Filter Transmission Curves

[jmccormac01]

Hi Guys,

I'm building a prototype telescope and had some filters made by omega optical in the states. Is there a U.K based company that can test the optical performance of your telescope at different wavelengths. And also do filter transmission curves?

Cheers

James

jmccormac01@qub.ac.uk

[roundycat]

try this lot Oldham Optical Web Site. Not sure if they do or not but easy to find out.

Dennis

[Merlin66]

James,

I have a very good calibrated spectroscope and can produce a transmission curve between 4000 and 7000A of all/ any filter, if that helps you. Buil on his webpage has a detailed listing of the curves for most of the "common" astro filters...

[jmccormac01]

Hi Merlin,

The filters are custom made NIR 850-950nm, blocked above and below. I would need to test out to ~1.2 microns to make sure the blocking is in place at all wavelengths. I got a single curve with the pair of custom filters, but as you now both wont be exactly the same. We're trying to do accurate photometry and it would good to know precisely what both filters are allowing in.

Thanks anyway, I appreciate it

James

[narrowbandpaul]

i understand your concern james.

A university lab should have one. To get to 1.2um you may need a IR spectrophotometer, which is in chemistry. I am not sure how long their wavelengths are, as they use wavenumber in reciprocal cm.

A silicon based detector will only respond to 1.1um, so you can use a standard silicon based detector with a VIS/NIR spectrometer to get to 1.1um, but the response becomes poor.

Your best bet is a university physics or chemistry lab. Any uni worth its salt will have a spectromter lying around.....

I am interested to hear of the accurate photometry methods you are using.

Ally (ngc2403) and I tried this for our 4th year project. Only got as far as CCD characterisation, but never mind.

How are you calibrating your CCDs, for gain and QE, for accurate recovery of photon numbers through the filters......

great stuff

good luck

paul

[jmccormac01]

I'm doing a astrophysics PhD in the U.K., The dept has a spectrometer but its only good to 900nm, which is max throughput wavelength of the filters I had made. I should check out the chemistry labs that totally makes sense.

For the measurements were taking we need ~ 1 milli mag (or better, being the optimist lol) But the prototype CCDs haven't been on sky yet. We still doing some QC on them.

I'm currently testing them in the lab for Gain, ReadNoise, QE vs Temp, wavelength dependence on flat fielding, fringing etc. All with different clocking voltages and readout frequencies.

The set up is a low wattage constant current light source. I place one of several narrow bandpass filters in front of the source, shine a coloured spot onto a diffusion screen and take flat fields of the spot from behind the screen. Its working quite well. Hours of masking off gaps in the black out blinds etc means there is very little unwanted or scattered light.

Cheers

J@mes

[narrowbandpaul]

Sounds like a really fun phd.

What telescope are you using? What filter sets, ie johnson, SDSS, stromgren etc.

I would be really interested on the typical FWHM you get on stars at 500nm and 900nm say.

Have you got James R Janesick's, Photon Transfer book

Its such a good read.

To get a good accurate value for gain and read noise, PRNU and full well, linearity (important for photometry), a process of Photon Transfer can be employed. This book tells you all that and lots more. It will also tell you how well your flat fields are working

To get QE(lambda) you can use a calibrated silicon photodiode, have you access to such a thing.You will also need either a monochromator or interference filter for examining the qe dependance on wavelength.

Are you using back thinned CCDs?

Using something like 4000A light from a flat source, one can check the even-ness of the thinning, simply by looking at it. Sometimes the corners are over thin, and so have a higher QE for blue light, and appear brighter in blue flat field images.

Your phd sounds like a lot of fun......

what are you investigating?

paul

I start my phd in september-ish. CMOS and 3d Detectors

[jmccormac01]

Yea it is pretty cool, I'm really enjoying it.

The telescopes are Takahashi Epsilon 180 Newtonian Reflectors.

(500mm Focal length) We will initially only be looking through the custom filters I described before as we want to select a certain type of star.

We're looking for extrasolar planets, this is the testbed instrument for a larger scale version in the near future.

I havent read that book it might be worth looking at, thanks

They are specially coated thinned, back illuminated CCDs for high QE in the NIR. We will be observing red stars looking for transits of Ice Giant type stars and hopefully smaller with the final version of the instrument.

It is tons of fun, hands on which is pretty sweet!

CMOS chips, cool. what exactly will you be doing with them? I know they are quite noisy but tons of research is going in to make them behave more like CCD's.

What do you mean by a 3d detector?

Cheers

James

[narrowbandpaul]

tak epsilon 180's.....wow. See once you are finished can you pass a one too me.....:-)

are you worried about having a steep light cone from the f 2.8 optics?

I know they use lumigen as a coating for UV work, but the only way I knew of for NIR was deep depletion, whereby they use a much thicker epi layer.

what coating are you using and what wavelengths does it aborb to re-emit in NIR?

I find the NIR an underused part of the EM spectrum, especially for imaging.

I will be using the Diamond Synchrotron at RAL (rutherford labs in Oxfordshire) to investigate the UV and Xray response.

By 3D detector (i dont know much yet) but the gate electrodes are embedded in the silicon to provide a better Charge collection efficiency.

paul