"Squaring" a focusser

[erick (Eric)]

I have an 8" GSO OTA with a 10:1 GSO focusser.

I want to replace it with a second hand JMI focusser I have bought.

A quick look suggests that the mounting holes are interchangeable between the GSO and the JMI - beaudy!

The JMI is on a flat base, but I also bought a spacer that fits the JMI onto an 8" tube.

Now the JMI is sophisticated - well for me anyway - it has four small grub screws that can be driven into the spacer (loosen mounting bolts first) to tilt the focusser.

So, how do it get it properly squared?

Here is my thinking. The current OTA seems to be fairly well collimated so perhaps the GSO focusser is fairly well squared at the moment. So, remove the secondary, fit my collimation laser to the GSO focusser and see where the spot hits the opposite inside surface of the OTA. Crank the focusser in and out and see if it moves. Mark the relevant positions. Remove the focusser. Accurately measure the location of the centre of the focusser hole (from the mounting holes in case the large hole is off). Now locate that point on the opposite inside surface of the OTA. Does it coincide with where the laser struck? If yes, all is good. If not, then?

Fit JMI and insert laser. Adjust grub screws to have laser strike - where?

Or ignore all this entirely - swap over the focussers then worry about the location of the focusser relative to the optical axis by other means?

Advice welcome - thanks Eric

[erick (Eric)]

ps. I'm now doing my Google searches - sorry I did this back to front. Looks like lots of advice is out there.

[DavidU (Dave)]

I would think with the laser inserted into the focuser and wound out you would want the laser spot dead centre of the primary.(provided the scope is collimated.)

[erick (Eric)]

So I guess that is the another way, Dave. Start with scope fairly well collimated. Presume (??) the GSO focusser is fairly well squared. Check the laser dot is hitting centre of primary. Change over focussers without touching secondary or primary adjustments. Adjust focusser until laser dot hits centre of primary. Maybe monitor where the laser is striking the secondary for each focusser as well. If it moves substantially between focussers, that cannot be good?


Yes, and before anyone says it - the laser collimator will be well collimated itself.

[DavidU (Dave)]

Yes I saw you checking the laser in a V block at Snake Valley.
Also, It actually doesn't matter if the primary is out of collimation doing it like this. As long as the secondary is spot on.

[erick (Eric)]

Originally Posted by DavidU

Also, It actually doesn't matter if the primary is out of collimation doing it like this. As long as the secondary is spot on.

Yes, of course - got it now - thanks Dave.

[chrisp9au (Chris)]

Hi Eric,

I went through this exercise.

I decided that the best way to do it was with the secondary removed.

With my wife's dressmaking tape measure, I carefully measured (several times!) and marked the exact spot on the opposite side of tube, having first determined the exact circumference of the tube, and the distance from the front equal to the center of the focuser.
Then I installed the focuser, adjusting it until my laser hit the marked spot.

Then install the secondary and collimate, knowing that the focuser is square to start with.

Cheers
Chris

[multiweb (Marc)]

Originally Posted by erick

Fit JMI and insert laser. Adjust grub screws to have laser strike - where?

Or ignore all this entirely - swap over the focussers then worry about the location of the focusser relative to the optical axis by other means?

Advice welcome - thanks Eric

Hi Erick, I have a fair bit of experience now on this one both in Newts and SCTs. After having pulled everything apart. The focuser barrel/draw tube in my case was a problem. To check if the focuser mechanism was square in itself I spun it on a turn table and shimmed it until it was acceptable. Then for the SCT I stuck a collimated laser directly screwed in the mirror cell and removed the secondary. Check the red dot reflection on a wall while spinning the entire OTA. Shimmed the adaptor ring. When happy with that I did the same with the laser inside the focuser. Once you have that working secondary adjustment is only part of the standard collimation process.