8 replies to this topic

[Ben Diss]

Can someone check my math please.

 

I'm looking to do a slight adjustment to my camera tilt. NINA (Hocus Focus Aberation Inspector) says to move one corner towards the objective 28um. I'd like to know how many degrees to turn the grub screw, so I've done some math. This isn't my area of strength, so if I could lay this out and have someone check it over, I would greatly appreciate it.

 

Equipment is TOA-150B with Tak 4" focuser and Oasis Rose electronic focuser. ZWO ASI6200MM, EFW, OAG-L.

 

Focuser Steps per Rev = 27000
1 Revolution (mm) - measured = 24.4

Tube movement mm/Step = 0.000903704
Tube movement  micron/Step  = 0.903703704

M4 Grub Screw pitch rev/mm = 0.7
M4 Grub Screw pitch rev/micron = 700
M4 Grub Screw degree/micron = 1.9

Micron to move = 28.7
M4 degrees = 55.8

 

I've attached a small spreadsheet with these calcs.

 

Thanks in advance!

Attached Files

•  TOA150-OasisRose.xls   18KB   3 downloads

[klangwolke]

I’m getting errors trying to open the document, unsupported excel version. If it isn’t a lot, maybe cut and paste here. I think I follow and it makes sense.

[triplemon]

Assuming the grub screw is the one that tilts the image train and it is located about the same distance from the center than the image corner, all you need is the pitch of that screw and the corner focal plane offset:

One turn would move it out 0.7mm = 700um, but you only need 28um. So 28/700 = 0.04 turns or 15 degree scew rotation is what it takes.

 

If that screw is located at a larger radius from the center, it is proportionally more of a turn. So if e.g. the tilting grub screws are on a 4.5" diameter circle, but your sensor is only 1.5" diagonal, then its 4.5/1.5 = 3x as much of adjustment to get that amount of tilt.

 

If there are three tilt screws only, its a bit more complicated, as the directions of the adjustment are all different than the x/y directions that NINA calculates. But at least you get some idea how much to turn. The easiest way to find out for sure is to just turn half as much as you think it takes and re-measure the tilt you get. Then it will be very easy to predict how much of a 2nd adjustment it takes to be spot on after that.

Edited by triplemon, 19 May 2025 - 03:07 PM.

[Ben Diss]

I’m getting errors trying to open the document, unsupported excel version. If it isn’t a lot, maybe cut and paste here. I think I follow and it makes sense.

 

Try this one.

Attached Files

•  TOA150-OasisRose.xlsx   9.53KB   1 downloads

[Ben Diss]

Assuming the grub screw is the one that tilts the image train and it is located about the same distance from the center than the image corner, all you need is the pitch of that screw and the corner focal plane offset:

One turn would move it out 0.7mm = 700um, but you only need 28um. So 28/700 = 0.04 turns or 15 degree scew rotation is what it takes.

 

If that screw is located at a larger radius from the center, it is proportionally more of a turn. So if e.g. the tilting grub screws are on a 4.5" diameter circle, but your sensor is only 1.5" diagonal, then its 4.5/1.5 = 3x as much of adjustment to get that amount of tilt.

 

If there are three tilt screws only, its a bit more complicated, as the directions of the adjustment are all different than the x/y directions that NINA calculates. But at least you get some idea how much to turn. The easiest way to find out for sure is to just turn half as much as you think it takes and re-measure the tilt you get. Then it will be very easy to predict how much of a 2nd adjustment it takes to be spot on after that.

 

Right. I hadn't considered the grub screw not being congruent with the corner of the sensor. It'll take more movement of the screw. I'll have to measure that.

 

It's also unclear what is down/up/left/right from NINA's analysis.

[klangwolke]

File works now.  It wasn’t clear to me either which screw to move which direction. I’ve only done it once, and I remember figuring it out by moving a screw and rerunning the analysis to see the impact. Did you get it working?

[Ben Diss]

File works now.  It wasn’t clear to me either which screw to move which direction. I’ve only done it once, and I remember figuring it out by moving a screw and rerunning the analysis to see the impact. Did you get it working?

I have not. I went down a rabbit hole and now have to spend more money to get out of it. I looked at each focus run throughout two nights and found that the tilt was inconsisent. I suspect the imaging train beyond the focuser is flexing. No use trying to fix tilt when it changes based on the alt/az. So, I've ordered a couple of unobtanium M92 extenders to push the focuser back inside. Of the 74.5mm focuser range, 53.5 is hanging outside. I found a company in Belarus that makes a M92 40mm extention. Hopefully the tariff hit won't be too bad.

 

Thanks for looking at the data though. I'll post back here when I make some progress.

[t-ara-fan]

Your M4 screw specs have the units inverted. The screw is not 700 revolutions per micron. But your final calculated number was correct.

Takahashi sells a M92 extender that is 25mm long. I have one on my TOA for the reason you mentioned.

[Ben Diss]

Your M4 screw specs have the units inverted. The screw is not 700 revolutions per micron. But your final calculated number was correct.

Takahashi sells a M92 extender that is 25mm long. I have one on my TOA for the reason you mentioned.

Ah, yes. I corrected that. Thank you.

 

Here are the M92 extensions I found:

 

TKA36205 M92 extension 58mm

TKA31584 M92 extension 25mm

TKA26583 M92 extension 17mm

RafCamera M92 extension 40mm

Williams Optiocs M92 extension 66mm

 

I have the first one installed now, so adding the RafCamera 40mm extension will leave 13.5mm of the drawtube exposed.