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FLATS ISSUES CONTINUED - FLOCKING MY FOCAL REDUCER AND BLACKING MY FILTER BEVELS

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#1 cfosterstars

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Posted 31 January 2018 - 05:06 PM

I have posted a series of threads on my efforts to improve my flat frames to eliminate artifacts in my imaging. This data is mostly in the following post:

 

https://www.cloudyni...ring-artifacts/

 

Most of this thread discussed edge of filter light leakage issue with the ZWO 36mm filters and all the work that was put into resolving that issue. I then purchased a set of 36mm Astrodon filters and did a comparison to the ZWO filters and that data was covered in this post.

 

https://www.cloudyni...ontrolled-test/

 

The  conclusion of these two posts was that by either masking the edges of the ZWO filters or going to much better Astrodon filters that are fully coated to the edges you can eliminate the serious edge of image "vignetting-like" patterns in flats that is uncorrectable with out serious post processing of data. By masking the ZWO filter or using full-coated, better quality (and more expensive) Astrodon filters, I now can get FLATs that are good enough so that the artifacts can be removed in post processing. 

 

In other words, although the flats still had ring patterns, I have no uncorrectable issue with these flats calibrating my light frames. With the ZWO filters with all the edge-of-filter masking or with the AD filters that were fully coated, my calibrated light frames show no edge color rings that I could not correct "easily" with Pixinsight. Are they perfect? Well DynamicBackgroundExtraction (DBE in Pixinsight) is a wonderful tool. To my eye, I had good results with my ZWO filters and my AD filters - they were good enough. What remained in the flats were very weak (~1-2% intensity variation) concentric ring patters only visible with an aggressive screen stretch. Without stretching the flat the flat was -- well -- FLAT. You cant seen anything in my current flats without strongly stretching them. 

 

However, I wanted to see if I could fully eliminate them. I was convinced that the ring patterns - as weak as they were - were still the effect of the remaining reflections in my optical path. Flocking my focuser draw tube was my first attempt to determine where they were coming from and whether I could fully eliminate them. The results of this work was shown in this most recent post:

 

https://www.cloudyni...cuser-drawtube/

 

Did I need to do this - probably not. Good enough is good enough. However, that said, the previous reflection and filter issues show that reflections are non-linear effects. And even if weak - too weak to really notice - they are still there. I don't think it will hurt anything to fix them. The less I have of these artifacts, the less I have to do with DBE. At least that is the theory. This post will show the further step I used to eliminated them. I applogize up front for the long post and the number of breaks. There is a lot of data here so bare with me.

 

Even after flocking my focuser draw tube, reflections in the optical train of my system were clearly visible: This was through the OTA, focal reducer, filter wheel and LUM filter with the flocked focuser

 

Flocked through LUM
 
There is still a bright ring of reflection from what looked like the focal reducer and a ring on the filter edge. I believed that the ring of reflection on the filter was due to Astrodon not blackening the bevel edges of their LRGB filter (36mm E-series). The source of focal reducer reflection was not so clear cut. So I took down the optical train for inspection.
 
These are a set of threads internal to the Williams Optics Flattern IV that look a lot like the threads that were on my focuser draw tube:

 

Focal reducer threads
 
Same issue - same fix. I was able to apply flocking to these threads:
 
flocked FR threads
 
There also appeared to be some reflection from the inner opening of the reducer flange so I also applied flocking to that surface:
 
reducer flange flocking
 
I then reassembled the focal reducer and looked at the reflections. There was a marked improvement:

 

Through the F/R after flocking
 
More in the next post..
 
 

 



#2 Kaos

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Posted 01 February 2018 - 01:53 PM

Do the uneven edges shown in the last image create any shadows on the sensor or any other artificats? I order the flocking material from scopestuff and an going to attempt to flock my Hotech Self Centering FF. I plan on starting there since you seemed to get the best results from doing that. I may end up doing the draw tube as well. One other question is there any real risk of the material flaking over time creating dust issues?

 

Thanks

 

Kaos



#3 cfosterstars

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Posted 01 February 2018 - 02:31 PM

Do the uneven edges shown in the last image create any shadows on the sensor or any other artificats? I order the flocking material from scopestuff and an going to attempt to flock my Hotech Self Centering FF. I plan on starting there since you seemed to get the best results from doing that. I may end up doing the draw tube as well. One other question is there any real risk of the material flaking over time creating dust issues?

 

Thanks

 

Kaos

Yes they did. I have not posted it yet, but I went back and re-did that flocking to remove the overlap and the issue is fixed. This wont matter for the draw tube since it is way outside the beam path, but for the flange - since it is a choke point, it does matter. I dont think that this will flake, I have found it to be very durable so far. You do need to be neat in trimming the edges. I also took a sharpie to the edge of the paper backing so that there was no white showing.



#4 cfosterstars

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Posted 02 February 2018 - 12:28 AM

OK. I have now finished some more data crunching to add to the above images. 

 

Prior to re-assembling my imaging train, I decided to spend some time determining exactly what degree of vignetting my rig actually had and what my "ideal" flat should look like. I also decided to re-assemble my rig piecemeal and take flats at each point to see if and how they would deviate from this unobstructed baseline configuration. 

 

I first dug up my 2" compression adapter for my ORION EON OTA and used it to install the ASI1600MM-C directly onto my OTA without my Williams Optic Flattener IV or my filter wheel. This was the optics train:

 

Optical train without the filter wheel or focal reducer

 

This is what the optical train looked like from the camera perspective.

 

OTA without the F/R or filter wheel
 
There was a thread from the flocking sticking out and at some point I will have to go after this and get rid of it. You can also see the extra piece of flocking that I used to cover the gap in the larger piece that flocked the bulk of the focuser draw tube. This seam is way out of the optical path and should not cause any obstruction.
 
With the camera re-installed, I took flat frames with my Alnitak EM panel and two tee shifts for diffusers. The intensity was much higher on the sensor than usual since there were no intervening optics, but it worked. I processed all the flats (40 frames and 40 flat dark frames) in pixinsight. I then used a STF stretch of the flat:
 
BareOTAFlat LF STF
 
Without the STF stretch, the FLAT showed NO variation at all that I could see and even with the STF stretch, it is basically featureless except for dust motes. This is an anlysis of the MasterFlat frame of 40 flats and 40 flat darks of my EON 115mm APO without my Williams Optics FR, filter wheel or filters using CCD Inspector. The purpose of this flat was to quantify the degree of illumination non-uniformity across my sensor with minimal optics in the imaging train.
 
BareOTA CCD
 
This shows that my OTA exhibits about 3-4% total intensity variation. I then removed the 2" compression adapter and reinstalled my WO flattener between the OTA and camera. This was the imaging train:
 
optical train through the focal reducer after flocking and painting
 
This is a view through the WO and the OTA from the perspective of the ASI1600MM-C camera: I was rather dismayed to see that I still had a bright reflection even with the flocking of the flattener:
 
reflection off the focal reducer flange
 
This ring was coming from the inside edge of the flattener flange. This edge was anodized black, but very smooth so it reflected a lot of light.
 
Focal reducer flange
 
However, I still took flats at this point to see if this reflection was causing any issues in the flat frame. This is the STF-stretched MasterFlat frame of 40 flats and 40 flat darks of my EON 115mm APO with just my Williams Optics FR installed but no filter wheel or filters. This was before addressing the remaining flange reflection. There was little noticeable difference between this flat and the flat without the flattener. 
 
WO Flat LF STF
 
However, this is an analysis of the MasterFlat frame my EON 115mm APO with just my Williams Optics FR after flocking but without the filter wheel or filters using CCD Inspector. This quantifies the degree of illumination uniformity across my sensor with the flattener optics in the imaging train. The non-uniformity was increased by the flattener to about 7%:
 
WO OTA CCD

 

This was clearly a bit worse but not catastrophic. The flange edge reflection bothered me. I first tried to flock this edge, but it clearly caused some vignetting, so I removed the flocking and used my BLACK 2.0 paint on the flange edge. This picture was taken before the paint fully dried. After it fully dried, It was quite flat and much less reflective than previously:
 
Focal reducer flange after painting with BLACK 2.0
 
The flange actually looked much better to my eye. I re-installed the flattener and this is a view through the flocked and painted WO and the OTA from the perspective of the ASI1600MM-C camera:
 
Image through the OTA with the fully flocked and painted focal reducer
 
The flange reflection was gone and I then took another set of flats. This is the STF-stretched MasterFlat frame of 40 flats and 40 flat darks of my EON 115mm APO with just my Williams Optics FR installed but no filter wheel or filters. This was after addressing the remaining flange reflection.
 
Flocked painted WO LF STF
 
This flat looks better than prior to painting the flange. This is an analysis of the MasterFlat frame of my EON 115mm APO with just my Williams Optics FR after flocking but without the filter wheel or filters using CCD Inspector. This quantifies the illumination uniformity across my sensor with the flattener optics in the imaging train after eliminating all reflections from the flattener. This has improved back to about 5% non-uniformity.
 
Flocked And painted WO CCD
 

I then removed the LUM filter from my filter wheel and re-installed the filter wheel in the imaging train. 

 

ASI1600 filter wheel

 

This if the view through the LUM filter position back through the now flocked and painted flattener and the flocked focuser drawtube:
 
View through the OTA with the flocked focuser drawtube, flocked/painted focal reducer, and the filter wheel with no filter in place
 
This shows that the flocking and painting have significantly reduced the total reflections in the optical train. This is the STF-stretched MasterFlat frame of 40 flats and 40 flat darks of my EON 115mm APO with my Williams Optics FR installed and filter wheel installed but without any filters. This was after addressing the all reflections I could determine in my optical train.

 

Flocked painted WO FW LF STF

 

This is a nice featureless flat frame with just the slight vignetting of the OTA and dust donuts - no ring pattern is visible. This is an analysis of the MasterFlat frame of 40 flats and 40 flat darks of my EON 115mm APO with my Williams Optics FR installed and filter wheel installed but without any filters using CCD Inspector after addressing the all reflections I could determine in my optical train.

Flocked And painted WO And FW CCD

 

This shows that with all the reflections addressed to the best I could do, the optical system shows a little over 4% non-uniformity prior to any filters being installed.

 

More to follow in the next post.


Edited by cfosterstars, 02 February 2018 - 12:38 AM.


#5 Kaos

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Posted 02 February 2018 - 12:47 AM

Did you flock or paint the spacers that it looks like you have installed? Looks like the work you did resulted in a nice decrease in the lower left. My Hotech Self Centering FF is not a reducer so I don't have the flange issue, but I think your tests show that I may benefit from flocking the inside of the Hotech and my spacers. 

 

Kaos



#6 cfosterstars

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Posted 02 February 2018 - 12:54 AM

Did you flock or paint the spacers that it looks like you have installed? Looks like the work you did resulted in a nice decrease in the lower left. My Hotech Self Centering FF is not a reducer so I don't have the flange issue, but I think your tests show that I may benefit from flocking the inside of the Hotech and my spacers. 

 

Kaos

No, I did not do anything to the spacers. I could not see any reflections from them so I left them alone. I have a lot more data to analyze before the next part of this post. Basically I say to look through the OTA when pointed at a bright light and just peal the onion. If it glows, paint or flock it. That was my approach and I will show later what it all got me. It is just taking time to do all the analysis. Right now, I am trying to actually get some imaging done - if you can believe that. Its nice for a change to not focusing on the hardware so much. Once I finish the run on my current target then I can get the pictures through all the filters to go with the flats data. I also want to complete an image to show what all this bought me.


Edited by cfosterstars, 02 February 2018 - 12:55 AM.


#7 rockstarbill

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Posted 02 February 2018 - 01:42 AM

Great work here. Can I ask something though? Have you tested any of these flats against lights? 



#8 freestar8n

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Posted 02 February 2018 - 01:47 AM

Thanks for the update and congrats on cleaning things up further.

 

I don't use PI and I don't use CCDI - but one thing I hope is clear is that the rings shown in your images are simply contour lines - and have nothing to do with your problematic ringlike variations in intensity - correct?

 

If so - you are showing very smooth and flat flats with no evidence of rings - and only about 4% vignetting.  That is a very good flat if I am understanding what you are showing.

 

It still doesn't mean you won't have a pupil ghost or some other issue imaging - and doing actual calibrations of lights is the way to tell that - but right now your flats certainly look good and better than mine - and at the same time mine work well and don't show rings in the final result.

 

Frank



#9 cfosterstars

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Posted 02 February 2018 - 10:24 PM

Great work here. Can I ask something though? Have you tested any of these flats against lights? 

Yes I have. I am working on the next section of this post to report on how well the flats calibrated my lights. Sort answer: they work fine, but they did before. The real question is will I have less in my raw stacked flats to have to fix with DBE. The problem with that is that it will be a very subjective response. I really have no way of measuring how much improvement I am getting. I am working on image processing for the before flocking data. I have been so busy with testing and fixing, I have not had time to process any of my data. I now have a huge backlog of processing to do. Bare with me - life gets in the way. I have a job and a family that limit my time to play...



#10 cfosterstars

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Posted 02 February 2018 - 10:34 PM

Thanks for the update and congrats on cleaning things up further.

 

I don't use PI and I don't use CCDI - but one thing I hope is clear is that the rings shown in your images are simply contour lines - and have nothing to do with your problematic ringlike variations in intensity - correct?

 

If so - you are showing very smooth and flat flats with no evidence of rings - and only about 4% vignetting.  That is a very good flat if I am understanding what you are showing.

 

It still doesn't mean you won't have a pupil ghost or some other issue imaging - and doing actual calibrations of lights is the way to tell that - but right now your flats certainly look good and better than mine - and at the same time mine work well and don't show rings in the final result.

 

Frank

Frank,

 

The contour plots generated in CCD Inspector only measure intensity variation. They are not very selective and are just to judge overall intensity variation so that I can have a numerical quantification of any changes. The CCDI contours are not the ring patterns since they are currently a much weaker signal than the mechanical vignetting of my optical train and are much less than the ~4% variation that CCDI measures. The stretch flats are the only way to see any of the ring patterns. None of the flat I have show so far have been with filters in the optical path and show practically no ring patterns even by eye. One issue is that if you look hard enough at anything looking for something that you want to see, you my think that you see it even if its not really there. For me it is really only relevant to  do side-by-side comparisons back to the previous flats that showed the ring patterns. If you do that - the flats I show without filters are flat with NO rings.

 

As I responded to Gemini, how well these flats work is what I am analyzing now. I will publish the actual flats for each filter and a view down the OTA through each filter with the final flocking as soon as I can get the data crunched. I will also show some fully processed images I am working on with the new rig.

 

Chris



#11 cfosterstars

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Posted 03 February 2018 - 10:29 PM

So to this point, I have basically done about as good a job on the OTA and reflections from the focal reducer as I could. I have very little reflection from the optical train and now I moved on to the filters themselves. Any degradation of the flats at this point is either due to the filters themselves or a interaction with what little reflections I have and the filters.

 

I also investing in a new filter wheel. I was having a lot of communications issues with my ZWO EFW and was considering getting a replacement. But it started behaving well only after I ordered a new Starlight Xpress filter wheel. I arived while I was in the middle of a significant data collection project - that I still hope to post on B33 and the flame with the AD filters prior to all this flocking work - so it set on the shelf for a while. The wheel comes in a nice case that is nice and sturdy:

 

 

Starlight Xpress SX USB filter wheel
 
The SLX wheel is quite a bit heavier than the ZWO and more robustly built. It also opens with a set of thumb screws instead of the cross point scres on the ZWO and is easier to open. You can also swap out the type of adapter flange that you need on either side of the wheel. If you get one of these wheels it pays to think through the flanges carefully. I hand to jury rig the connection the first time and have a flange on order to make it less of a kluge.
 
SX USB filter wheel
 
There is also a central thumb screw since this wheel has interchangeable carousels for different filter sizes.
 
SX USB filter wheel with cover removed
 
The carousel just lifts out and that is how you access the filter seats and attachment screws for the filters on the backside of the carousel.
 
SX USB filter wheel 36mm filter carosel
 
The filters are held my a set of four lips on the one side and two teflon washers and screws on the other side. One issues is that the filter numbers are only printed on the other side of the wheel from the screws attachments. I ended up putting the filters in a different order that the ZWO wheel and that resulted in redoing the filter setup in sequence generator pro.
 
SX USB filter wheel
 
The attachment method makes the filters appear squared off. So I started to remove my 36mm Astrodon filters from the ZWO filter wheel. I plan on re-installing the ZWO filter set with the new filter edge masks into the ZWO wheel for use with a 2nd ASI1600MM camera at some point for my other OTAs.

 

This was the ZWO filter wheel with the LUM filter removed
 
So I removed the E-series LUM filter. This shows the edge bevel of the filter and not that the edges in not blackened line like the 36mm narrow band filters from Astrodon. I dont understand why that dont blacken the edges of the LRGB filters. I blackened the edges of the LRGB filter with a sharpie marker - sharpies are like duck tape - nothing can really improve on them.
 
Astrodon E-series 36mm LUM filter with uncoated bevel
 
This is the LUM filter after I blackened it. I did not really know if this would make any difference, but it was not likely to hurt anything either:
 
Astrodon 36mm E-series LUM filter after edge bevel blackening.
 
Here is the Starlight Xpress 36mm carousel being loaded with my Astrodon filters. The fit very securely and snug in the wheel.
 
Starlight Xpress 36mm filter carousel fully loaded with 36mm Astrodon filters
 
After tightening everything down, this is the filter wheel carousel from the other side showing the squared off look that this wheel gives to the filters:
 
Filter carousel front side
 
I installed the carousel into the wheel housing, put the cover back on with the thumb screws. It was certainly easier than the ZWO to install the filters. One note is that this filter wheel has a 29mm backfocus vs. 20mm for the ZWO. I need to change the spacers between the camera and filter wheel and the focal reducer to get the backfocus correct for the reducer. Also this heavier filter wheel require complete rebalancing of my mount - what a pain. I also had several USB glitches with this wheel. You need to be sure to disconnect the wheel driver in SGP prior to unplugging the wheel or SGP locks up.

 



#12 gunny01

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Posted 04 February 2018 - 07:59 AM

 

So to this point, I have basically done about as good a job on the OTA and reflections from the focal reducer as I could. I have very little reflection from the optical train and now I moved on to the filters themselves. Any degradation of the flats at this point is either due to the filters themselves or a interaction with what little reflections I have and the filters.

 

I also investing in a new filter wheel. I was having a lot of communications issues with my ZWO EFW and was considering getting a replacement. But it started behaving well only after I ordered a new Starlight Xpress filter wheel. I arived while I was in the middle of a significant data collection project - that I still hope to post on B33 and the flame with the AD filters prior to all this flocking work - so it set on the shelf for a while. The wheel comes in a nice case that is nice and sturdy:

 

 

 
 
The SLX wheel is quite a bit heavier than the ZWO and more robustly built. It also opens with a set of thumb screws instead of the cross point scres on the ZWO and is easier to open. You can also swap out the type of adapter flange that you need on either side of the wheel. If you get one of these wheels it pays to think through the flanges carefully. I hand to jury rig the connection the first time and have a flange on order to make it less of a kluge.
 
 
 
There is also a central thumb screw since this wheel has interchangeable carousels for different filter sizes.
 
 
 
The carousel just lifts out and that is how you access the filter seats and attachment screws for the filters on the backside of the carousel.
 
 
 
The filters are held my a set of four lips on the one side and two teflon washers and screws on the other side. One issues is that the filter numbers are only printed on the other side of the wheel from the screws attachments. I ended up putting the filters in a different order that the ZWO wheel and that resulted in redoing the filter setup in sequence generator pro.
 
 
 
The attachment method makes the filters appear squared off. So I started to remove my 36mm Astrodon filters from the ZWO filter wheel. I plan on re-installing the ZWO filter set with the new filter edge masks into the ZWO wheel for use with a 2nd ASI1600MM camera at some point for my other OTAs.

 

 
 
So I removed the E-series LUM filter. This shows the edge bevel of the filter and not that the edges in not blackened line like the 36mm narrow band filters from Astrodon. I dont understand why that dont blacken the edges of the LRGB filters. I blackened the edges of the LRGB filter with a sharpie marker - sharpies are like duck tape - nothing can really improve on them.
 
 
 
This is the LUM filter after I blackened it. I did not really know if this would make any difference, but it was not likely to hurt anything either:
 
 
 
Here is the Starlight Xpress 36mm carousel being loaded with my Astrodon filters. The fit very securely and snug in the wheel.
 
 
 
After tightening everything down, this is the filter wheel carousel from the other side showing the squared off look that this wheel gives to the filters:
 
 
 
I installed the carousel into the wheel housing, put the cover back on with the thumb screws. It was certainly easier than the ZWO to install the filters. One note is that this filter wheel has a 29mm backfocus vs. 20mm for the ZWO. I need to change the spacers between the camera and filter wheel and the focal reducer to get the backfocus correct for the reducer. Also this heavier filter wheel require complete rebalancing of my mount - what a pain. I also had several USB glitches with this wheel. You need to be sure to disconnect the wheel driver in SGP prior to unplugging the wheel or SGP locks up.

 

  The zwo fw was not very well constructed (it was an accident waiting to happen) and I did as you did with an SX replacement.  It is great.

 

  I would add caution with regard to blackening the edges of the filters.  When you go to clean them with 91% ipa, that greasy kid stuff is going to ooze and create a mess the next time you clean.  With that gorgeous fw, I never had any issues leaving the edges alone.  Too bad the camera didn't work...........another story.



#13 cfosterstars

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Posted 10 February 2018 - 07:24 PM

 

 

So to this point, I have basically done about as good a job on the OTA and reflections from the focal reducer as I could. I have very little reflection from the optical train and now I moved on to the filters themselves. Any degradation of the flats at this point is either due to the filters themselves or a interaction with what little reflections I have and the filters.

 

I also investing in a new filter wheel. I was having a lot of communications issues with my ZWO EFW and was considering getting a replacement. But it started behaving well only after I ordered a new Starlight Xpress filter wheel. I arived while I was in the middle of a significant data collection project - that I still hope to post on B33 and the flame with the AD filters prior to all this flocking work - so it set on the shelf for a while. The wheel comes in a nice case that is nice and sturdy:

 

 

 
 
The SLX wheel is quite a bit heavier than the ZWO and more robustly built. It also opens with a set of thumb screws instead of the cross point scres on the ZWO and is easier to open. You can also swap out the type of adapter flange that you need on either side of the wheel. If you get one of these wheels it pays to think through the flanges carefully. I hand to jury rig the connection the first time and have a flange on order to make it less of a kluge.
 
 
 
There is also a central thumb screw since this wheel has interchangeable carousels for different filter sizes.
 
 
 
The carousel just lifts out and that is how you access the filter seats and attachment screws for the filters on the backside of the carousel.
 
 
 
The filters are held my a set of four lips on the one side and two teflon washers and screws on the other side. One issues is that the filter numbers are only printed on the other side of the wheel from the screws attachments. I ended up putting the filters in a different order that the ZWO wheel and that resulted in redoing the filter setup in sequence generator pro.
 
 
 
The attachment method makes the filters appear squared off. So I started to remove my 36mm Astrodon filters from the ZWO filter wheel. I plan on re-installing the ZWO filter set with the new filter edge masks into the ZWO wheel for use with a 2nd ASI1600MM camera at some point for my other OTAs.

 

 
 
So I removed the E-series LUM filter. This shows the edge bevel of the filter and not that the edges in not blackened line like the 36mm narrow band filters from Astrodon. I dont understand why that dont blacken the edges of the LRGB filters. I blackened the edges of the LRGB filter with a sharpie marker - sharpies are like duck tape - nothing can really improve on them.
 
 
 
This is the LUM filter after I blackened it. I did not really know if this would make any difference, but it was not likely to hurt anything either:
 
 
 
Here is the Starlight Xpress 36mm carousel being loaded with my Astrodon filters. The fit very securely and snug in the wheel.
 
 
 
After tightening everything down, this is the filter wheel carousel from the other side showing the squared off look that this wheel gives to the filters:
 
 
 
I installed the carousel into the wheel housing, put the cover back on with the thumb screws. It was certainly easier than the ZWO to install the filters. One note is that this filter wheel has a 29mm backfocus vs. 20mm for the ZWO. I need to change the spacers between the camera and filter wheel and the focal reducer to get the backfocus correct for the reducer. Also this heavier filter wheel require complete rebalancing of my mount - what a pain. I also had several USB glitches with this wheel. You need to be sure to disconnect the wheel driver in SGP prior to unplugging the wheel or SGP locks up.

 

  The zwo fw was not very well constructed (it was an accident waiting to happen) and I did as you did with an SX replacement.  It is great.

 

  I would add caution with regard to blackening the edges of the filters.  When you go to clean them with 91% ipa, that greasy kid stuff is going to ooze and create a mess the next time you clean.  With that gorgeous fw, I never had any issues leaving the edges alone.  Too bad the camera didn't work...........another story.

 

Thanks for the warning. I only you 5% IPA in DI water and not 91%. I work at a wafer manufacturing FAB in Austin - Cypress FAB25. We use %5 IPA solution for removing organic residues on all surfaces for tool cleans. This is far more stringent than what is required for optics or astrophotography. Using more concertrated solution leaves more residue on the optic than it removes unless you quickly rinse. With 5% IPA, it will dry basically clean and you have more time to get the rinse done if you want to. I rinse with pure DI water that I get from my work. It is 16Mohm UPW - it get dirtier from any container. We use sharpie markers for TEM sample preparation and it leave basically a carbon layer that is not very soluble if you let it fully dry. However, I will be very careful when and if I need to clean them. 




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