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Collimation Woes

Posted: 24 May 2016

A friend read about my collimation difficulties and sent me a link to a Duncan Mask, which is supposed to make collimation easier by improving the diffraction pattern you see at the eyepiece. (Thanks Fred.) I made one from black construction paper on Monday afternoon, 23 May 2016. Took about 90 minutes to measure the telescope, draw the lines, and do the cutting. I added some tape to the edges to create handles to remove the mask from the telescope. There are also three dots (not visible in the photos below) for indicating where the collimation screws should be when the mask is on the telescope. The right photo shows the mask on the telescope.

photo photo

Did the mask help? Continue reading.

Open: Monday, 23 May 2016, 1817 MST
Temperature: 90°F
Session: 967
Conditions: Clear, breezy

Equipment Used:
12" f/8 LX600 w/StarLock
Wireless AutoStar II handset
2" 24mm UWA eyepiece
1.25" 15mm eyepiece
2" 2X PowerMate

D7200 DSLR

1916 MST: Jupiter was naked eye visible high in the sky several minutes before sunset. 1924 MST: sunset. The breezes were calmer now.

LX600 ON, StarLock OFF. Took a quick look at Jupiter, 102X. Three moons were visible.

Next, slewed to the star Spica to begin the telescope optics collimation.

1930 MST: Jim, a local amateur astronomer friend, came by to assist in the collimation. The job is easier when there are two people; one to adjust the collimation screws and the other to watch the changes in the eyepiece. Since we both have successfully collimated other telescopes we expected that the process would only take a short while.

Initially we tried to use the Duncan Mask, but even using a magnification of 325X we could not get the "Y" diffraction pattern large enough or clear enough to make any determinations of what adjustments were needed. We agreed to try collimation without the mask and just use the out-of-focus star (Spica).

Our first adjustments were to make the diffraction rings in the way-out-of-focus star image as "concentric" as we could. Unfortunately, that moved the shadow of the central obstruction way off center. Once we had the rings "concentric" we viewed Jupiter; incredibly bad view. So we knew we had to try a different approach.

We returned to Spica and this time we got the central obstruction shadow as centered as we could in the "donut" in the out-of-focus image. But now the rings were significantly not concentric. We viewed Jupiter. The view was better BUT each of the moons appeared "double" when in focus. Argh. And as the focus was changed in and out the moons would stretch first one way (showing something like "--") and then 90° the other way (like "|"). Neither of us had ever seen a telescope do this.

At this point after nearly three hours of trying to collimate the telescope, we gave up. Something is just not right now with the telescope. I will be asking for some expert advice. Stay tuned...

Before Jim left we took quick looks at Mars and Saturn. The views were not very good. A dark area was visible on Mars but Cassini Division was not visible in Saturn's ring. 2218 MST: Jim left Cassiopeia Observatory. Thanks to Jim for the help.

I returned to Spica and mounted the D7200 DSLR at prime focus to take some images of the diffraction rings. These cropped images (1 second, ISO 1600) show how the rings and central shadow looked out-of-focus on both sides of in-focus:

photo photo

I then used the Astrocap Focusing Mask to precisely achieve in-focus. This is how it looked (cropped image):


I then took a 1 second, ISO 6400, exposure of Spica and some surrounding fainter stars. The exposure was intentionally kept very short to avoid trailing due to any tracking errors. As seen in this photo with the magnified inset, the same "stretched" effect that was visible with Jupiter's moons is visible in the stretched star image:


2252 MST: after a totally frustrating night I took final quick looks at Mars and Saturn, 102X and 271X. Seeing was steady but the views were lousy.

I began closing up for the night, wondering what the next steps would have to be with the telescope. The good news is that the next few nights are forecast to be clear. The bad news is that with Mars approaching its closest approach to Earth in 10 years my largest telescope is crippled.

Close: Monday, 23 May 2016, 2304 MST
Temperature: 62°F
Session Length: 4h 47m
Conditions: Clear

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