UHC & OIII Filter Focus

[eliot]

I'm normally an imager so not greatly experienced with visual astronomy.  However recently whist imaging rig is running I have been having fun learning how to star hop again with an old 12" dob a friend gave me when he upgraded scopes.  The mirror is in desperate need of recoating, but with a Baader Zoom and 2" Meade 34mm eyepieces the images are nice and sharp.

I have been trying to find the Veil Nebula.  Unfiltered from my (darkish) garden I have struggled.  I bought second hand 2" UHC and OIII filters from flea bay to help me find it.  They appear to be the kind that OVL sell.  The OIII filter wasa great help but the nowhere near as good as I had read it would be.

One thing I had noticed was the how soft the image looks with both filters.  With some delicate focusing I can get the image to be OK, but nowhere near as good as unfiltered.  However If I take the filter from the back eyepiece and hold it up again the front the image appears much sharper.

The scope isn't amazing fast (f/5), but the barrel of the eyepiece is quite long.  Could this be caused by a distortion in the light cone caused by the filter glass?  I would have thought restricting the light bandwidth would have made the stars tighter if less luminous?

[vlaiv]

Ah yes - if filter causes blur when in front of focal point (converging light beam close to focus point) but not when placed between eye and eyepiece - than it is very poor quality filter in terms of parallelism of surfaces and polish (general optical quality).

In front of your eye, image is already amplified and small aberrations on amplified image will not be resolved by your eye. In converging light cone before eyepiece any aberrations will be amplified by eyepiece (same as image)  - and things will go blurry if you don't have decent optical quality filters.

[Louis D]

I have both a late 90s era Lumicon and a Zhumell OIII filter.  The Lumicon shows the Veil nebula as bright, crisp, and detailed in a 15" f/5 Dob.  The Zhumell is more of a light pollution filter by comparison.  The Lumicon appears to be a mirror when viewed directly, the Zhumell looks more like a weird green filter.  I'm not even sure the Zhumell uses dichroic coatings because of this.  The Lumicon was over $150 20 years ago.  The Zhumell was $16 five years ago.  You make the call on manufacturing quality based on that alone.

[RT65CB-SWL]

I have the Explore Scientific UHC & OIII filters after researching that the OVL ones are not that good.

I can now safely say, that I can split the Trapezium in M42. 

[Stu]

44 minutes ago, Philip R said:

I have the Explore Scientific UHC & OIII filters after researching that the OVL ones are not that good.

I can now safely say, that I can split the Trapezium in M42. 

Agreed, I think The ES ones give good performance for a reasonable price.

[JTEC]

+1 for the ES filters in terms of value for money. Having tried most, including the Lumicons, etc, the DGM NPB filter is now my preferred UHC-type filter - and they’re not massively expensive at least in the 1.25 size. I’ve also tried the Televue (by Astronomik) OIII and thought it was the best OIII I’d ever used, again, including the Lumicons. Admittedly, personal preference and other system characteristics can play a part in this. 

[eliot]

Hmmm.  I had suspected as much.  Thanks for the recommendations.