becomes extremely useful/powerful.  All the errors are doubled the test can tell with certainty if your optics are closer to ¼ wave or ⅒ wave.  Though it cannot stipulate a quantitative value for the lens/mirror - only data reduction software crunching output from an interferometer can do this. 

 

How many times have you heard or read that XYZ refractor has "perfect" or "text book" Fresnal (star test) pattern?  I see it here, I see it in the "reviews" in Astronomy and Sky and Telescope  In the real world, there are no such things as perfect star test patterns outdoors in typical seeing with typical optics.  Even in the unlikely event the optics are perfect, unless its a long focal length triplet (or modern Petzval), there will be chromatic effects that cause the diffraction pattern to be unbalanced/dis-similar.   

 

Star testing is easy to do, but can be difficult to interpret as most lenses suffer from multiple aberrations that manifest themselves within the star test.  It can be a chore to discern what is really happening.  But, when combined with the the autocollimation test, a powerful set of tools are now available to the amateur.  When these test methods agree, you can begin to make reasonable assumptions on the goodness of an optic.

 

Each test has its strengths and weaknesses but when combined and in agreement can give a very accurate qualitative assessment of absolute performance.  I like to use the star test to judge coma, astigmatism zones and spherical correction (in that order).  Autocollimation is great at assessing spherical correction, zones, surface condition, focus shift as a function of wavelength and astigmatism (in that order).  These two test are complimentary 

 

Finally, two things struck me when I was learning to perform autocollimation testing.  (1) I have never seen a telescope that did great under test perform poorly under the stars.  (2) There are few sources of errors when testing in autocollimation.  In other words its easy to do with a high degree of accuracy.

Edited by peleuba, 30 October 2020 - 07:43 PM.

[fate187]

Paul is absolutly right. After setting up the test, probing the scopes and doing additional reading here on the forums and telescope optics site made me finally understand how the test works and why there is certain bowing of the ronchi bars, depending on the optic's properties, good or not so good . A very satisfying experience, after reading quite often about this method and wondering what was going on.

 

Star testing is still something I need to learn. It may be more difficult for me, because weather condition need to play along to get consistent results. With DPAC you just set it up and repeat. As long as temperature is stable and the optical set is square, the results will be identical. Cannot do that so easily during star test imho.

 

Another problem with star testing is, that I don't know what to look for and that I lack a proper bad sample , in order to see a difference. I do have the Suiter book though.

[gregbradley]

What is DPAC?

Greg.

[fate187]

Double Pass Autocollimation. Read Peleuba's post #8 in this thread

[Scott in NC]

What is DPAC?

Greg.

You can read about it above, but it stands for Double Pass Auto-Collimation.

[gregbradley]

Thanks Scott.

 

Greg.

[fate187]

Ok next scope in the line was the LZOS130 f/9 before the TEC140ED was press forward .

 

LZOS130 f/9:

RGB:

Red:

Green:

Blue:

 

Null:

RGB

And the separated channels:

R/G/B:

 

It is quite obvious, that the surface is rougher compared to the Tak, LZOS123 and TEC140. Inside focus you will also notice, that the bars have a "J" shape towards the edge. This hints towards a turned down edge ("tde"). I will try to find out how far this edge goes towards the center of the lens. I already got a nice method from Jeff B to evaluate this. Despite the rough surface and the additional small central zone, you will notice, that the lines inside and outside are very very straight. This means, that spherochromatism is very well controlled. Only in blue there is a very small hint of overcorrection.

 

This scope was bought for planetary observation. I need to use it more regularely. It performs very well with the bino and does not need additional GPC, because it has loads of backfocus .

Edited by fate187, 31 October 2020 - 11:35 AM.

[David Lim]

Hello Fate187, thank you for starting this thread and all the valuable information that you have shared with rest of us.  I am enjoying this very much.

 

Even though your LZOS has rough surface areas under Null and turned down edge, it has perfect (to my eyes) color correction which is very impressive.  I have LOMO 80/600mm (Russian) which also has similar rough surface area like yours under knife edge test.  I will post it when I get a chance.  In meanwhile, I will share some of my own results below.

 

David

[David Lim]

Scopes: FC100 circa 1991 and TSA102 circa 2006 comparison

Knife Edge (Foucault) is a slitless setup.

Ronchi screen: 250 LPI (10 lines/mm) from Edmund Optics

 

 

The knife edge null shows some subtle surface roughness on both scopes but the general appearance is flat without much hills or holes.

Both scopes have slight spherical overcorrection (Ronchi).

TSA has better color control than FC100 as expected (triplet vs doublet).

Blue correction is the worst in both scopes (see next post).

Slight astigmatism is due to my setup.

Edited by David Lim, 31 October 2020 - 03:31 PM.

[David Lim]

Continuation of FC100 vs TSA102

 

 

[fate187]

Hi David,

 

thank you for the results on your scopes. A proper focault or knife edge set up is something I have yet to realize. But time will come.

 

Before venturing into the realms of the large CFF I would to show what can happen to a test, if your mirror is not up to the task. Initially I got an 8" flat, that I quickly used to DPAC everything from TAK100 to CFF185, but the TEC140, because the TEC was fitted for AP including flattener and camera equipment.

 

Testing the LZOS130 and CFF185 was a real bummer with this mirror. However,I was not aware of the mirror/flat being the issue, so I thought I have to very bad lens samples .

 

Please compare these exemplarely selected images inside focus:

Red: 8" left, right 13" flat.

Green: 8" left, right 13" flat.

Blue: 8" left, right 13" flat.

 

Notice, that there is this lens seemed to be a lot more overcorrected. . 

To make things worse I also tested the large CFF. I selected those images to clearely show, what a bad flat will do to your lens. This is not reflecting the true optical quality.

Blue:

And null is making things worse:

.

It was only after I got the 13" flat shortly after, that I was able to identify the 8" flat being the problem. I tested the Tak100 and immediately saw what looked like a similar zonal error with the TAK, that tested perfect on the 13" flat. It also tested well with the 8" flat not being radial centered to the flat.

I deducted, that the 8" flat has a large zonal error in the center and it only good for small optical systems being tested radially more towards the edge of the flat. The 13" flat is much better optical quality, and all results posted this far where done with this flat.

What I want to emphazise with this, is just be careful with first results you get from a test set up. Scrutinize issues and look for problems causing them, before doing final judgement .

[fate187]

Ok final scope is now the CFF185, tested with the large 13" flat .

 

RGB first, inside/outside:

Red:

 

Green:

 

Blue:

 

Null RGB:

And separated channels

R/G/B:

 

This lens is undercorrected in red and green (slightly?) and overcorrected in blue. Still spherochromatism (line bending) is well controlled.

Due to the size of the lens and aspherization done to it at CFF the surface is not as flat as a my Tak100 or TEC (or LZOS123). Photographically, I am very satisfied with this scope and I am more and more liking it for visual purposes as well. It has a lot of backfocus. You only need a 1.25xGPC with certain eyepieces requiring more back focus. This in combined use with the Zeiss apo bino and Gutekunst ADC, so lot back focus already use.

 

This concludes the "basic" test of the scopes have. I will continue testing each lens with different visual backs (bino, ADC, flattener) to learn about the influence on the optical system.

[jahnpahwa]

this was a really enjoyable and informative thread! Thanks so much for taking the time

[David Lim]

Michael,

 

Thank you for pointing out about the optical flat.  I bought mine used from a seller and he stated the optical flat (6 inch Zerodur uncoated) came from Lockwood and had <1/10 wave.  I don't have any way of verifying the quality of my optical flat.

 

You mentioned in initial post (#1) that you "probed the surface" of your flat with Tak.  Can you please explain how you did this?  

 

Any other way of testing optical flat would be greatly appreciated.

 

I have been in search of larger size optical flat 8-12 inch on ebay but the quality of the flats are in question and new ones seems astronomically expensive.

 

David

[fate187]

Hi David,

 

What I did can be seen in the first picture of my first post here in this thread. I used the Tak100 (with really good lens surface) and did DPAC null testing on various points on the flat. You see the "systematic" pattern (encircled) the areas I covered with one test area. After that I moved the mirror a bit (turned it to be correct) and then did the same null test again looking for features, that were not there before. Since the Tak100 is flawless, I presumed it had to be the mirror surface. In fact I found some streaks (also shown in post one) in a distinct area of the mirror. I marked this area.

I didn't test the 8" flat, because I was not aware of it being an issue. Test results looked good with the Tak100 and LZOS123. Only the CFF was really bad. I thought it was really the lens, due to the symmetry and the distinct pattern. But using the Tak100 centered on the 8" flat (just as I have naturally with the CFF due to the size) I immediately saw the same flaw in the small Tak as well.

Obviously the 8" flat is good for testing of small lenses (< 4-5") with the scope being placed of center to the mirror.

[David Lim]

Michael, thank you for the explanation.  That was helpful.  Thank you!!  

[Jeff B]

this was a really enjoyable and informative thread! Thanks so much for taking the time

Ditto!

[Tyson M]

Awesome testing. I love seeing this kind of work done by amateurs to get a feel for sample variation with various brands.

Well done.