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Author Topic: Medium format 67 lens  (Read 591 times)

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Offline tomhow

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Medium format 67 lens
« on: 11:05:29, 18 June, 2017 »
So, nearly solstice and the season begins, time to get off my arse and do something.

Last year I was pondering trying out a Pentax 67 format lens on my Atik 490 for some wider field stuff.

During the spring I acquired a 135mm F4 Pentax 67 lens. Which then sat about for a few months until I made up an adaptor to attach it to the 490+filter wheel. The primary attraction of the 67 format lenses is the enormous flange focal distance: 85mm. Plenty of room for guiders and filter wheels and so forth. Much easier than trying to cram everything into the 44mm on a more typical canon lens.

I've chosen to put a Pentax 67 to 645 adaptor on the back of the lens. This gives an easy flat surface to drill into and bolt onto.  Then comes my fitting (complete with OAG) and then the filterwheel + camera.

A chunky stepper motor + arduino +belt gives motorised focus: That all worked nicely. A good V curve was achieved.

Below is 8x900s of the NAB with the 3nm filter last night (gosh don't you have to stay up late?!)

As you can see there is some coma around the edges and probably a bit of tilt in X.

Issues with tilt I expected: Getting all those fits to stay flat is tricky, plus I am wondering if the belt is yanking the lens off true slightly. Although I'm not sure about that, as you can see the lens would be pulled in Y by the belt.

Anyhow, the coma is a bit more annoying. So I inspected the lens this morning, and found it has come to focus around about the 4 metre mark, as you can see. Nowhere near the infinity mark. Now obviously I've aimed to make the optical path a couple of mm inside the prescribed FFD to enable a two-sided Vcurve, but I think I've overdone it.

Does anybody think this screwy focus position might contribute to the coma? I'll be trying some spacers I guess.

Measuring this morning I think I am 3.5mm inside the correct FFD. The ha filter will probably lengthen the light path by nearly a mm, but i think i need to get at least another mm further out. NB the lower close up of the focus is the image i am referring to. The top image was before i tried focusing.

Anybody have any experience in these matters?

 
 
 
 
 
 
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

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Offline chris.bailey

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Re: Medium format 67 lens
« Reply #1 on: 13:45:39, 18 June, 2017 »
That looks promising Tom. I suspect, based on my very limited trials with such lenses is that the closer you get focus to the infinity mark the flatter the field will become.

Chris
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Offline tomhow

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Re: Medium format 67 lens
« Reply #2 on: 13:50:40, 18 June, 2017 »
I'll chuck a 2mm spacer between the filterwheel and the adaptor and see what happens.

Oh goody, I love making thin flat spacers. Not.
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

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Offline psjshep

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Re: Medium format 67 lens
« Reply #3 on: 21:04:07, 18 June, 2017 »
Well! That looks impressive.... project project'd I'd say!

Oh goody, I love making thin flat spacers. Not.

Yeah... know that feeling :laugh:

Phil
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Offline tomhow

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Re: Medium format 67 lens
« Reply #4 on: 21:20:32, 18 June, 2017 »
That's what a 2mm spacer gives us (dark enough @ 9.15pm to focus on vega, just)

That gets us from the 4 metre mark to nearly the 8 metre mark.

I guess a bit more methinks.

Not sure i can stay awake long enough to see how it changes the coma.
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

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Offline tomhow

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Re: Medium format 67 lens
« Reply #5 on: 21:27:37, 18 June, 2017 »
Well! That looks impressive.... project project'd I'd say!

For sure the project has reached the endless fiddling stage.
It might be a tad over engineered, but you use the materials to hand....

Yeah... know that feeling :laugh:

Anything involving parting usually makes me say "why can't I just use the saw??"
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Offline psjshep

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Re: Medium format 67 lens
« Reply #6 on: 22:11:12, 18 June, 2017 »

Anything involving parting usually makes me say "why can't I just use the saw??"


 :harhar:
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Offline Einari

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Re: Medium format 67 lens
« Reply #7 on: 06:04:50, 19 June, 2017 »
Yeah I had similar problems with mating Samyang 14mm with my SXV-694.
Had to use spacers in 1mm. Was lazy and ordered a set of them.
Tapio

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Re: Medium format 67 lens
« Reply #8 on: 22:35:06, 21 June, 2017 »
Anything involving parting usually makes me say "why can't I just use the saw??"
Feel ya pain Tom.... I always part off with great trepidation !
Simon

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Offline tomhow

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Re: Medium format 67 lens
« Reply #9 on: 20:35:37, 22 June, 2017 »
On Chris' suggestion I've been experimenting with imaging some graph paper. After half an hour looking for graph paper it dawned on my to use a grid pattern on a tablet.

I can defo see some improvement when closing the aperture one click - but that kills the light levels by a factor of 1.6

So I've experimented with my own aperture step down adaptors on the front - even taking it from F4 to F4.7 shows an improvement and a lot better light levels.
Guess I am waiting for some clear skies to see how the real stars look.
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

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Offline tomhow

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Re: Medium format 67 lens
« Reply #10 on: 09:46:45, 06 August, 2017 »
Finally got another go!

A bit more like it this time. Moved to a 150mm medium format lens, stopped to about f4.3 using a step down ring on the front. 17x900s with Atik 490 + Astrodon 3nm

Pleased with this - stars look ok. I'm going to make up another step down ring to see how it responds. Will aim for about f4, maybe f3.8.

 
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

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Offline tomhow

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Re: Medium format 67 lens
« Reply #11 on: 10:02:41, 13 August, 2017 »
widened the stop down ring to about f3.9 and yet another go.  Stars actually seem better this time. Which makes no sense!

4x1200 (and then clouds!)

 
 
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

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Offline Nimbus

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Re: Medium format 67 lens
« Reply #12 on: 10:08:25, 13 August, 2017 »
Definitely producing the goods, looks like you have a great set up. As soon as I am done with NGC700/IC5070 the Veil complex is next on the list. Jim
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