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judebox11

1.25inch filter on 2 inch eyepiece

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Hello, I was wondering if I could use a 1.25 inch UHC filter on a 2 inch eyepiece by putting on top of the part you look through on the eyepiece, 1.25 inch would cover all of it I think. I heard you could do this with moon filters so I was wondering if it could work like this?

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As long as the filter does not touch the top lens (as Ben warns above) it does actually work. One technique of finding challenging nebula is to move a UHC or O-III filter in and out of the view though the eyepiece and see if the nebula can be spotted "blinking" on and off.

An issue that I've found when I've tried this is that the filter gets in the way of putting the eye at the right distance from the eye lens (ie: the eye relief distance) to enable the full field of view to be seen. Also the filter glass seems to attract annoying and distracting glare and reflections from any light sources nearby more readily than the eye lens of the eyepiece does.

 

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25 minutes ago, Ben the Ignorant said:

You might scratch the lens.

 

3 minutes ago, John said:

As long as the filter does not touch the top lens (as Ben warns above) it does actually work. One technique of finding challenging nebula is to move a UHC or O-III filter in and out of the view though the eyepiece and see if the nebula can be spotted "blinking" on and off.

An issue that I've found when I've tried this is that the filter gets in the way of putting the eye at the right distance from the eye lens (ie: the eye relief distance) to enable the full field of view to be seen. Also the filter glass seems to attract annoying and distracting glare and reflections from any light sources nearby more readily than the eye lens of the eyepiece does.

 

Thanks for the replies, I cant imagine it being THAT sensitive that the filter just touching the lens would scratch the glass, are they really that fragile? I thought regular low strength steel/aluminium is softer than glass. ( obviously steel is stronger, but using Rockwell scale of hardness)

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By experimenting a bit, you can actually use a 1.25" eyepiece ahead of the 2" eyepiece and have it work with barely noticeable vignetting.

 

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1 minute ago, judebox11 said:

 

Thanks for the replies, I cant imagine it being THAT sensitive that the filter just touching the lens would scratch the glass, are they really that fragile? I thought regular low strength steel/aluminium is softer than glass. ( obviously steel is stronger, but using Rockwell scale of hardness)

The coatings on the lenses could get scratched and that would notably devalue the eyepiece if you ever want to sell it, although it would still work fine.

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Posted (edited)
3 minutes ago, John said:

The coatings on the lenses could get scratched and that would notably devalue the eyepiece if you ever want to sell it, although it would still work fine.

Ah I forgot about the coatings, thanks for the help

Edited by judebox11

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Posted (edited)
58 minutes ago, judebox11 said:

I thought regular low strength steel/aluminium is softer than glass.

Where many soles rub on granite stairs the stairs are worn out yet rubber is softer than granite. Anything can scratch anything if it rubs hard or often enough, and aluminum in astro gear is never bare, it's always anodized, meaning it's coated with the aluminum's own oxide, which is the hardest material next to diamond.

Edited by Ben the Ignorant
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4 hours ago, jock1958 said:

If you used one of those adaptors with a 1.25” filter on either a 2” eyepiece or diagonal would it reduce the AFOV significantly?

It would depend on the field stop of the eyepiece to some extent.  If it is less than 27mm, there shouldn't be any vignetting at all.  If it is bigger, there would probably be some, but it might be very difficult to detect with the human eye.

It would also depend on the distance between the reducer and the field stop.  The farther away the better.  If you put it on the front of the diagonal, you'd probably never notice the vignetting.  I use a 2" diagonal on a 127 Mak with a 27mm diameter rear port.  Since the port is pretty far from the field stop, vignetting is all but undetectable to my eye.

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You can always experiment with a cardboard mask with a hole the diameter of the clear aperture of a 1.25 inch filter cut in the centre. Put that on the end of the eyepiece barrel and see what the effect on the AFoV of the eyepiece is.

 

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