Configuring MFP2 for a lens. Need help please.

[Samuel Tumlad]

Hi all. First of all, thank you for the fantastic project that this is and I am absolutely grateful to Mr. Brown.

My project is based on the TMC22xx + Nano PCB. I have the focuser moving, and I find that the needed claculations for adjusting related to the Trinamic drivers easy to follow. I digress.

The motor is a Wantai Stepper Motor NEMA 17 with 200 steps. The lens is a Nikkor 180 ED @ f/2.8. I have 3D printed a sleeve pulley to fit the helical focuser grips that turns the entire thing into a 132T pulley connected via a GT2 timing belt (350mm) to a 16T pulley on the motor shaft.

That gives me 8.25:1 ratio. The focuser is unable to turn a single 360 on the Nikkor. From closest focusing distance to >infinity the lens body contracts and moves about 24.8mm (measured with caliper).

I tried the technique of a user who posted here and tried to "jog" the focuser from minimum focus to >infinity and I was able to get 3485 steps (Motor Speed set to Medium at 1/4 step on theMFP2 windows app) which doesn't seem to be anywhere close to the 17000 steps that user observed so am dumb founded.

Now, I have all those numbers but I don't know what to do with them.

I have attached an imaged of the pulley/belt system if that helps

[brownrb]

Hi Samuel
The rules we apply to refractors etc do not really equate to what happens with a telescopic/fixed lens on a DSLR

because the total focuser range (probably over 1 or less complete rotation) (in mm) is significantly less than most telescopes.

The faster the lens the more gearing/reduction is required. For an f2.8 lens this means significantly more gearing or reduction is required.

Whereas a refractor might have 60cm of travel, those camera lesses are much much less (often by a factor of 3-4). And there is a fine point in focus where probably a very small 2-3 degree turn is the difference between focus and out of focus.

You are starting with a NEMA 200 step motor, then gearing it and then using microstepping. My thoughts on microstepping have significantly changed over the years and now I prefer to have full stepping at all times.

And to make matters worse, the amount of rotation (or steps) is not linear over the lens travel, as one gets closer to hills and then to the sky, the amount of lens travel becomes much less, and approaching stars even less. Meaning to use those lens for astronomy, the useful focus range is often less that 10 degrees of rotation, and often less than 5.

lenses are not really designed for sky gazing, most of the lens rotation is geared from close up to portrait shots and then to panoramic views. Significantly less lens rotation is dedicated to the hills-sky-infinity. If you use cameras to take photographs then you find this out pretty quickly.

I am qualifying this by only talking about the fast f/8 lenses as I have not used wide angle lens at all (in astronomy).

I doubt whether you can get enough steps with the current setup that you have. I have personally used Canon f2/8L 70-200 USM with a GT2 belt connected to a NEMA-17-PG27 half stepped which gave good results, but focusing is always a challenge on such setups. I cannot remember what the steps were, but they were well above what you have. From memory somewhere on the plus side of 18000. That was a 70mm (closer to 90mm with a ring gear) diameter lens using a GT pulley on the NEMA shaft of 20T.

BUT, there is always a but, the useful range of focusing (to the sky) the useful range of steps was in the vicinty of less than 50 steps between focus and not in focus when used for astronomy.

The maths is simple - 18000 steps from closeup to the stars. But only 50 of those were useful steps in focusing on the stars.

I also used the same setup on a sky camera using an f1/2 lens and could not achieve focus most of the time (an indication that I required more reduction).

The key take away on this is that you can always get good results form a highly geared system, but if you do not have enough steps, then no amount of things you do can ever fix the focus problem,

Cheers
Robert

[Samuel Tumlad]

Thank you very much for the response Robert.

In my case what could be the solution? Should I step down to f/4-f/5? Should I increase or eliminate microstepping? Change to a 400 step NEMA 17? Increase or decrease gear reduction?

And also, it is unclear to me still how, if possible, do I use the numbers above and make sense of it to plug it into the MFP2 system. The PDFs have clear instructions and examples on setting up telescopes but only a brief and very narrow section on lenses.

Regards,
Samuel

[brownrb]

my thoughts are below, an opinion, to each their own
For a fast lens you cannot go past a nema 17 pg5. It has grunt as well as good reduction, giving a high number of steps.

For wide imaging, the steps near to focus is over such a small range, like less that 1 degree can be the difference between in focus and out of focus. So the most geared motor is required. The faster the lens is the more redunction you need

I do not bother with step size, and usually set backlash to 0.
I use motor speed slow, I enable delay after move, having a time of 50-100

I have the camera catching images in a loop so I can see what the focus looks like.

I set up the focuser - the focuser is adjusted to near focus like on the house next door. Manually, Then I connect the belt etc and I set that position as 0.

Then I set maxsteps to like 10000 and slowly jog the focuser out towards best focus. I go a bit past that till the image starts to go out of focus again. then I add about 500 steps to that, then I set that steps as maxsteps value.

Once that is set I move back towards 0 until the image goes through best focus and stop just as the image starts to go out of focus.

Then it is power up with the imaging software, and away we go.

Regards
Robert

[Samuel Tumlad]

I see, thank you for the information Mr. Brown.

I would eventually replace the motor then. But for now I'll see what I can do with the materials I have.

I really appreciate the all the help. :)

Regards,
Samuel