filter astigmatism problem
Posted 02 February 2013 - 07:29 AM
(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.
So is there absolutely anything I can do with this piece of glass?
Posted 02 February 2013 - 12:33 PM
Everyone needs to know .
The shame of it all !
Posted 02 February 2013 - 12:54 PM
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.
Posted 02 February 2013 - 01:28 PM
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):
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.
Posted 02 February 2013 - 03:38 PM
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...
Posted 02 February 2013 - 10:33 PM
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.
Posted 03 February 2013 - 02:23 AM
Posted 04 February 2013 - 10:53 AM
Posted 04 February 2013 - 11:01 AM