DGM NPB for a 6" Dob?

[JJDOBBER79 (Jas)]

Hi there.
I am thinking of buying this DGM NPB filter. Its only $75 and from what I have read on the forum it comes very highly regarded. Is there any reason why this would not be suitable for a 6 inch dob.(because of the aperture) Also I will unfortunately be using it with my cheap EP's that came with it. Is it still likely to be effective? Thanks in advance.


http://www.ebay.com/itm/NPB-1-25-DGM...item257f183e92

[mental4astro (Alexander)]

All filters work by eliminating the undesirable wavelengths, and transmitting the ones we want. Astronomical nebula filters of all types, transmit a very tight set of wavelengths. Two things happen here: one, the percentage of light transmitted as a whole is small; two, the wavelengths that are transmitted, our eyes aren't the most sensitive to. When you get down to "ultra high contrast" nebula filters that the NPB is, the transmission is even less.

The only way around this is just by sheer brute aperture, which is the only way to get enough light through that is bright enough for our eyes, particularly with the UHC types.

An NPB filter will work in your 6", but as you infer, the image won't be very bright. For your aperture, an OIII and even a broadband nebula filter would be a better option. An OIII filter is also the best option in light polluted areas.

I have an NPB, an OIII and a broadband filter. The broadband I don't use much, and really only with my smaller scopes. The NPB gets used only at a dark site with my 17.5". The OIII is my most used filter, both at home and out bush.

Fellow IIS member, John Bambury, has written a very good article on astronomical filters you might like to go over. It discusses not only nebula filters, but also coloured filters and their applications.

I think you would be best served with an OIII filter, rather than an NPB or other type of UHC filter. GSO also makes some decent OIII filters too. My 1.25" OIII is a GSO, and I'm very pleased with it.

Mental.

[JJDOBBER79 (Jas)]

Hi Mental,
I actually posted this in the wrong forum a couple of days ago not realizing that there was a specific equipment one. I ended up buying the NPB and I wish I had read your advice first, I probably would have gone for 03 instead. Anyway, I hope it still is a noticable improvement from EP alone. Probably something that I will use when I upgrade aperture (hopefully soon) Speaking of which, Is there a forum where ISS members buy/sell on here. I thought I read something like that. Might be thinking of going to a 10". I am happy with dobsonian as I dont want to get into imaging until I know my way around the sky. Probably years. Thanks for your advice. I will let you know how the NPB goes in my 6".

[mental4astro (Alexander)]

Hey Jas,

Mate, look towards the bottom of the forums listing page & you'll find the IIS classifieds. To be able to sell you need a minimum of 5 posts, which you've already gone over. To buy or reply to an ad you only need to join IIS.

You can still get yourself an OIII filter. While there is overlap between an OIII & UHC, there are differences that make having both good if you already have just an UHC.

[JJDOBBER79 (Jas)]

Thanks Mental,