Filters at Rock Bottom Prices

[mercedes_sl1970]

Originally Posted by Omaroo

Today - I just received my two 1.25" O-III filters I ordered through Robs eBay site, which now add to another 2" I bought from Rob at Omega through astronut. Great international service I'll say. The TWO 1.2's landed at my door today for US$88 including shipping. Not bad.

I tried the 2", mounted at the the base of my 2" extension tube in the 8" Newt the other night (with John "astronut"), and was pleasantly greeted with a great contrasty view of the Eta Carinae, Turantula and Orion nebulae. It certainly made a huge difference to the dark lane areas - as advertised. By mounting it to the extension tube I can use the same filter for 2" and 1.25" EP's as usual by merely swapping the adapter above it.

I bought two of the 1.25" so that I can use both at the same time on the bino-newt that is in the works.

I guess that the other filters mentioned might be better in some way - but my human frailties and eyesight will probably prevent me from getting any better value from them. For the price, these are great.

Thanks John - a great pickup!

Just curious about what minimum size scope/bino these could be used with? Eg. 100mm BT45 binos or 80mm refractor?

Thanks

Andrew

[Don Pensack]

Originally Posted by astronut

Not to worry Tony. the O111 HB is a fairly wide band filter, but it's main operation is in the O111 line of the spectrum.
Here is a explanation from the supplier


"Precision Interference Filter for EBAY Astronomers that selectively transmits the Hydrogen beta and Oxygen III emission of Galaxies. Specifically produced for EBAY customers, this filter has had significant development to produce excellent transmission of the Oxygen lines at 496.9 nm, 500.4 nm, as well as the 486 Hydrogen, while attenuating most of the background light from other sources. This is a relatively wide pass band nebula filter. The filter is Mounted in a standard 48 mm eye piece ring. Specifications follow: Central Wavelength, To pass 494, and 486 to 500.4 nm with typical 90%, as well as > 50% (Typically 50% of 656.3nm; Size, 1.78 +0-.006 inches (45mm); mounted in 48mm eye piece ring. Thickness 2.1 mm. Filter supplied with actual spectral data."

This bandwidth is better described as a "UHC" filter. True "O-III" filters do not pass the H-Beta band at 486nm. That's fine, as a UHC filter is generally advantageous on most nebulae. On those nebulae for which a true O-III filter produces the best image (like the Veil, for example), a "real" O-III filter may be preferred.

But you can't beat that price--even for a UHC filter.

[Omaroo (Chris Malikoff)]

Originally Posted by mercedes_sl1970

Just curious about what minimum size scope/bino these could be used with? Eg. 100mm BT45 binos or 80mm refractor?

Thanks

Andrew

Mechanically: These filters act like any other - they just screw into the base of the eyepiece you choose to use - or barlow - or extension tube using the standard thread.

Optically: I'm not sure that's an easy one to answer. The smallest aperture scope I've used these on is my 200mm f/5 reflector. Given that they should should only effectively block light from a specific part of the spectrum I can't see why they'd restrict it across the rest at all - making their use on smaller scopes a "darker" experience....

Anyone?

[skies2clear]

Originally Posted by Don Pensack

This bandwidth is better described as a "UHC" filter. True "O-III" filters do not pass the H-Beta band at 486nm. That's fine, as a UHC filter is generally advantageous on most nebulae. On those nebulae for which a true O-III filter produces the best image (like the Veil, for example), a "real" O-III filter may be preferred.

But you can't beat that price--even for a UHC filter.

Agree with Don here. I looked into this and also found these are not "true" OIII filters, and since I already have UHC and the DGM NPB filters, I decided not to get this. However you can't argue with the price, and I'm sure they do work very well,

Clear skies

[Don Pensack]

As with any nebula filter, the bandwidth allowed through is narrow, i.e.the brightness of the "overall" image will be reduced 99%. That's less of a problem for a large scope than it is for a small scope.

Additionally, many small scope users tend to use them at higher powers, where the images are already darkened by magnification.
Keep the usage of such filters to 10X/inch or less (i.e.40X for a 4" scope) and the image will likely not be too dark.

These filters also have a different transmission at oblique angles than they do on axis. That means the nebula may be dimmed significantly if looked at off axis. If the nebula, just about the only thing still visible through these filters, disappears off axis, this is unlike the behavior of normal eyepieces where one can have one's head move around a bit and still see the image. Nebula filters require the head to be held much stiller. In practice, people don't have much of this problem with this wide a bandwidth (usually it's the line-band filters like a pure H-Beta), but to a newbie it can be disconcerting. When I explain to visitors about looking through my scope when I have a nebula filter in place, I explain this characteristic and no one has a problem with it. But people who have trouble holding their heads steady experience this more than others.
To avoid the problem:
--sit when viewing
--use low powers (larger exit pupils)
--adjust the eyecup up if it is adjustable.

But the nebulae themselves are dimmed only an inconsequential amount (0.1 magnitude or less), so there is no effective minimum size scope in which they can be used. A UHC filter will improve even the view of the Orion Nebula through a 50mm refractor.

[mercedes_sl1970]

Thanks, everyone. Guess I will just have to make the investment and give them a try.

Andrew