Problem using Astrodon 1.25” filters in FW8S-Aluma

Problem summary: 1.25” filter mount tops contact the housing or filter insert bottoms rub against drive chain and circuit board mounting screw.

Long description of problem follows:
I purchased an Aluma 694 with FW8S-Aluma new directly from Diffraction Limited in May, 2019. I loaded Astrodon 36mm filters for LRGB and Astrodon 1.25” Ha, SII, OIII using SBIG 36mm to 1.25 inserts. I initially loaded the 1.25” filters with the O-ring under the insert. However, the filter wheel would not turn under the housing when it was mounted because the tops of the 1.25” filters were hitting the FW8S-Aluma housing and the motor made a buzzing sound when the wheel would not turn. I changed the O-ring position to the top of the insert, and the filters rotated with no apparent initial problem in the FW8S when the housing was mounted.

I ran tests with the system indoors and outdoors over several weeks as I worked with the camera_FW8-S with no apparent problems. This week as I was working on a problem with my autofocus system during the day and the temperature rose above 95F, the wheel froze again and the filter wheel motor made a buzzing sound.

I shut the system down and took the housing off. The bottom edge of the 36mm to 1.25” insert projected about 1 mm below the filter wheel and was hitting a mounting screw of the FW8S-Aluma circuit board and was dragging across the plastic drive chain.

After the camera cooled, I noted that the bottom edge of the insert only dragged across the top of the screw and the drive chain. I noted that there were tiny flecks of plastic in the inside of housing from the bottoms of the inserts grinding material off the top of the drive chain during previous operation.

I am hoping that I am not the first person with this problem and that there is a simple solution.

Alternatively, it appears that either there needs to be a lower profile 36mm to 1.25” insert that does not protrude below the FW8S-Aluma filter wheel or there needs to be a smaller diameter O-ring that does not force the tops of the 1.25” filter mount against the housing of the FW8S-Aluma.

I’m looking forward to the possibility of your suggestions to solve the problem. Thank you

 

Thank you for responding so quickly.
Response to questions:
1. They are the same insert that you list. I purchased them from OPT where the item is listed as SBIG Filter insert 36mm to 1.25" Item #SB10105
2.
a. Total Top to Bottom height of filter cell: 10.50mm
b. Height of cell above screw thread: 7.67mm

 

The filter wheel jammed again, and I had to remove the 1.25” filters and inserts from the FW8S-Aluma to prevent further damage to the drive motor and other parts.

Before removing the 1.25” filters, I went through the process of trying to seat the filters into the insert deeper so that the tops of the filter cell wouldn’t hit the case when the O-ring was mounted under the insert. However, the tops of the 1.25” filter cells still hit the case when it was closed.
So I re-mounted with 1.25” filters with the O-ring on top of the insert. The bottom of the insert continued to rub over the top of the drive chain and a screw on the circuit board. During one startup process, the wheel jammed when the bottom of the insert locked on the screw.
I have included a photo of the marred circuit board screw (see arrow in photo) where it had been repeatedly hit by the filter insert and bottom of the filter. The drive chain is to the right of the screw in the photo.

 

Last night I removed all three of the 1.25” filters and the inserts and installed a scavenged 36mm Ha filter from my ST-8300 so that I can make some progress with my Aluma 694. The inserts have been removed from the 1.25” filters, and the filters have been boxed. My FW8S-Aluma is running smoothly now that I don’t have $1414 worth of new 1.25” Astrodon filters and SBIG inserts installed.

You will understand that I am not going to reassemble and re-install the 1.25” filters to make the photographs or do the additional diagnostic tests with the FW8S-Aluma that you request since every time I take the filter wheel apart, I risk damaging EXPENSIVE 36mm Astrodon LRGB filters. I have already chipped one of them running the problem investigation so far. You should have all the parts you need on hand to answer your questions.

I would be happy to mail you the three inserts for inspection. However, I doubt that this is a problem area. All three inserts protrude slightly below the bottom surface of the filter wheel when installed without the O-ring below them and drag across the drive chain. (Note added: I can see very fine linear striations in the finish on the bottom of the inserts where the surface is dragging along the drive chain. They are similar in overall thickness - 3.74mm, 3.76mm and 3.74mm. The depth of the inner ring that protudes into the filter hole varies from 1.29 to 1.38mm in the first, 1.24-1.34mm in the second, and 1.26 to 1.38mm in the third.) When the 1.25” Astrodon filters are tightly screwed into the inserts, the lowest part of the threaded cell protrudes slightly below the lower tip of the insert (<0.3mm). I think that it is the lower filter cell edge that is hitting the circuit board screw and causing the wheel to lock up. (Note added: Inspection of bottoms of the filter cell with a loupe revealed very small areas of finish loss down to bare metal. This is consistent with the interpretation that the filter cell is hitting the screw. Finish loss down to bare metal was not found on the bottom of the insert.)

The O-ring or another spacer needs to be installed under the 36mm to 1.25” insert to prevent the insert and filter from dragging over the drive chain and circuit board screw.

It seems to me that the most important potential variable that might be worth examining is the height of the filter cell above the screw thread for various 1.25 filter manufacturers. Do Baader and other 1.25” astroimaging filters have heights of the filter cells above the thread that are significantly less than 7.67mm? (Note added: I measured heights of filter cell above the thread on a Televue visual 1.25" filter. It was 7.79mm.)
Are there others reading this with available 1.25” astro-imaging filters who can help by measuring the filter cell height above the threaded portion for their filters and by reporting back?

 

Hi Colin,
Below I list the requested Astrodon part numbers for the 1.25” narrowband filters. I also give measurements of height of the cell above the screw thread for some 1.25” filters that I have on hand (mostly for visual use.) Finally, I provide photos with labels that show the filter and insert details that were described in my previous post.

1.25” Astrodon filter part numbers:
5nm Ha: SKU: HA5_27R
5nm SII: SKU: SII5_27R
3nm OIII: SKU: OIII3_27R

1.25” Filter: Height of cell above screw thread:
Astrodon 5nm SII: 7.67mm
Astrodon 3nm OIII: 7.81mm
Televue Nebustar filter: 7.77mm
Televue OIII filter: 7.90mm
Orion Moon filter (from early 2000’s made in Japan): 7.64mm

Photos pertaining to my previous post follow:

 

Note added: I need to add a note to clarify a potentially confusing sequence of events. The new 1.25” Astrodon filters that I purchased and installed have very poorly machined threads. They did not smoothly mesh with the SBIG insert threads. (The less expensive Televue and Orion filters mesh much more smoothly with the insert threads than do the Astrodon filters.) Two of the Astrodon filters were very snug in the inserts before the threaded lower tip protruded beyond the insert. These were run in the filter wheel for several weeks in a higher position than the filter insert lower surface which resulted in the finish striations on the bottom of the insert as it dragged over the drive chain. Only after posting the first entry in this thread did I go back and apply higher torque to the Astrodon filter cells to advance the threads and force the lower part of the cell beyond the lower level of the insert. The third filter had poorly machined, undersized threads and meshed very loosely with all three of the inserts. It protruded below the insert when screwed in at the start of the installation. It is the filter with the most prominent marks from hitting the circuit board screw. I know that this is potentially confusing. However, it explains why the fine striations from the drive chain occurred on the bottoms of the inserts from several weeks use instead being on the bottoms of the filter cells.

 

Thank you for the update, Colin
Because I have a need for reliable FW8S-Aluma function at a remote imaging site in the near future, I have sold the 1.25” Astrodon filters and SBIG 36mm to 1.25” inserts and purchased 36mm narrowband filter replacements.
Your ongoing research and possible product modification and/or revised guidelines for filter use should be helpful to future customers.