Newtonian for Imaging at prime focus - no secondary.

[Tommohawk]

I posted some while back on this, and now have something definite to report and a few further questions.

I now have a flex tube 250 fitted with an F4 250 mirror. I converted the spider to hold an omegon 385 osc camera. Mechanically and optically this has all worked out nicely. Precise collimation is a piece of cake. However there remains a complication.

To enable focusing I replaced the collimation screws with toothed drive pulleys connected by a belt. I also beefed up the springs. This works quite well however when focusing there is slight sideways image shift. Presumably this is due to either uneven threads or eccentric pulleys. Or it may be due to friction where the collimation screws pass through homemade nylon bushes. In addition quite a lot of torque is required to turn the pulleys. I think this might prove too much for the stepper motor method which I had planned. I may be able to adjust the camera position so the springs are less compressed whilst still supporting the mirror sufficiently.

I have come up with two possible solutions. First, replace the pulley system with lead screws driven by stepper motors. But most lead screws have a 2 mm or greater pitch which wouldn't make for fine focusing. Also I can't easily try out a manual version beforehand.

Secondly I could focus at the camera using a helical focuser suspended by the spider. But this will be tricky to adapt to motorised focusing. 

Somebody must have been this way before! Does anyone have any thoughts or ideas please? 

[andrew s]

In the old days the fine focus on Schmidt cameras was done with micrometer heads at the mirror.

You could try 3 at the base of one at the top with steeper motors for movement.

Regards Andrew 

Edited July 5, 2020 by andrew s

[Tommohawk]

1 minute ago, andrew s said:

In the old days the fine focus on Schmidt cameras was done with micrometer heads at the mirror.

You could try 3 at the base of one at the top with steeper motors for movement.

Regards Andrew 

Yeah now you mention it I think had something like that on the OMC200. Thats a great idea, especially if I could find a way of mounting a single one at the camera. The objective with this design is to minimise central obstruction though, so mounting at the camera gets tricky. Definitely something to think about, thanks for the idea.

[Chriske]

Hi,

We did a similar thing years ago. We installed a Nema17 stepper above the secondary mirror of a CAS
The stepper and mirror were surrounded by 6 very small ball bearings. Focusing was done with a M3 threaded rod.
There's a problem focusing with a CAS secondary mirror. Moving that secondary mirror is very tricky. Alignment of that mirror was no problem in our case, because of these 6  ball bearings.
Biggest problem is the the threaded rod itself. Most of these commercial threaded rod are not 'regular' enough at all. And that is not all : we discovered that over a certain length the threads of two different rods differ. One time we did a test with a M6 threaded rod of two different sources. Over the length of 1 meter the treads of these rods differ 1.2mm(!!!!!)
What do I mean with 'not regular'. Well we tested a M3 rod for that secondary mirror focuser. We installed a dial gauge aside the unit. The test was to see if for every single and precise rotation of that rod the mirror would move each time the same distance up or down. Needles to say all was springloaded and test were done very thorough.
Well, no the distance for one single rotation of that M3 threaded rod did differ sometimes 0.02/0.03 mm. So we never proceeded with that project.

The only way to overcome this problem is cut your own threads.
most commercial threaded rods are 'rolled'(not cut) at very high speed. For one threaded rod 1 meter long it takes about two seconds to finish.

Now that I come to think of it maybe, just maybe you also have 'bad' rods -of different locations-..?

[Tommohawk]

On 05/07/2020 at 18:04, Chriske said:

Hi,

We did a similar thing years ago. We installed a Nema17 stepper above the secondary mirror of a CAS
The stepper and mirror were surrounded by 6 very small ball bearings. Focusing was done with a M3 threaded rod.
There's a problem focusing with a CAS secondary mirror. Moving that secondary mirror is very tricky. Alignment of that mirror was no problem in our case, because of these 6  ball bearings.
Biggest problem is the the threaded rod itself. Most of these commercial threaded rod are not 'regular' enough at all. And that is not all : we discovered that over a certain length the threads of two different rods differ. One time we did a test with a M6 threaded rod of two different sources. Over the length of 1 meter the treads of these rods differ 1.2mm(!!!!!)
What do I mean with 'not regular'. Well we tested a M3 rod for that secondary mirror focuser. We installed a dial gauge aside the unit. The test was to see if for every single and precise rotation of that rod the mirror would move each time the same distance up or down. Needles to say all was springloaded and test were done very thorough.
Well, no the distance for one single rotation of that M3 threaded rod did differ sometimes 0.02/0.03 mm. So we never proceeded with that project.

The only way to overcome this problem is cut your own threads.
most commercial threaded rods are 'rolled'(not cut) at very high speed. For one threaded rod 1 meter long it takes about two seconds to finish.

Now that I come to think of it maybe, just maybe you also have 'bad' rods -of different locations-..?

Yeah I think trying to get accuracy at this kind of level is just not going to happen.... well, not in my kind of budget anyhow. BTW I cant quite get my head around the mechanics of your secondary focuser? 

Anyhow, when I think about it with my set up I have a 5000mm FL scope (including the powermate) and trying to keep the image stable on a chip of only 4.1mm height. so the tiniest inaccuracy with  the synchronization of the 3 collimation screws will cause image shift. 

I wonder about focusing at the camera position ie where the secondary would normally be using a helical focuser, and mounting the focusing motor on the outside of the OTA, with a belt around the focuser ring. Obviously the belt would be in the light path, but it would be pretty thin and could be removed once focused. Trouble is I think any amount of sideways tension from the belt would cause movement and consequent image shift - the spider is pretty secure, but probably not secure enough.

Anyone have any ideas?