(IAAC) [amastro] Re: Update on Filter/Object Comparison Project

The following interesting post was seen on the 'amastro' email list yesterday,
from avid visual deep-sky observer David Knisely. David is not a current IAAC
subscriber (for some unknown reason!), so if you wish to follow up this post,
please MANUALLY put 'KA0CZC@navix.net' in the "Cc:" line of your reply.

Clear skies all!
Lew Gramer <owner-netastrocatalog@jovian.com>

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Date: Wed, 05 Jan 2000 09:08:13 +0000
From: "David W. Knisely" <KA0CZC@navix.net>
References: <200001020146.SAA06126@safety.lowell.edu>
X-Mailing-List: amastro@egroups.com
Subject: [amastro] Re: Update on Filter/Object Comparison Project

               by David Knisely, Prairie Astronomy Club

     The following is an interim report of observations in progress
regarding the comparison of performance of various filters intended for
nebulae.  The instrument used is a 10" f/5.6 Newtonian, working at 59x,
70x, and 141x, as well as a few unaided-eye observations using the
filters hand-held and looking up at the sky (for Rosette, North America,
California, and Barnard's Loop). The filters used were Lumicon's
DEEP-SKY, UHC, OIII, and H-Beta, and were all mounted in a modified
Lumicon multi-filter adapter.  This allowed rapid comparisons between
filters, thus avoiding judgement problems caused by the time needed to
change filters or reports from inaccurate anecdotal accounts.  
Observing was done from a dark-sky site (visual naked-eye limit 6.5 to
7.0).  For detailed descriptions of the objects, see any of the various
observing handbooks.  Each filter was given a 0-5 point "Score"
performance ranking behind it for each object observed; Example: OIII
(4) means the OIII gave a large improvement in the view over non-filter
use and contributes 4 points to its overall score total. 
                              "Scoring" Legend
        (5): Very Large Improvement   
        (4): Large Improvement  
        (3): Moderate Improvement   
        (2): Slight Improvement  
        (1): No improvment or fainter than no filter.   
        (0): Much worse than no filter (object marginal or not visible).
As of January 5th, 2000, 55 objects had been observed, consisting of 39 
diffuse emission nebulae and 16 of the major planetary nebulae.  The
scoring results so far are given below:

                   *** SCORING TOTALS FOR 55 OBJECTS ***
                 UHC...... 196 points, average rating 3.56   
                 OIII..... 187 points, average rating 3.40   
                 DEEP-SKY. 124 points, average rating 2.25        
                 H-BETA...  69 points, average rating 1.25   

                        GENERAL TRENDS IN RESULTS
     So far (with a few notable exceptions), the numbers show the UHC
and OIII are the filters of choice for viewing nebulae, and to some
degree supports the general recommendation that if only one filter can
be purchased, it should be the UHC.  In performance characteristics, the
UHC filter tends to reveal a slightly larger and/or brighter area of
nebulosity with many emission nebulae than the OIII does, while the OIII
filter will often yield somewhat more contrast and dark detail on a
given object.  The OIII also tended to be a bit better for "blinking"
small planetary nebulae than the UHC was, while the H-beta often hurt
the view of many planetaries.  The inclusion of some planetary nebulae
may have slightly inflated the score of the OIII filter, since in
general, the OIII often does a bit better on those objects.  The H-Beta
tended to be most useful on a more limited number of objects than either
the UHC or the OIII filters.  The Deep-Sky filter almost always produced
at least some gain in contrast for nearly every object observed
(especially when some skyglow was present), but rarely produced a
spectacular improvement of the view.  Filter comparison results for
specific objects are available on request in a much longer version of
this article.
- -- 
David Knisely  KA0CZC@navix.net
Prairie Astronomy Club, Inc.  http://www.4w.com/pac
Hyde Memorial Observatory, http://www.blackstarpress.com/arin/hyde
*   Attend the 7th Annual NEBRASKA STAR PARTY   *
* July 29-Aug. 5th, 2000  http://www.4w.com/nsp *

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