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The Pentax ED80 flattener guessing game.


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Hi,

I have been trying out some imaging using a SW ED80 with the SW field flattener and a generic M48/T adaptor for my Pentax K500 camera. It all fits together, however, I am seeing pretty severe field curvature issues still and I believe it is due to the distance between flattener and camera sensor not being the right 55mm. There are specific adaptors (presumably perfectly sized), for Canon and Nikon cameras, and I have found one for a Sony camera as well, but nothing specifically for a Pentax.

This means I am going to have to try and gauge the distance myself, and I have tried with simply unscrewing and adjusting focus/distance this way, but even if by some lucky guess I get it right, it is not an option to have a loose fitting wobbly DSLR hanging off the end of the imaging train. 

Does anyone know if there are specific adaptors for the ED80 flattener and Pentax K mount cameras and if not, would a set of spacers (like the Baader ones from FLO) help me in any way to try and get it right myself?

Cheers

Matt

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After processing the image that came out of this wobbly affair, it is much better than I first feared, although this is almost certainly beginners luck and the fact that the target was high in the sky so gravity assisted in securing things in place and the wind was kind.

M39:MOJ:2014-07-23

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If you have an M48-T2 adaptor for Pentax fit, then it should automatically achieve the right distance when attached. But.... probelm is, the Pentax isnt catered for.

So, how to make your own?

Ok, buy one of these:

http://www.firstlightoptics.com/adaptors/t-rings.html

And one of these:

http://www.teleskop-express.de/shop/product_info.php/info/p6863_TS-Adaptor-from-M48--2--filter-thread--to-T2---length-6-3mm.html

When you get the t-ring, you can remove the t-threaded part of it by loosening the three small grub screws - once loosened it should just fall out (put it somewhere safe). Next, get the M48-T2 adaptor and drop that in where the t-thread was (with M48 thread pointing outwards), tighten up the grubscrews again (making sure its all level) and there you go...  one M48-Pentax adaptor.

But make sure you have done the grubscrews up tight because there wont be any safety undercut on the M48-T2 adaptor.

Edit: Additionally, you will find it hard to get perfect corners on a large chip, even if you get the spacing right. The way around that is not to focus on a centrally placed star, but move it out to about 2/3rds away from the centre - then focus it. You will find that will often dramatically improves your corners (with a lens or refractor).

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The spacing is definitely critical with APS-C sized sensors and above - stretched stars in the corners and/or bad focus in parts of the frame.  You can usually crop out the worst of it for smaller targets (which I did for a while until I got my adaptors sorted out - different problem than yours but same result).  If you have too little spacing, then using t-extension tubes or spacer rings might help, but if you have too much then you need a shorter adaptor.

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Thanks. Looks like a home brew solution is in order. I have a Pentax K-mount T-ring and an M48-T2 adaptor which I used last night. The issue is as this is a custom solution, it doesn't match up distance wise. When tightly screwed in, the image was a right old mess. When I tried loosening everything to get a few more mm in the length, it appears to have made a big difference, so hopefully I just need some spacers rather than new thinner adaptors, although comparing my adaptor to a Canon manufacturer one, mine is already longer. I will simply keep playing around. If it works, I won't question it too much...

I also very much like the idea of focusing on 2/3rds off center. Doesn't that mean that one corner would be much worse than the others, or does it automagically focus on a circle/square around the 2/3rd off center line?

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If you have this kind of spacing issue, 2/3rds is the way to go (and even if you don't it is a good safety tip).  Assuming it is just a spacing issue causing a curved field (i.e. everything in the optical path is otherwise collimated and orthogonal) then if you focus at the centre, the corners will be most out of focus.

By focussing at the 2/3rds point you should have the best compromise as everything on a circle centred in the middle of the frame and passing through the 2/3rds point will be in focus - moving towards the centre and out to the edges/corners you go out of focus, but it should give the best average focus over the whole frame  (i.e. it's the least worst outcome).

Your mileage may vary if you have any tilt or if the camera sensor is not centred on the optical axis, so experiment to see where it works best.  Also bear in mind that if you plan to crop out the corners due to the elongated stars, you should figure out roughly where you plan to crop and use that rectangle as the outer edge for the 2/3rds measure (since you don't care about the stuff outside it being out of focus).

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