Helical Focuser from an old lens

[The Thing]

I have dismantled an old 135mm lens so I can use the focusing mechanism as a non-rotating helical focuser for my guidecam on an OAG. All well and good.

However I can't get the helical bits to screw back together again - I can get the threads in the right position but they lock and won't engage smoothly as they were when they came apart.  Is there a trick I am missing? There are many (>10) threads starting rather than the usual one on a 'normal' threaded item.

[RobertM]

Does this help ?

http://www.cloudynights.com/item.php?item_id=1405

[MarkH]

Hi Duncan, after reading that article (which as an engineer I didn't think was particularly clear on how to accomplish the task(no offence Robert)) I assume that Robert's pointer was at the fact that it's a left hand thread.  I assume you either remember that from disassmbly or have visually inspected the thread ends to establish the fact.

[MarkH]

Just had another thought Duncan, depending on the production method there might be only one position out of<10 that would fit. Sooooooo you might have to try every one of those positions before you find the right one. 

[The Thing]

Thanks guys, the article is one I have already read, it's not long on details and concentrates on how fantastic a machinist the guy is.

I thought of that as well MarkH, I think I have tried most if not all positions. I am thinking that it only screws on from one (the 'other') end in which case some more disassembly is needed to take out the black machined tube lining and the bits at each end which I really want to keep as one end is a T-thread (old Tamron style t-mount lens) and the other will hold the QHY5L-II in place. I have tried unscrewing these bits but I suspect they are glued and I don't want to damage the ali helical threads. I'l let you all know how I get on and if successful post a picture.

[The Thing]

Well, I bought another lens on fleaBay as the other just will not go back together. This one is was a Pentax SMC 50mm f2, nice and short, already broken, £10.50 delivered.

Turns out there was sand in the focusing screw and so someone had tried hard to turn the focus ring and released it from it's fixings. since it was held on by three screws with washers and it had just slipped and not broken anything it was salvageable.

I dismantled and cleaned it, re-greased and re-assembled (easy this one) without glass or iris, fitted part of the Brighstar OAG output metal work as a T thread and part of an old 1.25" to 0.96" adaptor to hold the QHY5L-II and now I have this! A non-rotating helical focuser with 6mm travel for £10.50!

[RobertM]

That looks really neat but is there too much back focus used up - if you go too far back with the main camera (to match) then won't you get a mirror/prism shadow on the imaging camera.  Could be you've already taken that into account but I thought I'd mention it jic.

Robert

[The Thing]

That looks really neat but is there too much back focus used up - if you go too far back with the main camera (to match) then won't you get a mirror/prism shadow on the imaging camera.  Could be you've already taken that into account but I thought I'd mention it jic.

Robert

With my QHY5 I was using a 20mm T extension tube and using  the Brightstar coarse adjustment nearly fully extended.

It should be OK as I've been using a de-lensed barlow tube to get the QHY5L-II far enough away to focus! QHY sell/provide an extension tube to mimic an EPs  length as the chip is quite close to the business end of the housing. I've been using that as well with the Brightstar coarse adjustment about half way extended. If anything I think I will need to add a short T extension! The QHY can mostly disappear inside the lens body if needed as well. The shortest extension of the lens innards is about 15mm from the main body of the Brightstar so without the EP extension the QHY could be quite close if needed. What I need now is a clear sky to try it properly

[MarkH]

Duncan, I assume you went for a short focal length lens to get a finer adjustment/revolution, making the bigger adjustments with your tubes. The reason I say this is zuiko made a one touch zoom lens (I have one) which both zoomed and focussed on the same ring. Killing two birds with one stone so to speak. Not that I would....... kill two birds that is.

[The Thing]

Mark, FYI for an Off Axis Guider the light path to the guide camera chip has to be exactly the same length as to the DLSR CCD. To achieve this you need maximum flexibility in the overall length of the light paths.

I didn't find any mention of zoom lenses when I researched this mod. I chose this lens because it was cheap, physically short, and had a good sized aperture meaning that the clear path through the lens body would be a good size, and indeed it can take a 1.25" lens holder with a couple of mm to spare all round which for my purposes was ideal. If I wanted to have a t-mount on the front end for a DSLR or something that would have to be fitted where the front lens element would normally be, making the physical length of the lens body even more important.

[MarkH]

Now this is interesting. What are the advantages/disadvantages of off axis as opposed to sepparate scope, or combi camera ( sorry if my terminology is stil amature). I can see where you are comming from now with the extension tube.

[The Thing]

When you have a separate guide scope mounted either on top of the main telescope or to one side there are several considerations.
1)The guide scope must be absolutely solidly fixed so that it can't move even a micrometer relative to the main scope (flexure) as this will show up in images.
2) Seperate guide scopes are usually have a much much shorter focal length than the main scope. This is very much the case with a 2000mm SCT and a 190mm finder/guider scope. The way round this is to choose the cameras (guide and imaging) so that they both give arc second per pixel ratios that are reasonably close e.g. 4:1 on their respective scopes. This means to use a finder/guider and an SCT with a DSLR the guide camera needs to have very small pixels.
3) With an SCT and some other designs where the main mirror moves to focus the mirror can shift obliquely tiny amounts as it moves in and out because the focus knob acts on one side of the mirror only. And then when you think you are focused the mirror will settle back a bit as the actuating rod is a bad fit. The mirror will also shift as the scope tracks and the weight of the mirror acts though different lines. This is also a form of flexure and will affect images.

An Off Axis Guider allows the guide camera to use the same optics as the imaging camera. There is no flexure and mirror shift is automatically corrected as the image shifts the same amount for both cameras. The major downside is a much dimmer guiding image and a small field of view so finding a guide star can be problematic. Also the focusing is critical as the two cameras needs to be in focus  i.e. parfocal hence my desire to have a helical focuser as the Brightstar OAG is hard to adjust for the guider. Also the old lens body has focus markings so focus should be fairly repeatable. The OAG guide camera needs to be accurately focused so that more guide stars show up.