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Old 10-03-2008, 12:39 PM
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T-adapter just arrived. Tips for tonight?

Well Marjfra finally shipped the t-mount adapter, so now have the old Olympus OM1 hooked on and focusing well. Tonight shall see the 1st steps into official astro-photography!

Without the cable thing we're currently limited to 1sec exposure with ASA 400 film at the mo but good practice until the motors arrive.

So any tips fellow people?

Imagine some sort of light pollution filter would be mandatory?
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Old 10-03-2008, 02:38 PM
Zuts
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Originally Posted by Astro78 View Post

So any tips fellow people?
Take an umbrella

Paul
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Old 10-03-2008, 03:19 PM
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From memory, with an OM1, if you set it to 'B' and without a shutter release cable, you'll actually get about 3 or 4 seconds. Best way also is to set the self-timer and lock the mirror up to minimise vibration.

But without the release cable and limited to a few seconds, you won't get much unless to stick to lunar and planets etc. For those you use shutter speeds of 1/10 to 1/1000, depending on f-ratio etc. And for those you might as well use fine grain film, not 400 ASA.

But why not just get a release cable - they are only about $10 -$15 or so. Any decent camera shop should have one. Then you can do whatever long exposures you want.
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Old 10-03-2008, 05:19 PM
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Good point, and have just ordered a chinese cable.

How do you calculate f-stop when not using a lens or eyepiece, oh let me guess, f5 = my mirror. Right?

A lower film ASA would be good but shouldn't i go the other way, ie. 800+
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Old 10-03-2008, 11:03 PM
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Ordered a Chinese cable? I thought any camera shop would have one one the shelf... I have about 3 lying around somewhere...

Film - depends on what you are taking photos of. For bright objects - the moon and palnts - hey, they are in full sun! - then fine grain film is good as you still get reasonable shutter speeds and with fine grain you can enlarge then more. You can get tables in various books that tell you the correct shutter speed for film/f-ratio combinations for the various objects. Have alook in a library.

For deep sky, faster film is generally better.

As for f-ratio - if it's prime focus, then the f is the telescope f - so f5 or whatever. If you use a barlow or teleconverter, say a 2x, then add a stop or so - but it actually depends on the lens - film distance.

For eye-piece projection, the f-ratio is still f = focal length / diameter, but here it is the 'effective focal length'. There is a bit of maths involved, but you can approximate it by assuming that you get a magnification of 1x with a 50 mm lens on a 35 mm camera. Now if you have a 8" (200 mm) f5 with a 10 mm eyepiece, visually you will get (200 x 5) / 10 = 100 x magnification. But photographically, the magification is also multiplied by the ratio of the objective focal point to eyepiece : eyepiece to focal plane distance.

Confused yet?

Anyway, is this second magnification is 5x, then your overall magnification is 500 x or the equivalent of a 500 x 50 = 2500 mm lens. So now your effective f-ratio is 2500/200 = f12.5.

Probably best to get a book on the topic....
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Old 15-03-2008, 03:44 PM
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Thanks Susan that all makes sense, i had been confused by the extra magnification from the distance to the focal plane not realising this was exclusive to eye-piece projection. So thank you very much for explaining

Looks like i will have to move the primary mirror forwards a couple of cm's to achieve focus on the stars. Can make it focus but have to remove the locking screws and push the collimating screws right in, and that makes me nervous on the EQ type mount!

I actually want to extend the tube anyway to get a larger 75% illuminated area for the sensor or in this case the film. This is not something that people seem to talk about here - how important would this be for pics (if you don't mind me asking) ?

many thanks
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Old 15-03-2008, 04:57 PM
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I agree, just go to any camera shop, and you will pick up cable, I too have three that I don't use anymore, which were in fact used with a Olympus OM 4Ti. and OM2n

Leon
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Old 17-03-2008, 10:48 PM
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Thanks Susan and Leon.

Strolled into a camera store on the weekend, $90, yes $90 for the cable! They even showed me their purchase price of $60. This was Paxton's in Chatswood.

Funny enough the assistant said they just got it in a few days before and discussed how it would sit there for years and years....might still though as i didn't pull out the cash.

Do you guys want to sell any, please let me know?
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