Saturday, October 6, 2018

#163 - Friday, October 5, 2018 - A Foggy Night at Hidden Hollow (and I fixed my mount!!)

It was a chilly, rainy day at Hidden Hollow Star Party!  But it didn't rain all day, so between rain showers, I decided to try out the backlash fix for my Celestron AVX mount that I found in this YouTube video.

First, I needed to remove the casing around the declination axis, which was held on with Phillips screws.  Unfortunately, one of them is placed such that I couldn't get a screwdriver in straight because the mount head was in the way, so I had to remove the mount head.  I was working quickly because of the threat of rain, otherwise I would have taken more pictures and video - sorry!

Once that was removed, I powered on the mount and slewed in dec at slow speed so I could see how things were looking.  Sure enough, the backlash was very obvious - the motor drive gear had to turn a fair distance before engaging the worm wheel gear.

I removed the motor drive gear as shown in the YouTube video with a small hex wrench, and then I loosened the three hex bolts (just loosened a bit, not much) that hold the motor in place.  Then I put the motor drive gear back on, and pushed the motor closer to the worm wheel gear.  I tightened up the motor drive gear onto the shaft, and checked how it meshed with the worm wheel gear - there was still some gap, so I pushed it in a bit further.  I checked whether there was still backlash by slewing at slow speed (speed 3), and sure enough, the motor drive gear engaged the worm wheel gear right away!  I took off the gear, tightened the motor bolts back up, and then put the gear back on.  Then I slewed around at high speed to make sure it wasn't binding anywhere.  Then I put the case back on so I could put the mount head back on and see that slew around.  Everything was looking good until it started whining loud - the dec clutch hit the casing!  For some reason, I had to turn it down more than usual to tighten it.

Video showing the backlash before and after my fix.

I was about to start figuring out that problem when it started to rain hard, all of a sudden!  I quickly threw the cover over the mount, but the telescope with cameras attached was still sitting on the table.  I quickly grabbed it, threw my computer's dew towel over it, and dashed down the hill to my car, since it was too long to fit into my accessory bin.  Then I sprinted back up the hill to grab my DSLR and Vixen Polarie, which were still out from doing a timelapse run earlier.  They plus my intervolemter were pretty soaked, so I laid them out to dry in my car and ran back up the hill one more time to the education building.  I was thoroughly soaked!

The rain only lasted about 20 minutes or so.  After it ended, I put the mount back together with the scope, but since it was still threatening to rain some more, I decided to postpone working on the dec clutch problem until later that night.  

When I returned to the observing hill that evening, the sky looked fairly clear, which was surprising, but a fog had formed from the ground up to about 20 feet or so.  Unfortunately, the temperature was actually increasing slowly all night, so it stayed at 100% humidity all evening, and that fog never lifted.

However, since the sky was clear, I was able to test out whether my backlash fix worked.  But first, I needed to fix my dec clutch.  I removed the mount head from the mount again so I could see why I was having to tighten the clutch more than usual.  I thought at first that maybe the cylinder that the mount head attaches to was not quite circular, so I rotated the mount head with respect to the cylinder (the four bolt holes used to attach it are square, so you can rotate it 90 degrees at a time).  No luck though - I still had to turn the dec clutch far enough that it would hit the casing.

I got someone else to come over and look at it, and he suggested I remove the whole knob (it has a Phillips head screw holding it on).  Once I removed it, I could see that it had little notches in it that would allow you to orient it any number of ways with respect to the clutch bolt, so I just put it on rotated higher up and put the Phillips screw back in.  Now I could tighten it, and it wouldn't turn so far down as to hit the casing.  Fixed!

I re-polar aligned and re-aligned, re-focused, and slewed again to the Western Veil Nebula.  I calibrated PHD, which still showed a lot of backlash in dec during calibration, but then the guiding graph looked pretty flat and not crazy like before.  I got sidetracked in conversation, but then I checked on the incoming 3-minute luminance frames, and the stars looked round!  And the guide graph looked pretty good!

Weird-shaped stars because of bad guiding because of bad backlash in declination

Basically what stars should look like (ignore the noise, it's a raw frame)

Frame after frame looked this good.  It was hard to see the Western Veil Nebula itself due to the fog, but I had accomplished my goal for the evening, which was to see if my backlash was fixed!  Success!  I might have shed a tear or was beautiful...

After sitting inside and chatting for a while, I poked my head back out later, but the fog was still just sitting there.  I also tried to image the Andromeda Galaxy area with my Nikon D5300 on my Vixen Polarie, but I was having a hard time focusing - my 55-200mm lens is very touchy on focus.  I finally gave up and got into bed by about 1:30 AM.  I needed to be up around 8:30 the next morning to give my talk - "Astrophotography Without a Telescope."  

So I won't have an image, but at least I fixed my mount! :D  Now I need to figure out how long I can guide for.  Hopefully I can get a couple stars tonight to test that on.  Forecast is looking dicey.  On top of that, the scissor lift to look through Hidden Hollow's 36-inch reflector "Big Blue" is broken! :(

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