update on square filter anomalies and a bit of history of how we got here


Richard Crisp <rdcrisp@...>
 

Today I exchanged some emails with the gentleman that first reported the anomalous filter issue involving the AstroDon [OIII] square 50mm filters. This was Bob Anderson of Canada, a heck of a good imager and a darn decent human being too I might add.

I had been asked to help with the diagnosis of this problem by another person that was in the loop and the understanding I had at that time was that Bob was solely interested in a rapid resolution of his problem and wasn't especially interested in publicizing that it was his system that was having the issues with the AstroDon filters, what with filters being such a lightning-rod issue at times.

As had been disclosed earlier there is a zero cost fix that was proposed on Sunday for which I wrote a disclosure that I have since disseminated and perhaps you have seen it.

In the meantime Bob has returned to town from a business trip and I learned today and has no problem letting out that it was his system with the issue and is actually interested in being named as a part of this effort.

I certainly have no problem recognizing contributions and giving credit where credit is due and really am happy that Bob doesn't mind disclosing it was his system having the issue, because it allows a more complete story to be told and in my view the whole story is always more interesting than just the summary and the final result. I just wish he'd been around on Sunday when I did the writeup so we could have coordinated this a bit better, but unfortunately that was not the case.

this is Bob's system configuration:

TMB175 with custom flattener
FLI Proline 16803
FLI CFW5-7
Astrodon [OIII] and Ha 50mm Square filters.

The story of how this all came down is pretty interesting to me: as I mentioned I was contacted late last week to see if I could help. Up to that point I had not heard of any issues at all with the 50mm square filters.

But when I heard of this problem I had suggested to Bob that he try a number of things to gather experimental data including a mask that could be used to mask off signifcant parts of the filter wheel and filter. I had originally suggested that the mask overlap the filter and give a circular aperture. In that way reflections from the anodized surface would be eliminated as well as any edge effects of the filter including any reflections from non-AR coated surfaces, light leaks, and perhaps some unanticipated diffraction effects due to a straight edge.

Bob's initial results indicated the reflections from the anodizing were not likely the cause. Initially trying the full masking I had suggested he found that it eliminated the problem. But Bob took it one step further: he masked the edge only leaving the rest uncovered. He also found that eliminated the problem too. So he had a fix: leave the filter edges masked.

On Sunday I had some time available so I spent the afternoon reproducing his problem and trying experiements as I wanted to dig further into the cause and see what other solutions may exist. Unfortunately I didn't have the 16803 sized sensor, but had a KAF3200 available for testing using a Baader 50mm Ha square filter.

This is my system configuration:
Pentax 6x7 200mm f/4 Takumar lens
FLI PDF
FLI CFW4-5
FLI CM10 (KAF3200ME sensor)

The KAF3200ME sensor is too small to see the problems Bob saw on all four edges but it is big enough to exhibit it along one edge and that is all that I needed.

That one edge was sufficient to determine that I too could eliminate the problem by masking the edge of the filter. But there was a simpler solution that "just worked" out of dumb luck: flipping the carousel over in the filter wheel.

So I tried that configuration and saw that it was 100% effective in my system at eliminating the line.

So there's the story of how this got resolved starting with my involvement on Wednesday July 18.

I thought it was a fun exercise and just exactly the sort of thing that I enjoy doing and am happy that I got a chance to get involved.

there's a second reason for my interest in this and that was that plan to adopt 50x50mm square filters originated with me in conversations I had with Greg T and BobK of FLI back in early 2005 when they were first considering these larger sensors. I don't remember the exact date but it was in the late winter or early spring, probably Feb or March of 2005

I had raised the issue of filters to support this new class of large sensors and the anticipated high cost associated with either 72mm round or 60mm round emission line filters (about all I was using at the time were E-line filters, an at that time they were very expensive so I didn't want to see a csmera that needed $2000/filter hardware) and the fact that a lot of that filter area was wasted using round filters with square sensors. We did a lot of due diligence back in that time and I understand that Greg actually contacted a number of filter makers afterwards to test support for the concept. Greg can tell more of that story than me.

As late as Feb of this year, I had contacted Thomas Baader to alert them to the opportunity to make the 50mm square as I had been very very impressed with the performance of his other emission line filters and absolutely delighted at the very aggressive pricing compared to the status quo. So it naturally made sense for Baader to join in the party too, especially at the beginning of the production ramp. Why pay over a thousand dollars for a filter you can get for about $350-$400 from Baader? Obviously Thomas was keenly interested in this and sent me an early prototype which I used in my experiments.

Because I had so much involvement in the initial decisions to go the square filter route, I had a lot of interest in seeing it through to completion including any unanticipated technical issues that might arise.

As often happens, a simple upgrade often turns into a "project" and this apparently was no different. I am happy to see that it was only a short project and that everything being sold today by FLI and Baader is directly usable with no additonal parts or modifications; Just install the filter carousel upside down and flip over the drive sprocket. That's all it takes! Presumably the square filters made by others are equally usable with this idea but I haven't had an opportunity to try them. The one thing to be careful of is the thickness of the filters not being too thick. For example the Edmund scientific f50mm square filter I bought is simply too thick to work using the normal retaining mechanism. I think I could glue it into place with Silastic and may do that to test it compared to Baader. At $120 it is priced nicely but it may not give very good results due to them not using such great substrates.,

In fact one of the missing points in filter discussion is substrate quality. I plan to make an interferometer to test filters when I can devote the time. I have the optical flat now...

here's a link to the updated page:

http://www.narrowbandimaging.com/square_flat_anomalies_page.htm

enjoy, this sucker's in the freezer now!


Ray Gralak <rgr@...>
 

Would you PLEASE stop posting off-topic posts on this list.

-Ray

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
On Behalf Of Richard Crisp
Sent: Tuesday, July 24, 2007 6:22 PM
To: FLI_Imaging_Systems@...;
narrowbandimaging@...;
ccd-imaging-technology@...;
asa_Astrographs@...; ap-gto@...;
norcal_ai@...
Subject: [ap-gto] update on square filter anomalies and a bit
of history of how we got here
JUNK deleted...


dmwmpd <westergren@...>
 

Hi Richard,

I for one thank you for posting the information here since I am one
of many AP users getting into using a large CCD that requires 50mm
square filters. Using a simple no cost trick to make the filters
work better also help me use my AP scope and AP mount better too.

Don

--- In ap-gto@..., "Ray Gralak" <rgr@...> wrote:

Would you PLEASE stop posting off-topic posts on this list.

-Ray

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
On Behalf Of Richard Crisp
Sent: Tuesday, July 24, 2007 6:22 PM
To: FLI_Imaging_Systems@...;
narrowbandimaging@...;
ccd-imaging-technology@...;
asa_Astrographs@...; ap-gto@...;
norcal_ai@...
Subject: [ap-gto] update on square filter anomalies and a bit
of history of how we got here
JUNK deleted...