Rack & Pinion Focus Creep

*dadipentak* · 11-03-2009, 10:50 AM

It was a clear last night so I thought I'd try shooting the moon with the 1000mm f/8 & the 2x TC--especially now that I have a sturdy tripod. I figured I'd try both the remote and MLU. The problem was that the moon was high in the sky so I was shooting at a sharp upward angle and the rack & pinion focusing mechanism kept rolling back down, away from focus. I ended up shooting "hands on"--not the best bet for optimal resolution at that length.

Looking at the configuration of the lens, I'm a bit perplexed about how to mitigate this to allow precise and stable focusing while shooting at an acute angle from the horizontal. Any thoughts or oral traditions in this regard?

forensicscientist · 11-03-2009, 11:11 AM

deleted...

*wildlifephotog* · 11-03-2009, 11:39 AM

It's basicly the same as a regular refractor. The resistance to movement can be tightened up at the pinion. The plate can be removed and two small pieces of thin leather used to tighten up the assembly. I've had this problem on several large refractors.

BTW, it helps to put just a touch of vaseline on the leather.

*dadipentak* · 11-03-2009, 12:27 PM

Originally Posted by wildlifephotog

It's basicly the same as a regular refractor. The resistance to movement can be tightened up at the pinion. The plate can be removed and two small pieces of thin leather used to tighten up the assembly. I've had this problem on several large refractors.

BTW, it helps to put just a touch of vaseline on the leather.

I'm not sure what a regular refractor is but appreciate the suggestion. I'll investigate--thanks!

*wildlifephotog* · 11-03-2009, 12:40 PM

In the upper part of the photo, you'll see the same basic focuser style. This happens to be one I built. But the focusers are pretty much all the same.

*dadipentak* · 11-03-2009, 12:42 PM

Originally Posted by wildlifephotog

In the upper part of the photo, you'll see the same basic focuser style. This happens to be one I built. But the focusers are pretty much all the same.

Cool--thanks!

MattGunn · 11-04-2009, 01:58 PM

Wildlifephotog: I like the servo driven focus - is this a modified servo to get a cheep geared motor or are you focussing by remote control?

Dadipentak: you coulds gaffer tape tome elastic / bungee to the lens and wrap it round the camera body. If you adjust the tension it should be possible to get the elastic to take the weight of the camera and still have the focus move relatively freely.

*wildlifephotog* · 11-04-2009, 02:03 PM

It is a modified servo. It will turn 360 either direction. Controlled by a DIY battery pack and two way toggle switch.
The foam pulley is a RC airplane tire.

Ben_Edict · 11-04-2009, 02:55 PM

Originally Posted by dadipentak

It was a clear last night so I thought I'd try shooting the moon with the 1000mm f/8 & the 2x TC--especially now that I have a sturdy tripod. I figured I'd try both the remote and MLU. The problem was that the moon was high in the sky so I was shooting at a sharp upward angle and the rack & pinion focusing mechanism kept rolling back down, away from focus. I ended up shooting "hands on"--not the best bet for optimal resolution at that length.

Looking at the configuration of the lens, I'm a bit perplexed about how to mitigate this to allow precise and stable focusing while shooting at an acute angle from the horizontal. Any thoughts or oral traditions in this regard?

Hm, focus creep with a r&p focuser is pretty normal. In telescopes you usually have a simple lock-screw to stop this creep. I guess, Pentax never thought about using that lens for long expsoures or astro-imaging at all, because they sold dedicated scopes of similar specs (but with much better apo-corrction) - and these of course have that lock-screw...

But I bet you can add such a screw witin half an hour work (bore a hole, make a thread and use a nice thumb-screw).

Ben

*dadipentak* · 11-04-2009, 03:09 PM

@ Matt: Bungee & duct tape is my kind of technology but hardly seems like a recipe for precision focusing.

@ Ben: Or maybe there's a lock-screw and I just didn't recognize what it was for. I'll make a closer inspection. Surgery is a last resort (and I wouldn't attempt it myself in any case.)

Thanks to you both for the suggestions!

*wildlifephotog* · 11-04-2009, 03:23 PM

On the rear sight there is a screw holding it in place. If that hole goes all the way through, you'd only need to buy a proper threaded thumb screw.

*Ex Finn.* · 11-04-2009, 03:37 PM

A lock screw should be obvious and easy to spot, a tension adjustment screw will most likely be recessed.

Cheers. Mike.

*dadipentak* · 11-04-2009, 04:01 PM

Ok, there's no lock screw but I think I have a solution of sorts. I hooked a large rubber-band over the focusing wheels on either side of the lens and it seems to create enough friction to prevent any creep. I'll have to do some trials to see to what extent this will interfere with focus-acquisition.

UPDATE: here's a shot of the underside of the lens (with the red rubber band in place.) There's no lock screw or tension adjustment but there is access to the r&p mechanism. I have neither the tools nor skills to investigate that independently--I'll see if I can find someone who does. BTW, I have no idea what that screwy thingy at the upper left is for but it seems to have no effect on the issue at hand.

*Ex Finn.* · 11-05-2009, 03:29 PM

I would remove the plate (those are Philips head screws right?) and see if there is a way to put a friction device or brake on the pinion shaft.
Should not be too involved.

Cheers. Mike.

The thumb screw looks like it could be a collar lockscrew. for loosening and adjusting camera horizontal or vertical after mounting to lens

*dadipentak* · 11-05-2009, 04:35 PM

Originally Posted by Ex Finn.

I would remove the plate (those are Philips head screws right?) and see if there is a way to put a friction device or brake on the pinion shaft.
Should not be too involved.

Yep, I want to see what's inside that black box and, yes, those are Phillips heads. It's the kind of simple-looking operation I've proven so adept at bungling beyond the point of restoration. I'm going to get help with that--it's too valuable a lens.

Originally Posted by Ex Finn.

The thumb screw looks like it could be a collar lockscrew. for loosening and adjusting camera horizontal or vertical after mounting to lens.

Yes, that's it--very useful. Thanks!