UV Venus imaging

robbeh

Hi,

is there any information available regarding the UV characteristics of the ASi120MM-S, including sensor sensitivity and UV transmittance of the glass window in front of the sensor? I am using the Baader U Filter (http://www.baader-planetarium.de/sektion/s44/bilder/gross_ufilter_spektrum.gif) for UV Venus imaging with some success, but exposure times are very high (500-800ms, 60-80% gain), so seeing is a big problem.

Greets, Robert

image.jpg

Sam@ZWO

here is the curve of the sensor

~~and I attached the curve of the protect AR window~~

robbeh

Thank you for the images. The Baader filter has its maximum transmission at 350nm, if one extrapolates the curves its damn hard to get any UV to be sensed. Maybe it is possible to temporarily remove the glass in front of the sensor as this might multiply sensitivity by a factor of about 4 at 350nm?

Any hint about that, Sam? Thanks, Robert

fossil light

Hi Robert,

I used my ASI174MM with an old style Baader Venus filter (like the U-filter but with a small IR leak) on my 444mm Newtonian and got this image with an exposure time of about 30msec from memory and gain of about 75%. I had an Astronomik MC Clear filter as a cover window at the time and a Pierre Astro UV transmissive atmospheric dispersion corrector to correct for the low altitude at the time. I also had two APM 2.7x barlows stacked. Quite a lot of glass in the light path in total but all should have been transmissive in the UV.

What scope are you using? A lot of modern multilayer coatings have very poor transmission in the UV.

Cheers

Martin

robbeh

Hi Martin,

that's a really great image, whow. In the meantime I managed to remove the glass window in front of the sensor, easy doing as it is screwed not glued. Hope to be able to give it another try soon. I use a 200 f/5 Newton with a 4X Powermate. I think a big part of UV is lost in my Powermate, maybe I have to find UV optimized Barlows to gain better frame rates. First I want to get in touch with TeleVue in order to gather more information.

Robert

fossil light

Hi Robert,

I would be surprised if is the front glass seeing the transmission spectrum from Sam. Much more likely to be the coatings on your Powermate. You might want to have a look at the APM 2.7x barlow and the Baader 2x barlow. I would ask the question to APM and www.Astrograph.

The 2.7x is supposed to be 99% transmitting in the range 400nm-700nm but not sure what it does below that: http://www.apm-telescopes.co.uk/apm-coma-barlow.html although it can't be bad as that is what I used paired together to make a 6.25x barlow.

Have you got a standard TV 3x barlow? I used one for this image in 2012 with my 222mm Newtonian which needed 60ms exposure at pretty high gain though.

I just checked and I used a gain of 350 and 57ms for my recent image of 18th April.

Hope that helps,

Martin