I finally got a chance to do some comparisons of my different Mars filters.
Wednesday morning (09-08-05) I compared several filters in a DSH-6 (6" GSO dob). All were used with a 12.5mm Highlight Plossl with Ultima barlow (2.2x) for 200x. Observations were made on Lopez Island, WA, USA, under NELM starting at~6.0. Observations were made between 5:00 a.m. (total darkness) to 6:00 a.m. (pre-dawn). Seeing was good under clear skies. Mars was directly on the meridian at the start of this session. The line-up included the following:
Meade #23A Light Red
Orion #80A Medium Blue
Orion #15 Deep Yellow
Orion Variable Polarizing Filter 1%-40% Transmission
Baader Contrast Booster
23A - Under dark skies this filter increased contrast of dark surface features very noticeably. Not huge, but a definite improvement. The Southern Polar Cap was still visible but not enhanced. As dawn approached and my dark adaption failed this filter become too dark for use in a 6" scope. It would probably do well in larger scopes.
#80A - Reduced glare and slightly highlighted the SPC, but was no help with surface detail.
#15 Yellow - It did dim the glare some, but wasn't really helpful. I didn't like the yellow color either. Just thought I'd try it.
Orion VPF - Big improvement in very dark skies but became too much for a 6" scope as dawn approached, even at the lowest setting. Reduces glare and overall brightness. Best for seeing the SPC, even better than the 80A. Noticeable increased detail in surface features. Adds no false color! Overall this was a very close second to the Baader contrast booster in this 6" scope. I should note that during a similar test last Friday in my DSH-8 I actually preffered this filter over the Baader.
BCB - Best overall filter in this scope. Reduces glare markedly. Reduces overall brightness enough to enhance both surface features and the SPC. Lets more light thru than the VPF. It adds a bit of orange to the image or maybe it just allows the planet's true color to show thru. The 23A was close to this good at showing surface detail in very dark skies, but the edge went to this filter. As the sky lightened this was the last filter to remain useful.
No filter - As dawn approached and my dark adaption failed Mars was best viewed unfiltered.
If you're viewing with a barlow, try sliding the EP part-way out of the barlow and refocusing for increased power (poor man's zoom EP). When seeing allows, the increased power may dim Mars enough to view unfiltered. Try it!
Some afterthoughts:
I have a feeling that on very dark nights during opposition larger scopes may want more darkening than provided by the Baader Contrast Booster. The
VPF may be your best bet. The 23A light red might also be better suited to larger scopes than the 6" I used this night. Likewise the darker 25A red might also become viable with a larger scope on a very dark night. I think the key to proper Mars filtering may be matching scope size, sky darkness and dark adaption to filter darkness. Some flexibility will be necessary.
I did a little more testing last night with my 8" Hardin (GSO). Seeing conditions were good, 4/5, but the full moon was blazing almost like daylight.
I found that my variable polarizer showed the polar cap best. It enhanced surface features too, but not quite as well as the #23A light red. The Baader contrast booster helped with both the caps and the surface, but couldn't beat the VP on caps or the 23 red on surface detail. I'd hate to pick one at the exclusion of the others. They each show different details.
The #25 red also highlights the surface details, but I felt it was too dark for my scope. I'll need to try it again on a moonless night when I'm dark adapted. I also tried the #80A blue and #58 green. The blue helps a bit, mostly on the cap, but the green didn't do much for me.
