Orion Optics VX12 focuser upgrade

[jetstream]

On 22/08/2019 at 17:04, Piero said:

but also whether I'm using my instrument at its highest potential. 

Interesting formulas Piero and the coma free area is what it is regardless of how its calculated.  I would be very interested to see if your collimation method would result in all the reflections being stacked in a dual pupil autocollimator....

from what I understand the most accurate collimation results when all tools are in agreement, including the DP autocollimator- for us amateurs anyway.

[Piero]

On 25/08/2019 at 02:32, jetstream said:

Interesting formulas Piero and the coma free area is what it is regardless of how its calculated.  I would be very interested to see if your collimation method would result in all the reflections being stacked in a dual pupil autocollimator....

from what I understand the most accurate collimation results when all tools are in agreement, including the DP autocollimator- for us amateurs anyway.

Thanks, but checking both alignments on the 1mm aperture stop was suggested by Vic Menard on CN. It's a time saver instead of using the tuBlug. The only potential issue is that one should be able to see the 1mm aperture stop from the UTA. I still use the tuBlug when I first collimate the primary mirror. During a session, time to time checks are done only on the 1mm aperture stop, though.

Personally, I think that it's all about knowing the tolerances and making sure that the errors are within. I am happy with the collimation errors I get at 2x. An autocollimator allows one to check at 4x and 8x, although requires a light in nighttime. For me this is not practical. In addition, I think that level of collimation accuracy requires better mechanics than what I have, e.g. the focuser.

In my f5.94 dobson the focuser axial error at 1x is about about 2mm, which is modest considering a tolerance of about 9.15mm. This means that the focuser is completely fine.

Edited August 29, 2019 by Piero

[jetstream]

3 hours ago, Piero said:

time to time checks are done only on the 1mm aperture stop, though.

Do you find that collimation moves during a session?

[Piero]

2 hours ago, jetstream said:

Do you find that collimation moves during a session?

No, I've never found a collimation shift during a session, but I like to check just to be sure. It takes 1 minute, really. 

I do have a shift of a couple of mm when I rack the focuser in and out, though. To counteract this, I collimate the focuser axial alignment with the drawtube racked out about as much as the travel needed by my eyepieces in order to reach focus. Doing so, the miscollimation shift is compromised. 

Will you get a coma corrector one day? 

[jetstream]

6 minutes ago, Piero said:

No, I've never found a collimation shift during a session, but I like to check just to be sure. It takes 1 minute, really. 

I do have a shift of a couple of mm when I rack the focuser in and out, though. To counteract this, I collimate the focuser axial alignment with the drawtube racked out about as much as the travel needed by my eyepieces in order to reach focus. Doing so, the miscollimation shift is compromised. 

Will you get a coma corrector one day? 

Yes I might get one but I'm not sure of the effect on DSO... some say it should increase contrast, which could be true if the coma reducing effect increases contrast greater than any potential loss from all the extra glass etc. Could be a costly experiment...

In these focusers, even with tight fitting lasers and sight tubes leaving the focuser thumbscrews loose show how much the laser dot can move if touched...I always center my laser on the secondary with the same thumbscrew tightened first- and then tighten the same thumbscrew first with the eyepiece.

Glatters parallizer can take care of part of this, but where does it end lol!

I'm going to start a thread on "entendue", you might find this interesting, if not controversial.

[Piero]

40 minutes ago, jetstream said:

Yes I might get one but I'm not sure of the effect on DSO... some say it should increase contrast, which could be true if the coma reducing effect increases contrast greater than any potential loss from all the extra glass etc. Could be a costly experiment...

In these focusers, even with tight fitting lasers and sight tubes leaving the focuser thumbscrews loose show how much the laser dot can move if touched...I always center my laser on the secondary with the same thumbscrew tightened first- and then tighten the same thumbscrew first with the eyepiece.

Glatters parallizer can take care of part of this, but where does it end lol!

I'm going to start a thread on "entendue", you might find this interesting, if not controversial.

 

The coma tail can be quite large, but most of it is not visible as it contains too little light. It is still star light spreading away though. Therefore, off-axis stars will certainly look like dimmer w/o CC than w/ one. On the extreme side, off-axis dim stars could not be visible at all w/o CC. I might consider a coma corrector in the future, but before this, I need to get to know my dobson a bit more. 

I do follow a protocol similar to yours regarding focuser thrumbscrews.  

Looking forward to reading your new thread.

 

Ref. https://www.handprint.com/ASTRO/ae4.html 

Edited August 25, 2019 by Piero

[faulksy]

On 25/08/2019 at 02:22, jetstream said:

I tested my Moonlight with a weight simulating the Binotron 27's and 2 Docter 12.5mm eyepieces. This allowed my to watch the Catseye cheshire and Glatters laser as I racked the focuser in and out. After about 1/2 travel miscollimation began, enough to damage views.

What I did then is adjust the extension tube to allow 1.3x and 2.3x (Powerswitch) focus in the first third of focuser travel. I will then pull the bino out to focus for 3x with the focuser at half travel or less.

I've heard new Feathertouch's might sag more than the Moonlight and would like confirmation (or not) actually. I use the 45mm OCS with the bino's.

Edit: Oops forgot you had an aluminum OOUK tube- I tested my VX10 and it was the tube that flexed, lots. I have a different stock OOUK focuser than yours though. @faulksy had to reinforce his VX14 if I remember, to help with the issue. Mine flexes so much I don't binoview with it.

correct gerry. i wrapped a aluminum cylinder around the inside of the tube. and screwed it to the outer tube. no problems after that 😁