A gorgeous night last night with the cold clear polar air sweeping over us. Excellent seeing. I did a quick copmarison with the 12mm f0.8 and the 12mm f1.2. This was just a quick subjective test as M45 was coming into the fov.
In the lab the f0.8 looked as though it was producing a brighter image in dim light but on the night sky it was a different story!
The f1.2 could cleanly "see" fainter than the f0.8, maybe a magnitude, perhaps a touch less but very noticeable and with better quality stellar images. This might be to do with the IR correction and aberations we've talked about. However interesting to see the difference in action.
Many thanks for carrying out this "back to back" test - very interesting results. Obviously this is the opposite of what we'd expect (would expect the image from a 12mm f0.8 lens to be 2.25 (just under one magnitude) brighter than a similar focal length at f1.2).
As you know I have the following: 1 off Cosmicar / Pentax TV Lens EX ASP 12mm f0.8 1 off Cosmicar TV Lens EX 12mm 1:1.2
in order to see if it is the absence of the IR correction that is leading to "soft" images. Can't explain the lower limiting magnitude, unless the optical aberrations from such a fast lens are blurring the fainter stars to the extent that they can be no longer seen ...
In the meantime, do you have AGC on or off - just wondering if the faster lens was transmitting more light to the sensor and the camera was adjusting the gain?
Will advise when I've done some experiments (still awaiting delivery of the filter).
Hi William, You had me worried there fora moment however I've just checked and the camera was on manual. I did another test but had the idea of using a laser pointer to trigger the camera without obscuring the view.
The conditions tonight are not quite as good as last night, a touch of haze as the humidity is higher but a good night nonetheless.
I also tried a BG40 filter. These filters come with a warning that they are hygroscopic. They are not joking! One night with a little dew and the filter surface has crazed and gone partially opaque , it's now in the bin! In practice it made little difference so keep us posted on how the photo UV/IR cut goes.
The camera used was a Watec 902H2 Supreme and the lens was a HS607EX-ASP Cosmicar / Pentax TV Lens EX ASP 6mm f0.75 CS. Capture was UFO Capture, set to auto-trigger once per minute.
20131112_194700 is with the uv/ir cut filter removed, 20131112_194800 is with the uv/ir cut filter fitted.
As you can see, fitting the filter does lead to smaller stars but at the expense of rendering the fainter stars invisible. Not clear at this stage if the advertising claim of "99% Transmission average across the visible spectrum" is accurate of if the sensitivity of the camera is down to sensitivity at longer wavelengths. Probably the latter.
Either way it appears to be of little use (to me anyway) as I'd rather retain the ability to detect faint objects, even if they are a little softer than I'd like.
Bill - am happy to send it up to you if you wish to experiment :-)
Returning home after the festivities last night I had a look outside to get a breath of fresh air. Turned out to be clear(ish) so decided to try out the f0.85 with the corrector. The lens still has some spherical aberration but what a difference in the apparent magnitude. Can see +5 stellar now. Very encouraging, looking forward to getting a decent spell with it on camera. Cheers, Bill.
Excellent news - great to hear that you too have seen a significant improvement in LM.
As you know, UFO Capture allows the user to characterise lens distortions during the alignment procedure. I assume this is held somewhere as an equation that that corrects for position based on radial distance from the centre of the image. Just wondering if this equation could be utilised by yourself if you wanted to correct for aberrations to the spectral image ... ?