Refractor problem

[woldsman]

An old doublet refractor has taken a knock. When looking straight down the tube using a concenter, there is a crescent obscuring part of the aperture as drawn in the picture. I think either the objective is out of collimation or a baffle has shifted. From what I can tell the baffles are in place though. The objective isn’t cracked and there is no dent in the ota.

Before I start trying to collimate I thought I’d pause to get the benefit of the community’s advice. Thanks for reading.

Edited July 9 by woldsman
Typo

[John]

If you have a cheshire eyepiece you can check for objective tilt quite easily. Use the cheshire straight into the drawtube, no diagonal in place:

If you get the image on the left, it's more likely that a baffle has shifted / tilted. 

If you suspect that the focuser is not aligned, shining a laser collimator down the tube will reveal that - check that the laser exits the front objective element in it's centre. Ideally this check should precede the objective tilt check.

 

Edited July 9 by John

[woldsman]

36 minutes ago, John said:

If you have a cheshire eyepiece you can check for objective tilt quite easily. Use the cheshire straight into the drawtube, no diagonal in place:

If you get the image on the left, it's more likely that a baffle has shifted / tilted. 

If you suspect that the focuser is not aligned, shining a laser collimator down the tube will reveal that - check that the laser exits the front objective element in it's centre. Ideally this check should precede the objective tilt check.

 

 

36 minutes ago, John said:

If you have a cheshire eyepiece you can check for objective tilt quite easily. Use the cheshire straight into the drawtube, no diagonal in place:

If you get the image on the left, it's more likely that a baffle has shifted / tilted. 

If you suspect that the focuser is not aligned, shining a laser collimator down the tube will reveal that - check that the laser exits the front objective element in it's centre. Ideally this check should precede the objective tilt check.

 

Thanks John - very clear advice. Away from home today but will get the Cheshire out to check. Don’t have a well collimated laser but know a friend who does and will ask for assistance.

[woldsman]

Problem turned out to be a slipped baffle - fortunately the one closest to the focuser so easily reached. Now fixed!