Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

Sign in to follow this  
joe1950

Opinion on Double Pass Autocollimation test results

Recommended Posts

Greetings from cloudy and cold NJ!

I recently had a lens replaced on a 102mm refractor, 1000mm FL, f/9.8, due to undercorrection of something more than 1/3 λ. The Ronchi lines seen in the double pass autocollimation test with an oil flat were significantly curved. The test is, of course, quite sensitive and allows one to see spherical aberration very easily.

The new replacement lens is far better in that respect and a definite improvement over the original. It also shows a measure of undercorrection and I'm wondering if anyone familiar with the test and reading the images would hazard a guess as to the approximate wavefront quality of the lens. Nothing precise, just an estimate would be very much appreciated.

Here are the specifics:

Lens: 102mm, 1000mm FL, f/9.8 achromat.  Ronchi grating: 150 lines per inch or 5.9 lines per mm. Oil flat, Double Pass Autocollimation.

DPAC test.jpg

The distortions in the shape of the images are due to imprecise alignment and the variations in the line thickness from uneven illumination and slight oil movement. Of course, the 3 image diffraction overlap of the test gives a false impression of the poor edge quality.

As I say, anyone who may have a knowledge and experience with the test and an idea as to what these results might translate to would offer an opinion as the the wavefront quality, or a range of same, I would greatly appreciate the information, for nothing else than curiosity about the new lens and a feel for the test.

I'm quite happy to have received a much improved lens and I am sure it will be very satisfactory for use.

Thanks very, very much! 

Joe.

 

 

 

Share this post


Link to post
Share on other sites

Hello Joe.  As you say, a double pass test is very sensitive as it doubles the observed error. On that basis I would have thought the lens to be very satisfactory. A wavefront  estimate  is likely to be subjective but a significantly better quality would probably be provided by one of the high end companies. It's worth bearing in mind that a lens needs to be only half the quality of a mirror to provide equal results.  :icon_biggrin:

Share this post


Link to post
Share on other sites

That's a very good point, Peter! And I certainly don't expect near the finish of a premium lens maker. 

I'm sure it will do very well once the skies clear. Even the first lens that was more undercorrected gave good views. The moon was detailed and sharp to over 200x and the Airy disk pattern was as good as one can expect - as is the case with the new lens.

I do try not to nit pick the λ or the Strehl ratio and such. One could end up spending more time on that than actually observing!

Thanks very much for the feedback, Peter!

All the best, joe

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By A_D05
      So I am fairly new to the hobby, what I mean is I have a Celestron Astromaster 114 right now but its hard to use because of the non computerized equatorial mount as well as the red dot sight is bad. I am mainly interested in looking at DSO’s because they seem very interesting. My question is should i get a refractor or sct for observing dso’s and sometimes planets? I want a computerized one with tracking so I don’t have to take a long time finding nebulae and galaxies. Also, is there a certain type of filter to see color on nebula when not using eaa and just viewing with your eyes? I am looking to spend between $600-900
      thanks, 
      drew
    • By jadcx
      TS Optics Photoline 90mm Triplet
      On reflection (or should that be through the lens of reality?) this was overpriced at £800, so is now reduced accordingly  
      In excellent condition, I gave a small writeup about this when I bought it, and it is still an excellent scope.  However it has been losing out to the 60 and 76 Tak and now spends all of its time alone, safely flight-cased.
      Don't leave this scope to suffer a lonely and unused life.  Buy it and catch some great views this winter! 


      Payment: PayPal (buyer pays fees) or bank transfer (preferred).
      Postage: Not included.  Collection from Nottingham, UK is free (of course), otherwise you will need to arrange your own courier.
    • By endlessky
      I have already posted my first astrophotographic session report in the telescope review thread: Tecnosky 80/480 APO FPL53 Triplet OWL Series - Review. But since that is more of a general review/diary of my experience with the new telescope, I feel some of the issues I am having are being buried and they will probably get more visibility if I post them - in a more synthetic version - in a dedicated thread.
      So, a few nights ago (October, the 5th) I took out my new telescope for its first light. All the photos have been taken with the 0.8x flattener/reducer and the Optolong L-Pro 2" filter attached to the reducer. The camera is an astromodified Nikon D5300. The only processing the following pictures have consists in this:
      - AutomaticBackgroundExtractor
      - ColorCalibration
      - Stretch
      Here we have a 90s shot of M31.

      And here's a mosaic generated with the AberrationInspector script.

      What I do like:
      - tightest, smallest, roundest stars I have gotten since I started doing astrophotography at the end of January. Obviously comparing it to what I have been achieving with a kit 70-300mm zoom lens, these can't be anything else but better by orders of magnitude
      What I don't like:
      - star shape not consistent in all areas of the image
      - residual chromatic aberration, especially on stars that are not round: there's clearly some red and blue edges visible
      I didn't expect this from an apochromatic refractor, but maybe it's just because the stars are kinda "smeared", so not all light is focused at the same spot? I don't see this around the center of the image (or, at least, the problem is less pronounced). Maybe I have some tilting in my imaging train/sensor?
      I have been doing some reasoning about it and it seems like a combination of tilting and/or backfocus spacing. According to the following image about backfocus spacing:

      if the stars are elongated radially, the sensor is too close, if they are elongated tangentially, the sensor is too far. But to me it seems I have a little bit of both: in the top right corner, for example, the stars look radially elongated, in the bottom right, they look tangentially elongated. Top left they look tangentially elongated, bottom left also, but a little less. Seems like there has to be some tilting as well, otherwise they would all have a symmetric shape on all corners, correct?
      How do I determine - is there even a way - if the issue is due to tilting only, backfocus only, or the combination of the two? Is there a sure proof way of checking for tilting? Like, rotating the camera and taking pictures with, say, the camera at 0°, 90°, 270° and 360°? If there's tilting, the pattern of the star shapes should follow the camera, correct?
      I also tried splitting the channels in R, G, and B components, doing a star alignment of the blue and red channels with the green as a reference, and recombining the channels. The blue and red edges become a lot less evident, which is good, but obviously the star shapes remain the same.
      In my Telescopius gallery you can also find two other images, Capella and Capella Mosaic showing pretty much the same issues.
      Also, one issue with the guide camera: ZWO ASI 224MC. When attached to the guide scope (Artesky UltraGuide 60mm f/4), I can't seem to get a "sharp" focus, I even tried on the Moon, and the best I got was a soft lunar disc, with some major features visible, mainly by change of color/brightness (the maria, for example), but no details. The image still seemed blurred/bloated. Is it because of lack of IR blocking filter? I tried the same camera attached to the main refractor, with the L-Pro filter (which blocks UV and IR, as well) and I could focus perfectly. Do I need an IR block filter for guiding or even if the stars appear a little soft, the camera guides just fine?
      Matteo
    • By endlessky
      I have been waiting for this telescope for almost five months. Since May, 19th, to be precise. The day I went to the TS Italia store and saw for the first time the SLD model, model now discontinued. I even missed the last available piece just for a few days, once I finally placed my order, June, 25th. It was to be replaced by a newer model, available at the end of the Summer.
      Boy, am I glad I did miss it. The wait was definitely worth it. The new and improved model is simply beautiful. I fell in love with it as soon as I saw it on the Tecnosky website a few weeks ago, when they posted the product sheet. But in person, it's even more beautiful.
      So, the people from the store emailed me Friday, October the 2nd, telling me that it was finally available for pickup. I read the message only a whole hour later and it was soon going to be closing time. I started calling at 4:30 PM and I finally managed to get my phone call through at around 5:05 PM. The store closes at 6:00 PM and doesn't reopen until Monday. And it's 40 minutes away from where I live. I made it there in 35. There was no way I was going to have to wait till Monday, knowing my scope was only a few minutes away.
      So, here's the pre-unboxing picture:

      - top left, brown box, behind: Vixen clamp for guide-scope
      - top right, white box: 60mm f/4 guide-scope
      - top left, white boxes: T2 Nikon ring, 30mm spacer, adjustable spacer
      - center, behind white boxes: Optolong L-Pro 2" filter
      - right of filter: spacers mounted and already calibrated for 55mm backfocus, for eventual use of the ZWO ASI 224MC camera with the refractor
      - top right, Bahtinov mask
      - underneath the white boxes, top left: Losmandy bar to attach telescope to my NEQ6 Losmandy saddle
      - big box underneath all of the above: Tecnosky 80mm f/6 FPL-53 OWL Triplet, with carrying case and 0.8x 4 elements flattener/reducer
      - ZWO black case: ZWO ASI 224MC guide-camera / planetary camera
      - front left: Talisker 57° North and two glasses (don't mind the shape of the glasses, they are the closest to Whisky suitable glasses that I currently own...) ready for me and my wife to celebrate the end of the wait
      - front right: box for the aforementioned Whisky
      I actually waited for yesterday (Saturday, the 3rd) for the unboxing, because I wanted my best friend Omar to be present and help me with filming and taking pictures. We have been friends since we went to kindergarten and we always have had astronomy as a common interest.
      It just so happens, to my immense surprise, that my telescope is actually SN. 0001, so I own the first telescope ever produced of this new series. The certificate is also very promising, with a Strehl ratio of 0.974 and a Ronchi test that seems very well behaved. I like a little less the red edges on the lenses, but I guess only time and a proper visual - and astrophotographic - session will be able to tell.



      Obviously the "new equipment curse" didn't help, but we got almost a whole hour with clear sky patches and obviously I couldn't pass up the opportunity. I quickly setup with the bare minimum necessities for a visual observation and me, my wife and my best friend Omar - who helped with the staging, recording and directing of the unboxing event - took a quick look at the Moon, Saturn, Mars, M31 and Perseus Double Cluster.
      I can definitely understand now, even if the seeing wasn't perfect, and my eyepieces didn't offer enough magnification (25mm and 10mm give me 80x and 200x, with my C8, but with a native focal length of 480mm, even with a Barlow 2x, we could only achieve about 38x and 96x, respectively), what people mean when they say that an apochromatic refractor brings out the objects from the background sky. The contrast was stunning, the stars were absolute points, pinpoint, small and sharp (with my C8 they always have kind of a "blob" feeling), the contrast on the Moon was fantastic and I could see many details, despite it being almost full, and only at 48-96x. I think it passed the visual test with honors. I was also very happy to be able to see the Double Cluster all in the same field of view for the first time. Saturn was well defined, could clearly make out the rings - don't recall, in all the excitement, rush and cycling between me, my wife and my friend, if I saw the Cassini division, but I'll definitely try again next clear sky night. Mars was also beautiful, could clearly see its rusty red color, the polar cap and some darker, black features on the surface.
      I really can say it's a beautiful telescope, very well made and machined. The attention to details is really of another level, the paint finish is very nice and matte. Also very lovely all the different red and black anodized surfaces, they really give it a nice finish and personality. The focuser is also the best I have ever had on a telescope. Very smooth, precise, with no backlash. Coming from a C8 where every touch of the focuser throws off the image all over the place and the backlash is quite significant, I really appreciated how easy it was to fine tune focusing with a proper focuser, especially with the 10:1 focusing knob.
      I can't wait to be able to take the first pictures of some star field, to check if even photographically the telescope lives up to my expectations. I hope to get pinpoint stars corner to corner and that the backfocus won't be something too hard to make perfect.
      Here's some accessories.

      Optolong L-Pro 2" filter, Bahtinov mask, Losmandy dovetail to replace the Vixen one the telescope comes with, Nikon T2 ring and spacers to use the ASI 224MC with the correct backfocus directly on the telescope, instead of a guide-camera.
      Here's the 60mm f/4 guide-scome, with Vixen clamp.

      And the ZWO ASI 224MC guide-camera.

      Here's the mandatory celebration beer, at Corte dell'Orso (the Bear's Courtyard).
      It's a Belgian sour beer, lambic style. Oudbeitje by Hanssens Artisanaal, with added strawberries. A very nice beer, sour, tart and fruity. Could definitely taste the strawberries.
      Cheers!

      Here's a couple of pictures of the full setup, with everything mounted on my Sky-Watcher NEQ6 Pro.
      The setup is in its astrophotographic configuration: mount, telescope, guide-scope, guide-camera, filter, flattener/reducer and at the end the Nikon D5300 astromodified. All controlled by Astroberry on my Raspberry Pi 4 4GB, conveniently mounted on a bar across the two telescope rings.


      And finally a close up of the rig.

    • By Dippy
      Baader optical wonder solution is practically Isopropyl alcohol. Instead of £12 for a 70mL of it, buy a 1000 mL of Isopropyl alcohol for £22 (before pandemic it was only £5). They have also smaller bottles which will be cheaper of course. The Baader solution and Isopropyl alcohol don’t remove the toughest of fungi on optics, only a few of the less deep set ones can be treated with them. I have used both for cleaning eyepieces and on certain stage of cleaning several 8 to 12 inch mirrors. They both worked identical. When applied through an optical cleaning fabric, they remove ( dissolve) fatty oils and fingerprints on optical surfaces. I had cleaned a 12 inch mirror once which for some unknown reason had ice cream stain on it (cleaning followed standard operation procedure for cleaning coated aluminised mirrors).
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.