Filter comparisons

[Allan]

I have grown my filter collection recently with the intention of doing some comparisons to come up with a range that suits my observing. I had the chance to do some initial comparisons recently at a dark site. I have a Moonlite filter slider in my Dob so quick, back and forth observations were possible. These are just my initial thoughts, as my week long trip was cut short by weather, but they will form the basis of my decisions going forward. I have several filters under consideration and want to trim this back to the minimum number to give me a good range of options.

Firstly, my Lumicon H-Beta is a keeper because it's the only H-Beta I decided to buy. I mention it in passing to say how easy the Horsehead was seen using it. I rotated most of my eyepieces through the focuser, and the 21 Ethos was clearly the best combination with this filter on the Horsey.

I spent most of my time comparing my Baader and Lumicon OIII filters on various bright and planetary nebula. They showed virtually identical contrast on every object I viewed. The teal colour tone was also the same. What was clearly noticeable as I flicked the slider back and forth, was the brighter image from the Lumicon. It has a slightly wider bandpass than the Baader, and my new Lumicon has very good transmission figures, so that may explain the difference. I have owned my Baader OIII for a long time now and I like it very much. The build quality of all my Baader filters is just a notch above others I have used. But the Lumicon gives up nothing in comparison at the eyepiece, yet the slightly brighter image is a little more appealing. The result of this one is that the Baader OIII looks set to be culled out of my lineup.

The only other comparison I started was between my Lumicon UHC and DGM NPB. It's too early to say which way I will go with this one. The Lumicon UHC appears to be just a much brighter version of its OIII cousin. Switching between the two, the most noticeable difference is the colour. The UHC has the typical teal colour, while the NPB has a definite rusty brown tone. At this stage I am leaning toward the NPB, only because it offers a much different view to the Lumicon OIII, while the UHC looks too similiar to the OIII to provide much of a different perspective. But more time is needed on this comparison.

Anyway, I had a great time under dark skies doing my comparisons, which is what I enjoy doing.

[AG Hybrid (Adrian)]

Sorry if this is off topic. But, how did you get the filter slide on your Orion dob? Did you move your secondary veins?

[Allan]

Yes, I moved the secondary veins around 45 degrees. I bought the Moonlite filter slider plus base plate that suits my XT12g. Bolts pass through a spacer, the base plate, the tube and then into the filter slider to keep it secured in place. My Feathertouch focuser then attaches to the spacer that's on top of the base plate. I got sick of screwing filters in and out of eyepieces, so this setup works much better for me.

[Don Pensack]

These graphs can tell you a lot about how different filters compare:
http://www.karmalimbo.com/aro/pics/filters/narrow.jpg
http://www.karmalimbo.com/aro/pics/filters/o3a.jpg
http://www.karmalimbo.com/aro/pics/filters/o3b.jpg

Notes about filters:
Narrowband filters should transmit H-Beta (486nm), and both O-III lines (496nm and 501nm) at >90%, and very little else. Since bandwidths shift as the light goes through the filter further off-axis, to guarantee that you will see all three lines well means the bandwidth has to be > 15-16nm. A typical high quality narrowband filter is 20-25nm in width. Wider, and contrast is diminished by a greater amount of sky light entering the field. Narrower, and one of the lines might get clipped.
H-Beta is the blue version of light emitted by all nebula that emit Hydrogen alpha in the deep red. Our eyes can barely see the H-alpha light, and H-Beta is not emitted with the same strength, yet including H-Beta light in a filter makes the nebula appear larger in extent. Some nebula filters pass the H-alpha also (DGM), but this has little effect in scopes smaller than about 45cm or so.

Visual O-III filters should transmit the two O-III lines (496, 501nm) at >90%. This requires 10-12nm at least and a few good O-III filters are as wide as 15nm. The Baader clips the 496nm line, so it's more of a "photo-visual" filter, with a 10nm bandwidth, but not centered in between the two lines. Other O-III filters catch one or both of the two lines and vary a lot in bandwidth. You want both lines at >90% but very little else.
Nebulae that emit a lot of energy in O-III are many planetary nebulae, some supernova remnants, and small sections of some large nebulae. Wolf-Rayet stars tend to ionize the nebulae around them and also show well in O-III filters (like NGC2359, Thor's Helmet)

Hydrogen Beta filters should pass only the 486nm line with >90%. These are most effective on nebulae that emit most of their energy in the Hydrogen wavelengths. The 686nm line in the deep red is tough for our eyes to see well, alas, but the H-Beta line at 486nm in the blue is right in our sweetspot for nighttime viewing. These filters do really well to enhance contrast in H emission nebulae because of a 8-10nm bandwidth.
Photographic filters go straight for H-alpha, where there is more energy. Visual observers will prefer the H-Beta filter for certain nebulae.

Here is an article written by a consummate observer, comparing the basic filter types and how each works on the most popular nebulae (sorry for the northern bias):
http://www.prairieastronomyclub.org/...ommon-nebulae/

[pw (Peter)]

Thanks for the detailed explanation Don, very educational.