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 Post subject: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 3:05 pm 
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After watching Mike's little demo at Tuesdays tutorial I'm looking to use my Microsoft Cinema HD Webcam with an SLR lens rather than the telescope. Am I right in going for a Pentax M42 screw thread with something like a 135mm prime lens ? I've seen a few on ebay but just want to make sure before bidding.

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 Post subject: Re: using old SLR lenses with webcams
PostPosted: Sat Feb 16, 2013 3:38 pm 
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Phil, remember that you'll need to devise some method of mounting the lens in front of your camera. I just took the plastic rear-lens cap, drilled a hole in the middle of it and mounted it on a wooden batten:
Attachment:
woodenbracket1.jpg
woodenbracket1.jpg [ 100.47 KiB | Viewed 6331 times ]


The aluminium brackets which Brian or Gerald can make are much neater (and more secure), but they need to have a lens-mount machined on the front. Now if your lens has a Pentax/Practika "M42" screw mount then this is easy to machine, but if you want to re-use an existing lens with a custom bayonet mount, then you'll also need to provide them with a matching old camera as a donor for the bayonet flange. So if you're buying in a "new" lens then it's best to stick to ones with M42 or T-mount screw-threads on the back.
Don't forget to include provision in your bracket for an Infra-Red blocking filter between the camera and lens, otherwise you'll never achieve proper focus. And if you live anywhere near the centre of town, you'll need a Light-Pollution Reduction filter too if you plan any long-exposure work!

You can calculate what field-of-view you'll achieve with your camera attached to a lens of any focal-length using simple trigonometry, but I've found that 28mm, 50mm & 135mm lenses work well on webcams with the small (1/4") sensors.


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 Post subject: Re: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 4:14 pm 
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Thanks Ross,

Mike did mentioned that it was worth having a word with Brian or Gerald regarding the mount but that I would need to provide them with the lens at the same time. As I don't have a suitable lens at the moment I'm going to keep my eye out on e-bay over the next few weeks. I am watching one now which is an M42 135mm and I'll bid on it tomorrow if it doesn't get to pricey before hand.

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 Post subject: Re: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 4:23 pm 
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Phil,
What are you planning to image with a 135mm lens + microsoft webcam? In real life Saturn (or Jupiter) would require 4000mm of focal length.


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 Post subject: Re: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 4:42 pm 
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I'm probably looking more for general views of the constellations, Orion Nebula, galaxy etc. Just looking for a cheap way of doing some basic imaging really without having to set the whole telescope up.

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 Post subject: Re: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 4:51 pm 
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poconnell wrote:
Orion Nebula, galaxy etc. Just looking for a cheap way of doing some basic imaging really without having to set the whole telescope up.


You'll need a Long Exposure webcam to get the deep-sky objects (fortunately we have two of them available for loan to Members).
And we'll be including these techniques in our session on 9-April.


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 Post subject: Re: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 4:59 pm 
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So would I really be wasting my time trying to use the 135mm with the Microsoft Webcam then.

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 Post subject: Re: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 7:31 pm 
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The last I heard there was no way of modifying the MS life-cam for long exposure.

Still I notice you have an Eq5 mount in your set-up. So you can either borrow one of the modified web camera or purchase a Cannon SLR. With some whackery you can mount these directly onto the EQ5 mount.

Either way you are already half way to long exposures, as you have a mount you can track with! You just need the right camera. :D So do some reading around the Cannon 1100D, they are a good little camera and cannon is the pretty much the standard in Astronomy world.

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I choose to image with an 11" SCT Celestron in this decade and do the other things, not because they are easy, but because they are hard......


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 Post subject: Re: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 8:39 pm 
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Cheers Carl

I do have a Canon 450D but I only have slow zoom lenses. I've only really used it hooking it up directly to the Explorer 200p and photographing the moon but I was just looking for something small and quick to setup. My EQ5 is also manual, I haven't yet bought then motor drive, so I was looking to rig up a barn-door type adaptor to use first with the lens and webcam, but I may have got the wrong end of the stick from Tuesdays session.

What you said though about using the dSLR makes sense and as I already have the Canon it probably would be better to put the lens money towards a motor drive for the EQ5.

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 Post subject: Re: Pentax Lens Advice
PostPosted: Sat Feb 16, 2013 9:58 pm 
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Phil,
Your Canon is ideal. Start with the constellations. The society has a simple barn door tracker made by Ross. This is easy to carry out and just requires sighting on the pole. As regards lenses I use Pentax M series on my Canon. They are cheaper than Canons on ebay and in my opinion are sharper for stars - avoid zooms. The 28mm f2.8 would be a good choice. It needs stopping down to F5.6 but that's normal. You will need a Pentax to Canon adapter again cheap on ebay and probably a Skywatcher light pollution filter. These are 48mm so a step ring to 49mm (ebay again) is needed. The last item and you've guessed - cheap on ebay - is a timer. This fires off any number of exposures whilst you are nice and warm inside. Shoot raws at ISO 400 - about 7 x 4mins.

The fun really starts with processing!


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