Jump to content


Photo

filter astigmatism problem

  • Please log in to reply
8 replies to this topic

#1 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5266
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 02 February 2013 - 07:29 AM

so i got this Omega Optics 2" H-A filter with 10nm bandpass for....


$100.


(used) couldn't resist when the equivalent Baader is $300.

Well... it has terrible astigmatism. Even after loosening the filter retaining ring and everything. Glenn (?) said it's probably in the substrate.

The seller claims it worked fine for him - on his SCT - and that he already over-spent sending it to me (he used FedEx, when I was expecting USPS) so I expect no joy there. Sending it back would probably cost $20.

Here's a single 10-minute sub through the filter. Look at 'em funky stars! there also is that weird bright arc above every bright star. Even the dim stars have it, if I stretch the contrast enough.

Posted Image

So is there absolutely anything I can do with this piece of glass?

#2 starrancher

starrancher

    Mercury-Atlas

  • *****
  • Posts: 2960
  • Joined: 09 Jun 2009
  • Loc: Northern Arizona

Posted 02 February 2013 - 12:33 PM

Who is this rip off artist that sold you that junk ?
Everyone needs to know .
The shame of it all !

#3 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10487
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 02 February 2013 - 12:54 PM

Before getting all wrapped around the axle, test the filter, along the lines I outlined in your other, related thread. I find it potentially suggestive that the three visible geosat trails are curved. This suggests poor tracking or flexure, which could present as an astigmatic elongation on your stars.

For an element lying quite near the focus, the light cone for any one image point is very small. That such a tiny area of the filter should so aberrate, and to the same degree across the image, would imply a substrate having a rather wicked cylindrical curvature on one or both faces.

Hold the filter in front of a 50mm bino and examine any suitable point source, such as Sirius or a distant streetlight. Then rotate the filter to see if any aberration rotates with it (as much to eliminate your eye and bino ad the cause.) This is a sensitive test, as it utilizes the full aperture, and so high power is not required. If this test reveals no obvious flaw, the filter will perform very well when placed near the focus.

#4 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5266
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 02 February 2013 - 01:28 PM

Glenn, those are not geosat trails (I assume you mean you think they are satellites?)

Those trails are above every bright star. Rotating the filter moves the orientation of those trails but they are always near the stars.

Here's another fine example (please ignore the dirt on my sensor):

Posted Image

I just tried the binocular test. I can't see anything through the filter.

I thought maybe the filter cell was distorted even with the loose ring, so I swapped the filter into another (known good) filter cell from my IDAS LPS-V4. No joy.

I also tried using the sensor with the Meade DSI, hoping the center of the filter might be better than the edges. Same thing. The thing is darn useless... then again all the stars are diamond-like, maybe there's some software that will round them or something.

Really annoys me though that after spending so much money to get round stars, this cheap filter ruins it.

#5 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10487
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 02 February 2013 - 03:38 PM

Oops! Had I made note of the long exposure time I would have instantly realized those streaks could not be geosats; the exposure would have to be measured in seconds, which of course would not have revealed nearly so much. Must be my cold and bronchitis fuzzing my brain; that's my story and I'm sticking to it! :grin:

The streaks are clearly reflections, then. Odd how they've got so greatly elongated!

How far from the focus is the filter located?

What is the f/ratio?

These answers might provide clues as to the degree of misshapenness of the glass, which now appears to be considerable.

In lieu of a star, how about looking at a suitably bright light, such as a street light, through a bino? Even if nearby and well resolved, details will either be sharply seen or not. And again, rotate the filter to truly ascertain its being the culprit.

I'd be most interested to learn of the result...

#6 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5266
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 02 February 2013 - 10:33 PM

hi Glenn,

the filter was screwed onto the 2" nosepiece of a refractor field flattener. So it would be about 70mm from focus. the refractor is an f7.

the only place i can think a reflection would come from is the surface of the flattener lens. but my IDAS filter is just as shiny and i don't get reflections off it...

anyway let me try your suggestion.

#7 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10487
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 03 February 2013 - 02:23 AM

Based on the f/7 light cone and the ~70mm distance ahead of focus, the light cone for any one image point would be ~10mm wide where it transits the filter. If such egregious defects are indeed caused by the filter, and are made manifest by such a comparatively small portion of its aperture, implies a quite ghastly substrate. The bino test should confirm.

#8 John Boudreau

John Boudreau

    Viking 1

  • *****
  • Posts: 855
  • Joined: 06 Apr 2008
  • Loc: Saugus, MA

Posted 04 February 2013 - 10:53 AM

If it's a laminated filter, it's possible that the substrates are starting to separate. I had just that happen to a Schuler UV filter a few years ago and it developed strong astigmatism.

#9 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10487
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 04 February 2013 - 11:01 AM

A narrow-band dielectric filter should not be laminated; it relies on solely on the multiple coatings to effect the desired filtration.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics