Jump to content

focus issues....


Recommended Posts

Okay, so I took apart the focuser to replace the bearings. I've collimated to a degree using a bottle top with a small hole in it untill I get my Cheshire.

Before I did any of this I had my eye pieces all the way home in the holder and had room left on the focuser to extend further out than needed, IE the focuser would bring objects into focus before I ran out of travel. Since I've done the above I'm now finding that I almost need a bit more travel? I've resorted to not sliding the eye pieces all the way in to allow for a bit more length from the secondary to the eyepiece if that makes sense?

Have I done something wrong here? Is my collimation incorrect to the degree that it would affect the focal point? Is any of that even possible??

I'm confused dot com. Off out for a half hour or so now I can feel my fingers again!

Clear skies all

Link to comment
Share on other sites

Unless you have significantly moved your mirrors closer together whilst doing your collimation, that procedure shouldn't affect the focus position. Does the focuser still have the same amount of travel since you rebuilt it?

Link to comment
Share on other sites

I know that at least one adjustment screw seemed to be a long way in on the primary, so that might be an idea, kind of back them all down a bit so in effect the primary moves further towards the bottom. Thanks, I'll give that a go tomorrow. I know the collimation was quite a way out before, having bought it second hand I don't really know what has happened to it in the past, but the chap I bought it off seemed to know his stuff though. Having said that, it did travel back from Liverpool wrapped up in a sleeping bag in the back of my van, so that won't have helped, but I was gentle with the driving, so it shouldn't have been too bad.

I'll try backing it all up and re-collimating it and see what happens. I do have the same amount of travel as before I started so I don't think that's an issue. It makes sense if the primary is too far up the tube, I think instead of adjusting the two screws to effectively catch up, maybe I should have backed off the other one instead? I would say the approx amount would be 2-3mm, would that be enough to make the difference? Sorry for rambling, I'm thinking out loud lol.

On a different note, my phone says it's 0 deg C outside, I disagree! If it is zero I have no chance if it gets colder, I'm blinkin' frozen! Time for a brew and "wonders of the solar system" on BBC HD :D

Link to comment
Share on other sites

Hi there

You say you took the focuser apart.

If it is a rack and pinion type focuser have you put the rack on back to front.

On one of my focus tubes the rack is off set as in the fixing screws are set in at different lengths from the end.

It is possible to refit the rack the wrong way round, this will reduce the amount of outward travel on the focuser.

Graham

Link to comment
Share on other sites

Hi there

You say you took the focuser apart.

If it is a rack and pinion type focuser have you put the rack on back to front.

On one of my focus tubes the rack is off set as in the fixing screws are set in at different lengths from the end.

It is possible to refit the rack the wrong way round, this will reduce the amount of outward travel on the focuser.

Graham

It's a crayford type focuser, and I was carefull to ensure I made a mental note of anything that could go back the wrong way round, there was nothing that could go together wrong unless I had bits left over, so I'm confident it's together correctly. From the above it does sound like I may have a migrated primary from collimating. I'll have a play later and pray for clear skies later to test it out. My cheshire should be arriving in the next few days, so that will help fine tune things, but this issue is related but not for any reason I had accounted for. I'm going to back everything right off and start from the very end of the tube and go from there. Although I do intend backing each screw by the same amount to try and keep it as close to collimation as I can so it's not too far out when I start collimating again.

Link to comment
Share on other sites

Had another quick go at collimating the scope. Without measuring I would say I took the primary about 5mm back down the tube. It took 7 turns to get the mirror all the way back down but I gave each one 1 turn back up to allow for a bit of adjustment space. It's now collimated as good as I can get it with my bottle top collimator, so untill the cheshire arrives that will have to do. Would 5mm make enough difference to affect the focus length?

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.