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for small telescopes... like mine... 4.5 inch reflector... these filters works?

i just came from a dark seein site... and saw m33...very subtle.. just wondering if i could enhance the view.

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Ozzie,

Filters will work with your 4.5" scope, but remember that filters do not help much in seeing galaxy detail. Filters work best for emission nebulae and certain narrow spectrum objects. Under dark skies, broad spectrum emitters such as galaxies (e.g. M33) don't benefit much from using filters.

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Lotteries are a tax on people who don't understand math - Snorkler

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for small telescopes... like mine... 4.5 inch reflector... these filters works?

i just came from a dark seein site... and saw m33...very subtle.. just wondering if i could enhance the view.

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for small telescopes... like mine... 4.5 inch reflector... these filters works?

i just came from a dark seein site... and saw m33...very subtle.. just wondering if i could enhance the view.

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A dark site is where nearly every filter works best. A broadband, or LPR/LPB filter might slightly enhance SOME galaxies, but at the sacrifice of a lot of brightness in star images.
You're better off *slightly* bumping up the power. Though M33 is very large, and has a very low surface brightness (a lot lower than hundreds of other galaxies), it does benefit from slightly increasing the power as this will darken the background sky in the field of view. The HII regions in M33's arms become more visible this way. My 5" Mak can pick out 5 or 6 of them by using 100X, even though the galaxy is larger than my field of view at that power.

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Well, I never find the brightness reduction in broadband filters to be all that enormous, although there is some visible drop in brightness. However, again some of the larger more diffuse galaxies *will* benefit from the use of broad-band filters like the Lumicon Deep-sky (even under some fairly dark-sky conditions). The object which probably typifies this improvement is NGC 253, which at least in 8 and 10 inch instruments shows its mottled light and dark detail better when using the Deep-sky filter than without. The improvement isn't exactly huge, but it is there and is enough to justify the use of the filter. If there is slight skyglow from Aurora or weak low-level light pollution, the Deep-sky filter makes a somewhat greater difference. Of course, increasing the magnification also helps, as the M33 example illustrates. In my 10 inch, I usually study M33 at between 90x and 141x, as that brings up the scale to where the numerous star clouds and a few HII regions become more noticable. In fact, I have used the Lumicon UHC filter on M33 with my 10 inch Newtonian and could *still* see the spiral arms (helps bring out a few HII regions).

I am modifying the post to add in an image that shows what the Lumicon Deep-Sky filter does to NGC 253

Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info


Edited by David Knisely (01/12/06 02:01 PM)

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I was very surprised after looking at the light transmission curves of the Lumicon UHC and Lumicon OIII at how different the eyepiece field appearance was. The field in the OIII is a LOT darker (= more contrast) than in the UHC. In non-quantitative impressionistic terms: the difference in field brightness between UHC and the OIII is half again that between no filter and UHC. And yet the light transmission curves are not hugely different...so in the final analysis, organoleptically, our eyes are the final arbiter of performance in these filters.

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Well, if you look at the numbers, the UHC and OIII filter curves *are* very different. The UHC has a fairly flat-topped passband (near 90% transmission) with a full-width-at-half-maximum (FWHM) of about 270 Angstroms. The OIII has a similar high transmission, but its FWHM is only about 100 Angstroms, so it is significantly narrower. Also, the relative performance of the two filters depends a bit on the object in question, as some are helped by both filters while others are helped more by one than the other. Generally, the UHC will show the largest area of nebulosity at a slightly higher brightness level, while the OIII will often provide greater contrast, a darker sky background, and more dark detail than in the UHC, but sometimes at a slightly lower brightness level. With an object that has significant OIII emission, the OIII may provide better performance than the UHC (like on the Veil or some planetary nebulae for example), but in objects with a "balance" of OIII and H-Beta emission (like M42 or M8), the UHC may be a better choice. For the best results, an amateur should consider having both a narrowband *and* a OIII line filter in their eyepiece boxes, as one is not necessarily better than the other. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

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Here are some definitive links you should review:

First, basic info about filters (how they work, how to define them) and which object is benefitted by which filter:
http://www.cloudynights.com/item.php?item_id=63&pr=2x9x42
and
http://www.cloudynights.com/item.php?item_id=387
and
http://pages.sbcglobal.net/raycash/filters.htm
and
http://www.astronexus.com/gatfaq/filters.php
and
http://www.astrosurf.org/lombry/reports-filters-dso.htm

Then, the test reports:
http://home.freeuk.com/m.gavin/grism2.htm
and
http://www.astroamateur.de/filter/

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A lot of these links are dead now. There used to be a good basic how-to about filters. Anyone have that link?

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Tom (Pegster)
DSH-8 (GSO Dob)
15x70 Oberwerks
SVP 100 f/6 achro
WO 66 Petzval
Sears Discoverer EQ 60/900
8x42 Regals




History is Philosophy teaching by examples.
Thucydides

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