DiscoDuck
08-06-2014, 07:10 PM
Hi,
I have been trying to collimate my GSO RC 8" via two alternative methods:
1. With a Glatter holographic laser based on techniques described in these videos https://www.youtube.com/watch?v=2EC5N5bfB9A. The steps I did were
(a) use the laser without the holographic attachment. Adjust the secondary to return the laser directly onto the centre of the laser faceplate (visible down the front of the scope) (as an alternative to using the technique discussed in step 2 of the videos, since the dot is easier to see than the reflected circles). This should align the secondary with the primary (assuming the focuser is straight and centred. Note that I do not have a tilt plate on my focuser (nor a secondary centre spot) and so have to skip step 1 of the video. More on this below!).
(b) Add the holo attachment and adjust the primary via the technique shown in the video (step 3). This should align the primary with the secondary.
2. (a) By using a collimation cap to centre the secondary under the focuser using the primary mirror's adjustment screws.
(b) Using the "hall of mirrors" method to align the secondary with the primary http://www.davidcortner.com/slowblog/20110815c.php
In both techniques, the steps are iterated a few times until they converge.
My question is re why the two techniques give different results, and which technique to follow.
Consistently after aligning with technique 2, the laser dot in technique 1 step (a) is returning below the centre on the laser faceplate. And pretty much in the same position each time (about 10mm out) - leading me to think that both methods are accurately repeatable at least. The view in a Cheshire eyepiece after doing technique 2 is beautifully symmetric. But after doing the laser technique, the Cheshire view is noticeably off-centre.
I'm guessing the difference is due to a tilt of the focuser drawtube with respect to the primary. So my questions are
(i) does this seem a reasonable explanation?
(ii) if so, is it worth fixing by buying a tilt plate? (I think I've worked out a procedure to do the alignment without having a centre spotted secondary, using the laser and hall of mirrors methods)
(iii) if not worth fixing, which of the two techniques do I follow for a better image?!! (I might add, I'm just using an APS-C size chip, so the tilt itself shouldn't be much of an issue for the sensor. The issue is more with how the presence of the tilt is affecting getting precise collimation). Or is the difference too small to affect my images, and either technique will do?
Thanks in advance for any suggestions/advice.
Paul
I have been trying to collimate my GSO RC 8" via two alternative methods:
1. With a Glatter holographic laser based on techniques described in these videos https://www.youtube.com/watch?v=2EC5N5bfB9A. The steps I did were
(a) use the laser without the holographic attachment. Adjust the secondary to return the laser directly onto the centre of the laser faceplate (visible down the front of the scope) (as an alternative to using the technique discussed in step 2 of the videos, since the dot is easier to see than the reflected circles). This should align the secondary with the primary (assuming the focuser is straight and centred. Note that I do not have a tilt plate on my focuser (nor a secondary centre spot) and so have to skip step 1 of the video. More on this below!).
(b) Add the holo attachment and adjust the primary via the technique shown in the video (step 3). This should align the primary with the secondary.
2. (a) By using a collimation cap to centre the secondary under the focuser using the primary mirror's adjustment screws.
(b) Using the "hall of mirrors" method to align the secondary with the primary http://www.davidcortner.com/slowblog/20110815c.php
In both techniques, the steps are iterated a few times until they converge.
My question is re why the two techniques give different results, and which technique to follow.
Consistently after aligning with technique 2, the laser dot in technique 1 step (a) is returning below the centre on the laser faceplate. And pretty much in the same position each time (about 10mm out) - leading me to think that both methods are accurately repeatable at least. The view in a Cheshire eyepiece after doing technique 2 is beautifully symmetric. But after doing the laser technique, the Cheshire view is noticeably off-centre.
I'm guessing the difference is due to a tilt of the focuser drawtube with respect to the primary. So my questions are
(i) does this seem a reasonable explanation?
(ii) if so, is it worth fixing by buying a tilt plate? (I think I've worked out a procedure to do the alignment without having a centre spotted secondary, using the laser and hall of mirrors methods)
(iii) if not worth fixing, which of the two techniques do I follow for a better image?!! (I might add, I'm just using an APS-C size chip, so the tilt itself shouldn't be much of an issue for the sensor. The issue is more with how the presence of the tilt is affecting getting precise collimation). Or is the difference too small to affect my images, and either technique will do?
Thanks in advance for any suggestions/advice.
Paul