Monday, August 18, 2014

Fixing Collimation on the SV4

In the previous post, I wrote about how images taken at an earlier session showed collimation/centering errors.

I picked up a laser collimation tool to help with the exercise of centering the focuser on the SV4 refractor.  The focuser has been removed from the scope several times over the last year or for flocking.

At the time I tested, I noted that the reflected dot from the laser collimator was offset by a significant amount.  With some judicious tweaking, I was able to keep the dot centered as the grub screws were tightened.  To seal the final adjustment, I ran a tiny bead of blue locktite along the seam to act as a removable glue.

Also while the focuser was off the scope, I removed the flocking paper and painted the internals with the flat black paint.  This worked better for the other metal parts and doesn't leave fuzzy bits to cause problems later.

After the focuser was centered, I figured that I'd try to identify issues of tilt with the DSLR.

I've read the method that Starlight Xpress suggests to fix tilt on their cameras.  The idea is to put the camera into a rotatable system, point a laser at the sensor and then note the position of the reflected spot as the camera is rotated.  If the sensor is tilted, the spot will wobble and wander as the camera is rotated.  Minimal movement will show a sensor that is perpendicular to the axis of rotation.

So I made a jig from a collection of stuff including the flattener, t-ring, and 2 inch drawtube extender with a set of 3-point rings for a 50mm finder.

Once I had the camera in place, I lined up the laser pointer.

Reflections of the laser were shined on a far wall, about 12 feet distant.  The initial test showed wobble of at least 6 inches.  After shimming, the deflection was reduced to less than 1 inch.  Not too bad from just a little collection of parts!

Thursday, August 7, 2014

Centering/Collimation Error

While reviewing the frames from the M8/M20 project, I noticed that all had a region in the upper left that was blurred.  The stars were streaked outwards and didn't look good at all.

So I ran CCDI on an example and this is what it shows:

When I did some measuring, it appears that there could be two things happening here:
Tilt and Centering.

Tilt makes sense because I've noticed the way that focus gets better when the camera shifts position.  I'd shimmed the camera body to avoid this whole issue.

Also, Centering makes sense, too, since I've had the focuser off the telescope several times.

So I put a collimation tool on the scope.  When using the Feathertouch-supplied 2 inch to 1.25 inch adapter, there was a slight deflection of the reflected laser.  I loosened the focuser and shifted it to show a centered dot.  What's annoying is that this centering wasn't uniform for the adapter - when rotated, the reflected dot wanders.

Tilt is easy to remedy.  I'll have to shim the camera adapter by wedging small bits of metal in the flange.

The final challenge will be to test and get some results from stars.

Saturday, August 2, 2014

Trifid Nebula from MBOSP

This is crop of the M8/M20 project.

M20 at MBOSP July 2014

The total data set is 22 lights - a 90% stack gives 19 lights.  All were done at Montebello over three nights: July 23, 29, and 30th. The crop was made as a custom rectangle in DSS and a 2x drizzle stack.  In processing in PI, I was happy with how it looked, but on importing to LR, the blue and red were garish so they were toned down a bit.

There were some general issues along the way. 

After the first session, I went up again on the 26th, but the weather didn't cooperate.  This was fine, because I learned that the shutter and mirror had let go because of the heat in the car.  The low-temperature hot melt glue had lost the ability to hold the small parts in place.  Thus, I had to fix the problem.  I went with small drops of cyanoacrylate adhesive.  It's a permanent solution and that's fine with me.  If anything, it reminds me that the choice to make changes to the camera to ONLY run as an astro camera is OK.

On the evening of the 29th, I found that something was amiss with the USB cable.  The tethering software wouldn't see the DSLR and the SSAG wasn't connecting. Very strange!  So, I checked the USB repeater cable and found that it was dead!  Luckily, I brought the old USB hub I had sitting around.  That worked OK - although it didn't like being powered by 5V.  This might be the reason why the serial cable stopped working properly.  I've ordered replacement parts.

On the evening of the 30th, I wanted to secure this replacement hub as the cable joint is weak.  I used a bungee ball and unfortunately, that got jammed by the DEC motor.  Only after several attempts to unbind the DEC axis gear did I realize the true problem.  Major disaster averted, lost about 1 hour under the stars.

Also on this evening, I noticed after finishing the M8/M20 target - and near the end of the night - that the cooler wasn't running.  Had to wiggle the connection with the powerpole to get it running again.  Later in the morning, I inspected that connector and saw that the metal contact wasn't properly seated.  This was likely the source of the problem that I'd noticed for some time.  Now it's fixed and all is good again.

During these nights I was running the dual setup, just like at GSSP.  The GM8 was plugging away with the Pentax 67 camera on it.  I put the IDAS LPS filter on the 200mm lens and let it capture several parts of the sky, repeating some of the targets missed at GSSP.  Since the mosaic worked so well in PixInsight, I thought it would be a good thing to continue.

I'll be scanning and posting those images later.