Pentax Lens Advice

For those interested in imaging and image processing with DSLR, CCD, webcam or film.
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rwilkinson
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Re: using old SLR lenses with webcams

#1 Post by rwilkinson » Sat Feb 16, 2013 3:38 pm

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:
woodenbracket1.jpg
woodenbracket1.jpg (100.47 KiB) Viewed 8497 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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DRatledge
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Re: Pentax Lens Advice

#2 Post by DRatledge » Sat Feb 16, 2013 4:23 pm

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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Re: Pentax Lens Advice

#3 Post by rwilkinson » Sat Feb 16, 2013 4:51 pm

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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Re: Pentax Lens Advice

#4 Post by cstone » Sat Feb 16, 2013 7:31 pm

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.
Celestron 11" Telescope
Eq6 Pro Mount
Eq5 Mount
EQ Mod
QHY5 Guide camera
Cannon 1000D SLR Camera

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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Re: Pentax Lens Advice

#5 Post by DRatledge » Sat Feb 16, 2013 9:58 pm

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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Re: Pentax Lens Advice

#6 Post by DRatledge » Sun Feb 17, 2013 12:25 pm

Shame about the Mac! IRIS on PC is the best I know for removing Bolton's light pollution. Most other packages can only do simple gradient removal which is fine for Arizona but not powerful enough for Lancashire. Still you should be ok for the constellations.

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Re: wide-field photography with DSLR

#7 Post by rwilkinson » Sun Feb 17, 2013 1:24 pm

Phil, our Loans equipment includes Pentax 50mm and Zeiss 135mm lenses, each fitted with a Canon-EOS bayonet adaptor, and as well as my barn-door tracker prototype, we also have the more sophisticated AstroMount.
If you'd like to borrow any of this kit, have a read of our Rules:
http://www.boltonastro.co.uk/members2/i ... ans-Rules/
and then contact Dean to arrange the loan.

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Re: dovetail adaptor for DSLR

#8 Post by rwilkinson » Sun Feb 17, 2013 1:28 pm

My latest wooden version includes a right-angle mounting, so that all images will have North at the top when using an EQ mount:
newbracket.jpg
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