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Old 24-09-2014, 07:53 AM
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PRejto (Peter)
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PRejto is offline
Join Date: Jan 2011
Location: Sydney, Australia
Posts: 1,296
I have continued experimenting with the blue filter and Hutech IDAS. I also was loaned an Astrodon blue which made for an interesting comparison with the Baader. The Baader cuts at 380 nm, and the Astrodon at 400 nm, thus one would expect the Astrodon to be slightly sharper. This was born out in testing though the difference is subtle. The Astrodon appears to concentrate the light a bit more but still showed quite a large halo effect. With the Hutech IDAS in place in front of both filters there is an improvement as the Hutech cuts at 410 nm. To my eyes the effect was better for the Baader; I'm not sure why but I saw it after several iterations.

I am going to be trying out the Baader Semi-Apo filter and depending on the result may purchase a Shott GG435 as recommended by Gerd at Astronomiks. That cut might exactly match the TEC140 lens specification. What remains to be seen is how overall colour balance works with such a filter in place. A look at the graph showing frequency response of various cameras easily shows why the effect I never noticed with my KAF8300 camera is now so obvious with the Trius.

The other issue that clouds all of this discussion is that of star size using the Trius. It's hard to separate the two issues so I ran a bit of an experiment* with the blue filter by gradually increasing exposure times in 30 second increments up to 5 minutes. The stars in question range from about 9 to 11th magnitude. I've plotted the results. Perhaps my test had too many variables; seeing, gradually lowering altitude, tracking (I was guiding), etc. Still, the average result points to a possible enlargement taking off around the 3.5 minute mark. The reason I don't put this on the doorstep of tracking is that the aspects in CCDInspector are still quite good at the 3-5 minute positions. Anyway, I put it out here for what it's worth. Personally I do think that my 5 minute exposures have probably been too long if I want to avoid even bigger and overly saturated stars. But, even 30 seconds gives saturated stars. I wondering if in using this camera one might need to take a whole range of exposures in order to simulate the effect of deeper wells?


* I focused with FocusMax before each of the three runs. This was with the Baader blue filter.
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Click for full-size image (Size Graph.jpg)
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Click for full-size image (materials_schott_GG435_1.gif)
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