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loosening filter retaining ring

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#1 orlyandico

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Posted 31 January 2013 - 10:00 AM

I got a no-name h-alpha 2" filter for $100 and... well it works. but it is astigmatic :tonofbricks:

Seems the retaining ring is a bit tight and I would like to loosen it and see if things improve. But it doesn't want to move and I don't have a filter ring removal tool.

Any ideas on how to loosen this?

on the other hand maybe even if loosened it would still be astigmatic. now I regret not getting a Baader or Astrodon or something....

#2 csa/montana

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Posted 31 January 2013 - 10:27 AM

Use a piece of the "rubberized" shelf liner to grab hold of the filter. This will give you a better grip on it.

Also check your kitchen; I have a tool that is plastic-coated that is used to open jars, etc. This works very well with filters!

#3 GlennLeDrew

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Posted 31 January 2013 - 10:31 AM

If a plane parallel plate of good optical quality is stressed or bent by the amount a filter cell could impart, it should not introduce astigmatism. It's more likely the substrate itself is at fault.

In any event, if you have an extra pair if hands available, one could support the filter cell while the other rotates the retaining ring using a pair of jeweler's screwdriver tips. Use the largest ones which will fit in the slots. The cell supporter should strive to not pinch or otherwise asymmetrically squeeze; try to exert uniform force.

It goes without saying that care is required!

#4 DaveJ

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Posted 31 January 2013 - 10:36 AM

Use a piece of the "rubberized" shelf liner to grab hold of the filter. This will give you a better grip on it. Also check your kitchen; I have a tool that is plastic-coated that is used to open jars, etc. This works very well with filters!


Hi Carol,

It's not that he can't get the filter off whatever it's attached to, he wants to remove the retaining ring that holds the filter element in the filter cell. The only safe way to do this is with a spanner wrench. I had the same problem last year and wound up getting a set of these Adjustable-Arm Spanner Wrenches. Problem solved. I've since used them for a lot of other optical projects that would have otherwise been impossible. Here's another set from Edmund Optics. Please notice that the company is EDMUND and not EDMUNDS as is a common mistake on CN. Edmunds is, in fact, a company, but it has absolutely nothing to do with optics! :grin:

#5 csa/montana

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Posted 31 January 2013 - 10:43 AM

:foreheadslap: Need more coffee! :roflmao:

Hey, some good came out of my booboo; your link to the spanning wrenches. :bow:

#6 Tim L

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Posted 31 January 2013 - 01:49 PM

Sometimes needle-nose pliers can be made to work if used carefully enough. Or maybe get an old credit card and cut it to the right width to fit across the filter into the ring slots.

#7 Billydee

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Posted 31 January 2013 - 03:19 PM

Orlyandico,

Sigapore is the camera captial. Go to any camera shop with a service department and I'm sure they will help you (most likely for free). They usually have a ton of spanner wrenches to fix lens (if you liked one buy it).

Luck, Bill

#8 nevy

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Posted 31 January 2013 - 03:47 PM

I reckon circlip pliyers would work.

#9 orion61

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Posted 01 February 2013 - 07:40 AM

:gotpopcorn:
I got sick of ruining things so I bought a jewlers Spanner wrench, best $40.00 I ever spent, i use it all the time, no more ripped up retaining rings or scratched/ruined lenses.

#10 orlyandico

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Posted 01 February 2013 - 10:42 AM

I used a vernier caliper. The jaws for measuring inside diameters worked great. Then a jar opener to grip the outside of the filter ring securely.

Loosened it nicely. Hope the astigmatism goes away. Otherwise its a hundred bucks down the toilet. Even at best focus the stars weren't round that DeepSkyStacker had trouble registering the subs.

EDIT: well, the filter retaining ring is nice and loose. Still got astigmatism. :tonofbricks:

since I bought it used... (I knew there was something wrong if a 2" 10nm H-A filter was going for a hundred bucks) i don't have much recourse. the seller said it worked fine on his SCT... but then SCT's have so much coma and field curvature it's likely these would have masked the astigmatism.

that, or imperfect tracking. that would also embiggen the stars and conceal the astigmatism.

oh well. should not have cheaped out. These deals always get me in the end..... :tonofbricks:

#11 GlennLeDrew

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Posted 02 February 2013 - 12:43 PM

Just to make sure the filter is at fault, try partially rotating it to see if the aberration rotates with it. I find it almost strange that an element lying near the focus, where the light cone for any one image point is quite small and hence utilizing only a small area, will still distort. Either the substrate has a varying refractive index, or it is a piece of window glass.

Another test; place it in front of a bino or small refractor, and examine any suitable land or sky target. If the aperture is larger than the filter's aperture, simply mask of the outer annulus of unfiltered objective. This is a sensitive test, for it uses the full aperture. The magnification need not be high.

For me this poses a fascinating problem, for which I would strive to divine the fault.






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