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Misaligned filters in the EFW?

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#51 jonnybravo0311

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Posted 10 March 2021 - 10:54 AM

This situation looks like an occlusion. One possible reason is that the filter wheel is not centered in its rotation. The another reason is that using the 1.25 inch filter is at the very limits of providing proper coverage of the large ASI294mm sensor. Due to the combination of a focal reducer,

 

the Antlia 1.25" filter cannot be completely covered. 1.25 inch  filter's effective aperture > 26mm, filter thickness is 2mm.

Thank you for your response. I would think the 1.25" is plenty to cover the sensor in the 294; however, I will certainly concede that there might be some tiny bits in each corner that are not. The problem is the L filter in particular seems to suffer considerably more so than the others. If you would be so kind, could you take a look at the other thread I opened here: https://www.cloudyni...65-lrgb-data/. 

 

I appreciate you taking the time to help with this.



#52 kwpickett

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Posted 10 March 2021 - 11:54 AM

I know this thread has been going for a while, but I just found it.  I'm having the exact same problem with my EFW not seeming to exactly center the filter. My flats look the same as yours.  In various forum searches, it looks like ZWO used to have a utility to center the wheel (different than the calibration routine, which didn't change anything for me, either). I haven't been able to find that software.  The forum links were dead.  

 

Please let me know if you find a solution!

 

Thanks,

Kyle



#53 imtl

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Posted 10 March 2021 - 12:04 PM

As I suggested to Jonny, I think the best thing to do now is to contact ZWO and get technical assistance on this matter.



#54 jonnybravo0311

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Posted 11 March 2021 - 12:27 AM

Following up. This is NOT an issue with the ZWO EFW. Thanks to the generosity and willingness to help me out, another CN member sent me a set of 1.25" LRGB filters that were no longer being used so I could try to troubleshoot the problem.

 

It is, in fact, the Antlia filters themselves. If you've gone through this thread, you'll have seen a whole lot of different screenshots of the Antlia L filter in various positions in the EFW. In each, there is a definitive cutoff on at least one corner. Far more than I would have expected from a 1.25" filter in front of the 294MM sensor. Tonight, I took the Antlia L filter out of the EFW and took a close look at the filter itself. I examined the housing and the fit of the glass in the mounted enclosure. What I saw was a small section of the circle that seemed not quite as circular as the rest. Curious, I loosened the threads and this allowed the glass to spin freely in the housing. As I spun it, I noticed the "not quite as circular" bit moved. I decided to take a few different test images with the "not quite as circular" portion in different positions. Here are the resulting screenshots:

 

Screenshot 1:

 

med_gallery_347158_16175_366162.png

 

Screenshot 2:

 

med_gallery_347158_16175_123396.png

 

Screenshot 3:

 

med_gallery_347158_16175_502641.png

 

Well... would you look at that. To add further proof here's a screenshot with a different L filter in that same position in the EFW:

 

med_gallery_347158_16175_29485.png

 

It looks like I will have to return the Antlia LRGB filters and consider what I will be purchasing going forward.


Edited by jonnybravo0311, 11 March 2021 - 07:46 AM.

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#55 Mike in Rancho

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Posted 11 March 2021 - 01:18 AM

Nice sleuthing!  As you may know, I had quite the saga chasing down a "not quite so circular now, is it?" problem myself.

 

What exactly is wrong, however?  The filter glass isn't an even field?  The frame, retaining ring, or any kind of spacers in the mount?

 

The known good L filter still seems to show a wee bit of corner cutoff and vignetting also, if I am seeing that right?  Just a lot more even and centered.



#56 jonnybravo0311

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Posted 11 March 2021 - 07:40 AM

The design of the filter is pretty common I'm assuming. Totally basing it on the 2 examples of mounted filters I've seen here smile.gif.

 

The mounting is a couple of pieces of aluminum. I'll call the bottom part the seat and the top part the fastener. The glass rests on the seat and is held in place by the fastener. On the glass, there is a ring of black on the very outside edge. I don't know what this is, but my guess is some kind of anti-reflective coating. Anyway, it is THAT circle around the glass of the filter that is not completely covered by the aluminum mounting. That's what seems to cause the "not quite circular" effect. When I spin the glass in the filter to different positions in the mounting, I get the results shown in the latest screenshots. These 3 screenshots are a small sample of the number of actual test images I took (and the only ones I actually remembered to screenshot). I must have assembled and disassembled that EFW and that L filter 30 times trying to spin the glass and find a "sweet" spot where the "not quite circular" part would be the least intrusive. The third screenshot above is about the best I could get things to line up.

 

Yes, the known good filter shows a tiny bit of cutoff in the corners. However, as you point out, it is evenly distributed. As to why it's happening at all? I don't know. LYAstro wrote in post #50

...The another reason is that using the 1.25 inch filter is at the very limits of providing proper coverage of the large ASI294mm sensor. Due to the combination of a focal reducer, the Antlia 1.25" filter cannot be completely covered. 1.25 inch  filter's effective aperture > 26mm, filter thickness is 2mm.

 

This could be true, I suppose. My purchase and my knowledge was/is based on the calculator at astronomy.tools telling me the 1.25" was plenty big enough, by people here on these very forums using 1.25" filters with this camera/wheel/scope, by ZWO providing a kit including the 1600MM pro and 1.25" filters, etc. According to the calculator at astronomy.tools, for my camera, scope, reducer combo, the minimum filter size is:

 

24.72mm if the filter is 10mm from the sensor

25.48mm if the filter is 15mm from the sensor

26.23mm if the filter is 20mm from the sensor

 

Last I checked, 1.25" is 31.75mm. This _should_ give me more than enough coverage. Maybe the mounting holding the glass is more than 5.5mm? I really don't know. What I do know is that the filter absolutely cannot be more than 20mm from the sensor, otherwise it would be flush up against the back side of the EFW because the total distance of sensor + EFW = 26.5mm.


Edited by jonnybravo0311, 11 March 2021 - 07:57 AM.

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#57 jonnybravo0311

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Posted 11 March 2021 - 02:14 PM

I've taken some pictures of the Antlia L filter. I think this first one illustrates the problem most effectively. It is a picture of just the glass portion of the L filter. You can see the black ring around the outside. Look closely at the right side of the glass and you'll see the black ring is NOT flush with the outer edge of the glass. This is the problem. That discrepancy causes the black surface to intrude into the light path.

 

gallery_347158_16175_930830.jpeg


Edited by jonnybravo0311, 11 March 2021 - 05:55 PM.

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#58 Mike in Rancho

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Posted 11 March 2021 - 02:30 PM

Ah yes.  Likely a padded spacer ring to protect the glass edges when the retaining ring is threaded down.  Not only is it off-centered, and has that bulge, but not a particularly clean circumference either - if any parts of that ragged edge stick out from the retaining ring.  On the far end of the telescope, something like that would also produce bright star halo artifacts.  I know!  Not sure of effects on this end, other than what you have noticed already.

 

Possibly DIY fixable if it can be aligned and maybe an x-acto taken to the inner bulge.  But then again maybe that ring was spit out of the machine slightly the wrong size and so will never fit right.  I would think sending it back is the right move.


Edited by Mike in Rancho, 11 March 2021 - 02:31 PM.

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#59 LYAstro

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Posted 12 March 2021 - 08:28 AM

Because the filter retainer is relatively hard material, we need to use this silicone gasket to prevent filter glass from being damaged by the hardness of the hard retainer. Antlia 1.25-inch filter has a larger luminous flux than other specifications, which can be 26.5mm.
The silicone gasket can be removed and the filter retainer can be used to fix the filter directly.



#60 sbharrat

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Posted 12 March 2021 - 08:59 AM

I've taken some pictures of the Antlia L filter. I think this first one illustrates the problem most effectively. It is a picture of just the glass portion of the L filter. You can see the black ring around the outside. Look closely at the right side of the glass and you'll see the black ring is NOT flush with the outer edge of the glass. This is the problem. That discrepancy causes the black surface to intrude into the light path.

 

gallery_347158_16175_930830.jpeg

So that says you are literally *on the edge* with 1.25 filters with this FW/camera... frown.gif



#61 jonnybravo0311

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Posted 12 March 2021 - 12:15 PM

Because the filter retainer is relatively hard material, we need to use this silicone gasket to prevent filter glass from being damaged by the hardness of the hard retainer. Antlia 1.25-inch filter has a larger luminous flux than other specifications, which can be 26.5mm.
The silicone gasket can be removed and the filter retainer can be used to fix the filter directly.

How would I go about removing this silicone gasket?



#62 imtl

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Posted 12 March 2021 - 12:18 PM

Send it to your vendor and get a new filter kit.



#63 jonnybravo0311

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Posted 12 March 2021 - 12:30 PM

Send it to your vendor and get a new filter kit.

Well... there's this tiny issue of stock at the vendor. Namely, they don't have a replacement set available :D.

 

I actually have the entire set packed and boxed ready to ship back and was going to head to the UPS store this afternoon. If the removal of the gasket is a simple procedure... then I can save myself the hassle of the RMA process and the who-knows-how-long wait until another set is in stock.

 

Granted, I do have another set of LRGB filters I could use in the meantime... but there's that whole issue of different filter thickness (my loaner set is 3mm thick, the Antlia SHO and LRGB are 2mm). OK... it's really more of an inconvenience than an issue since I can figure out appropriate starting positions for my FocusCube for the different filter thicknesses, and then use NINA's advanced sequencer to move the focuser to the right starting position for the given filter.



#64 imtl

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Posted 12 March 2021 - 12:49 PM

I would then suggest just returning the filters and getting a refund and buy them at a different store.

 

Regarding the different filter thickness. It's workable but will require big shifts. If you're not using OAG it is less of a problem.

 

Suggestion: You will image broad band targets with the thicker LRGB and narrow band targets with the SHO. Unless you want to mix the two groups, I don't see any issues with focusing.


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