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Liquid crystal shutters for meteor observing.

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BillW's picture
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Joined: 05/03/2014 - 08:50
Liquid crystal shutters for meteor observing.

Hi all,

I few years back I read a paper about an alternative to the standard rotating shutter used in meteor observing. Having gathered together a collection of older generation DSLR's I wanted to find some way of bringing them into use. Meteor "photography" is almost trival now compared to the days of film! However using DSLR's still has some difficulties. Apart from actually catching any meteors at all (nothing new there... ;-)) DSLR's seem to singularly good at recording satellites. Mostly it is possible to tell what is a bright meteor and what is a satellite. However on a still images there are sometimes ambiguities especially with faint meteors. Although ostensibly for measuring velocity properties of meteors the rapid chop can be useful for discriminating between satellites and meteors.

Meteors are fast thus giving the broken line appearence on photographs. Satellites, on the other hand are, relatively speaking much slower, therefore with a fast chop those chopped bits overlap and the satellite still looks like an unbroken line.

With the greater utility of DSLR's this is a good first guide to tell if you have a satellite or a meteor.

The mechanical shutter involves a larger shutter element driven by a motor. Some systems have proven very reliable but newer technology offers an alternative. Using a small liquid crystal element mounted in from of the lens driven by an appropriate signal generator can function in the same way a rotating chopper.

Here's a pic of one of the two 50mm LC shutters I use. For the moment it is simply held with a lens hood using "optical putty" aka BluTac!

To ensure correct operation the LC element must be driven with the correct plus and minus voltage. That is it needs to see a -volt signal then a +volt signal and so on. Not from zero volts to a max as this causes the LC component not to switch between transparent and dark fully, thus degrading the crystals. To achieve the correct voltages I use a programmable function generator. Using a two channel version allows me to drive both shutters from the one unit.

This particular unit needs a 5 volt power supply so I had had to get a DC/DC converter to take the 12v car batterey voltage down to the 5v needed. (No endorsement as they are tax avoiding bampots, but these things are very cheap from various dealers on Amazon.)

It took some 12 hours observing and 1000+ exposures over several nights to get a result but it works!

This is a (stretched) crop from the original frame. This was taken with a Canon 450D 30mm f1.4 lens 27second exposure at iso1600. Chop rate 20Hz. M31 can be seen top left to give some sense of scale. Due to my general laziness I forgot to correct the clock from BST to UT and there was a large amount of drift so there is some timing ambuguity. Mike Foylan caught what might be the same meteor on video. Long lasting 3-4 seconds and in the right part of the sky from his location, Co Meath Ireland, it looks pretty good.

The system seems to have promise but it has definite limitations. the LC shutter itself has a transparency of just over 40% so there is a magnitude loss to be reckoned with. A fast lens or higher ISO can mitigate this to some extent. It is more complex electronically than a mechanical job but easier to handle. A really useful aspect is the ability to change the chopping frequency easily. It's almost a way to optimise the results on a particular velocity of meteor. Time will tell....

Hopefully the Leonids will yield some more results. If so I'll post them here.

Cheers,

Bill.

PS. Thanks to Mike and the other Nemetoders for taking the time to check their images.

callump's picture
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Thanks for sharing

Hi Bill,

I noticed a couple of your twitter postings on this, so good to see a write up!

Where do you get the LC shutters from?

Cheers, Callum

BillW's picture
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Joined: 05/03/2014 - 08:50
Hi,

Hi,

The shutters were supplied by this company.

http://www.lc-tec.se/

However, I got mine via a friend in the Netherlands as he was ordering several for the same use. I don't know for sure but there may have been a minimum order quantity. A single shutter is around Euro 200-250 depending on model.

cheers,

Bill.

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Joined: 01/04/2014 - 13:02
The View from Ravensmoor

Hi Bill,

If you're confident that Mike's video image composite is the same event as your DSLR image then I caught it too (well, part of it). Will send pm to Mike and yourself to triangulate ground-track and confirm shower association (single-station has it as a Northern Taurid).

Best regards

William

BillW's picture
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Hi,

Hi,

Perhaps not "confident" as there is still a timing error but the trail length and relatively slow speed make it "definite" maybe....

Certainly was travelling in the correct direction for a possible N Taurid.

cheers,

Bill.

BillW's picture
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Thanks to the efforts of Mike

Thanks to the efforts of Mike and William this is looking like a good candidate. Whilst there is still a timing issue on my part the az and alt of my camera are in the correct direction.

Due to resizing the image to fit it's a bit difficult to see the numbers on the graphic. The meteor trail starts at 97 km and decends to 90km over a 30km path.

Image courtesy of Mike Folan.

cheers,

Bill.