1.91
#...
·
·
4
likes
|
|---|
|
Hi folks. Just putting something out there about the Antlia 2.5nm OIII filter in case anyone else has noticed. Recently I purchased 2" Antlia 2.5nm SHO Ultra filters. I've had a chance to try all filters and compare them to the Antlia 3nm version (I've had for a couple years now) on the same nebula (NGC3572). I have no issue with the 2.5nm SII and Ha filters. However, when directly comparing the 2.5nm OIII and 3nm OIII, the 2.5nm OIII had a number very small artifacts (small fuzz patches) and a couple of stars were noticeably brighter than they were in the 3nm image. I've attached some images to show what I mean (whole image, a fuzz patch, and brightened star in each 2.5nm and 3nm filter). If you jump between them, the artifacts and brighter stars become obvious. One of my fellow astrophotographers, who uses 50mm Antlia 3nm filters, recently imaged the same nebula (NGC3572). His 3nm OIII subs were largely the same as my 3nm OIII subs in that they both showed no artifacts or brightened stars. At this point, I began to suspect that the 2.5nm OIII filter is passing light out of band; likely in the near-infrared region (~700-1100nm). To test the theory, I took an OIII image of the same nebula (NGC3572) but with the Luminosity filter screwed to the back of my coma corrector (blocks the UV and near infrared). The resulting image gave no fuzz patches and the few stars that were brighter than normal, had returned to the level of luminosity seen in the 3nm OIII filter. For some additional evidence, I did an experiment whereby I pointed by imaging train at a light source (just a light bulb) and took frames through each 2.5nm SHO filter (~30kADU) with and without a lum filter (UV-IR blocker) in front of the imaging train. The results show that the OIII frames were duller by 1.36% with the lum filter (mean ADU reduced from ~31000 to ~30600). The SII frames were duller by 0.24%. The Ha frames showed virtually no change (<0.1%). So I can only conclude that the OIII filter is transmitting light out of the visible band. This would explain why some stars brightened appreciably in the OIII subs (those stars may give off a lot of NIR) and perhaps those "fuzz patches" are legitimate (i.e not artifacts) but not composed of OIII. I don't know if this is a problem for every 2.5nm OIII filter or just mine. I have engaged with Antlia and they're apparently going to test the transmission up to 1150nm (their spec chart stops at 850) but they are very slow and I don't know the results yet (been a few weeks). I'm hoping I just got a defective filter. Otherwise it could be an issue with the whole line up.       |
