Hi Martin & All,
Quote:
Originally Posted by BlackWidow
Thanks to all that gave advice on my problem. I think the O111 filter may be great for the hunters, but I did not like the green colour and it seemed to make everything very dark. I think that I am so new to all this that I may get a better use from a O111 later on. I went into my local store and spoke to a few others. As on this page most people suggested that the UHC filter may be better for my general use for filtering out light in my area and giving me a better view of Most Neb's. The store I purhased my O111 filter from agreed to take it back and exchange it for a UHC-s.
I hope I have done the right thing  I will let you all know as soon as I get the chance to try it out.
This is my best friends fault.....ALL OF IT...... If he had not shown me Saturn in his Mead ETX I would not be the owner of any Astronomy stuff, or filter selections  I think I've caught some sort of Bug! |
Congratulations on your new bug/obsession. Lots of sufferers here, you'll be glad to know that it is never fatal -- except to your bank-balance.
FWIW, with a 20cm telescope, I think you made the right choice with the filter by going for an exchange to the UHC -- however, be warned the UHC will still make bright nebulae look green.
To explain, there are four different types of visual general use filter for deep sky observing, The Deep Sky filter, UHC, OIII and H-Beta.
The deep sky filter is a broad pass-band filter that allows most light through but blocks most artificial light pollution. To quote the Lumicon website:
"The Deep Sky Filter is basically a general-purpose light pollution filter that restores a dark sky background for viewing and photographing deep sky objects from cities. It is the best light-pollution filter for all astrophotography on the market today.
Visually, the Deep Sky Filter is the most useful filter under light-polluted skies, from where it reveals star clusters, galaxies and nebulae."
See
http://www.lumicon.com/faq.php
My
personal opinion is that the deep sky filters are of no real use on galaxies and star clusters but perform well on nebulae. But if you are using it on nebulae, why not use a UHC?
The UHC is a much narrower pass-band that allows three separate emission-lines through -- The H-Beta line at 486nm and the twin OIII lines at 495 and 501nm. It dramatically enhances contrast on objects that emit on these lines and that includes most nebulae and the overwhelming majority of planetary nebulae. All three of these lines are in the cyan part of the spectrum
The H-Beta and OIII filters allow only the two above types of emission through -- H-Beta and OIII respectively.
The "problem" is that while these filters pass most of the "wanted" light at these bands (about 90-95%) and block most other wavelengths (this is how they improve contrast), they still do block
some of the wanted light.
See:
http://www.lumicon.com/filterspec.php
Because of that, I think the line filters (OIII, H-Beta) really only work well if you've got a lot of light available (ie a big 'scope). Personally. I wouldn't use them with less than a 10" 'scope.
On the other hand a UHC is very useful on 4-8" 'scopes because it has a wider pass-band.
But, remember because all three lines it passes are in the cyan part of the spectrum, the nebula if bright enough, will generally look cyan/emerald/lime green. The background sky will be much darker, but not as inky as with the OIII. Equally it will give a dramatic improvement if OIII and H-Beta emission is present. UHC filters will dim your view of clusters and galaxies etc -- don't use for these.
I own four filters for use on my 18" f/4.9: 2" -- UHC and H-Beta, 1 1/4" -- UHC and OIII. And what is my most used filter? The UHC's!
Best,
Les D
