February 20, 2018: Dark Site Disappointment and Mount Mastery

Ugh.  Just ugh.

As the eight long-time readers of this blog may remember (although they probably won’t), one of my absolute pet bugaboos is the complete and utter inability of weather forecasts to get it right.  They just can’t.  Not even frickin’ close.  But they give the illusion that they can, this illusion of supreme accuracy, by breaking down hour-by-hour what the weather will be.  What a joke.

Before I go on, most of the time I write this blog with you, the general audience in mind.  Infrequently I’ll write it for personal reasons – pretty much as an astro-diary.  This is one of the latter entries, and therefore probably isn’t as interesting as one of my, shall we say, triumphant entries that has more mass appeal.

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Weather forecasting woes

This past weekend was what my astroclub calls the “Dark Sky Weekend” because the moon was just past new.  The weather forecast for this past weekend was excellent.  Windy, cold, but very clear skies on both Saturday and Sunday.  The Saturday forecast was a little worse than Sunday; not in terms of clarity, but in terms of wind and cold.  The Saturday forecast was for sustained winds of about 18 mph, with temperatures getting down to around 34 degrees within a couple of hours of sunset.  That’s pretty rough.

Since it was predicted to and did hit SEVENTY DEGREES on Sunday here in Denver (no, that is not a misprint), the forecast for Sunday night was predicted to be a few degrees warmer than Saturday, in the high 30s early in the night.  Better than that, the prediction was for winds around 12 mph instead of 18.

I coulda gone out on Saturday, but I said, “Hey, less wind and warmer sure sounds nice to me.  Kind like less filling, tastes great.”  I went out on Sunday.

Here is the forecast for Sunday at the dark site.

LIES!!!  ALL LIES!!!

You can see where this is going.  The wind picked up at 7pm and didn’t quit.  We retreated to the warming hut to try and wait it out.  The others started drifting away, going home, but I really wanted to stick it out, because I had been skunked by the dark site before.  At 8:30pm the howling died down.  And the clouds had moved in.

Note the time of the forecast, right at the top – 1:42pm Mountain time.  The forecast was for less than 7 hours, and they still couldn’t get it right.  And this is soooo typical.  I’m really sick of it.  That partly cloudy to mostly cloudy transition from 1am to 4am was a storm front moving through, which gave us a few inches of snow.  Storm fronts don’t all of a sudden put the pedal to the metal and speed up.  When you can’t predict 7 hours in the future, something is very wrong with your industry.

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Mount/Scope Hits and Misses

I’m definitely still going through growing pains with the new scope and especially the “new” mount (which is actually 9 months old).  When last I used them in combo, I was having a lot of trouble with:

• the height – I didn’t extend the tripod legs, and was practically sitting on the ground to observe;
• the slewing – it plain wasn’t accurate, and this is worrisome for an $1100 mount;
• the counterweight, or lack thereof – maybe the slewing inaccuracy was because there was no weight on the other side?
• the finderscope – I had difficulty keeping it aligned with the main scope, because it wasn’t secure.

In using the scope – briefly, for less than an hour – I was able to figure out a bunch of stuff and learn some more about the scope and mount.

Hits

• Tracking is great!
• Focal reducer works great!
• Extending the tripod legs out all the way really helped keep me from sitting on the ground.
• Aligning the second scope with first (in this case, the 5″ Mak with the C9.25) was easy and accurate.
• Celestron’s prism diagonal works great!
• The view was bright and intense!

Discussing each of these in turn, I left the scope on M31 for half an hour while trying to wait out the winds inside the warming hut.  When I went back out after warming up, it was still dead on target.  That’s some good tracking, right there.

The focal reducer is terrific.  Obviously, it expands the field of view very significantly, while still providing a lovely view.  Even though I had the SCT and Mak side-by-side, I didn’t get to check out to see if the SCT with the focal reducer had a slightly wider field of view than the Mak, as the math predicts it should.  It was friggin’ cold and windy, man!

Extending the tripod legs out gets me another 6 inches in height, and that makes all the difference in using the finderscope without having to sit on the ground.  And mounting and aligning the second scope (my Mak) couldn’t be easier – it just slides into the second Vixen mount.  It’s then a simple matter to hold onto the SCT while manually pushing the Mak up or down slightly to get it in the right altitude position.  There is also an azimuth adjustment, but I didn’t need to use it.  The mount carried all three scopes – the C9.25, the 127mm Mak, and the ST-80 refractor – without any problems at all.

The diagonal has its own little story.  When I bought the scope it came with a stock diagonal that I simply tossed aside.  I figured that my 99% dielectric mirror diagonal would be better than any old “junky” stock diagonal that came with the scope.  However, after doing some research on it, it turns out that this stock diagonal is actually a prism instead of a mirror.  The prism diagonal enables the light path to be shorter than a mirrored one.

I had always wondered how something at the back of the optical path could affect the focal length.  In almost all catadioptric scopes, focusing works by moving the mirror up or back inside the tube.  By using a mirror diagonal with a longer light path through it, you have to move the mirror up closer to the corrector plate to shorten the focal length in order to compensate for the lengthening on the back end to bring the view to focus.  This is not where the designers of the scope intended the primary mirror to be, which is why they included the prism diagonal.  It allows the mirror to be in the proper position, resulting in a brighter view.

Of course, the view was brighter anyway simply because of being at the dark site where the Milky Way shines brightly overhead.  Going from the 5-inch Mak (which operates at an effective aperture of 4.7 inches due to an undersized primary mirror) to a 9.25-inch SCT gets me 1.5 magnitudes deeper.  Taking the 9.25 out from under the Denver light dome to the dark site gets me another 1.5 magnitudes – or at least that’s what all the math says.

Misses

• Gotta remember to screw in the stoppers in the tripod legs.  Whoops!
• Gotta remember that the screws holding the scope in the Vixen mount have to go on the bottom.  Whoops!
• Gotta remember NEVER TO BELIEVE THE WEATHER REPORTS.  Whoops.
• There was a bad cordwrap issue.
• There was a clutch issue.
• The finder was still wiggling around.
• The slewing continues to be very off.

Yeah, the tripod collapsed just as I was about to put the SCT on.  Good thing it happened before and not after!  As shown on the left of the photo below, the tripod legs fold completely in on themselves, right onto the shaft.  You unfold them down (up in the photo) into the extended position.  But then you gotta lock ’em in place by screwing in those knobs at top.  Forgot to do that and down it went, leaving a ding on the pot metal.

The various parts of the Orion Sirius Pro AZ-EQ/G mount.

Looking at the photo again, the second Vixen mount is floating in space on the top right.  In order for the mount to know which way to point and move, those knobs that hold the scope in place have to be on the bottom, not on top where they’re easy to view and grasp.  Otherwise, when you push the up button on the handset, the scope will move down.  Gotta remember that, too.

As I was slewing from Andromeda back to M42, the mount got stuck.  It just stopped, mid-slew and wouldn’t move.  I wasn’t quite sure what was going on, but was able to quickly figure out that I had a cordwrap issue.  The Sirius Pro has an inordinate amount of cables.  Have a look:

 

This picture does not even include the power cord, which is actually a series of three interconnected cords because the one that comes with the mount ends in a cigarette lighter socket.  This then needs to be converted from female to male; then there needs to be yet another cord to connect into the power source.  Starting at the bottom, there’s the “mount coupling knob” sticking out the left side – it screws a bolt from the pier into the mount head at top to keep it connected.  Then, on the left is the cord for the handset, which is long and always wraps around everything.

And then there is that cord in the middle, the one with the loop in it, which is just begging to wrap around something and get caught.  Which is exactly what it did.  That is the declination motor cable.  It takes power from the R.A. section and motor (the part with the label on it) and transfers it to the part with the declination shaft, which has its own separate motor.  I am going to have to figure out how to run that cable so it doesn’t get caught again.

The clutch issue was unexpected.  Take a look at that mount pic again – see where the right side of that declination motor cable comes in contact with the declination shaft?  There’s two bits of black there.  One bit, sort of on the side where the camera is, is to let you adjust how far out the dec shaft goes.  The shaft slides in and out of the housing above it.  The other black bit, which goes around that shaft, is the dec clutch.  You can release the dec clutch to make sure the scope is reasonably balanced.  If it isn’t, you can move the scope up or back in the Vixen mount slot to adjust this.  After you do that, you tighten it back up again.

Well, as I mentioned, it was windy.  Oh, did I not mention that?   Windy enough to cause all of us observing out there to retreat into the warming hut.  Regardless, ya still gotta have a dew shield on the front of your catadioptrics, and I had one on each of mine.  In that wind, the dew shields acted like sails.  When I slewed north, into the wind, to have a look at M81 and 82, the clutch couldn’t hold on against that much force, and the scopes tipped up, perpendicular to the ground.  Note to self:  don’t observe in high winds.

Then there is the finderscope.  Take a look at this picture and see if you can see what’s wrong here:

And no, it’s not the red-striped couch.

It’s that there are two rings to hold up the 6×30 finderscope, but one of the rings doesn’t have adjustment screws.  Hell, it doesn’t even have holes to screw adjustment screws into.  And without anything to support it, the back of the finder just flops around inside the ring, getting it out of alignment, which is pretty annoying.  What gives?

Well, the good people of the Facebook astronomy pages tell me that some finders just come this way – and instead of screws, they just use a rubber O-ring inside that back ring to keep one end of the finder steady in one position, instead of moving around in the breeze.  You remember, that 18 mph steady wind that the weather did not forecast?  But I digress.  Instead, because I’m cheap, I just put a coupla doubled up rubber bands there.  Seems to do the trick.  Nice and tight.

My correspondents are also telling me that I’ve got the rings mounted backwards – that the ring with the adjustment screws should be at the back.  I can’t see why this is a big deal.  The only difference is that you can make finer finder adjustments if the screws are at the back because you’ve got a longer lever arm going on. But it was more than easy enough for me to align the finder with the screws in front.  I don’t need pinpoint accuracy; I’ll take good enough.

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Slewing inaccuracies and cures

The slewing on the Sirius Pro just isn’t accurate, even with the 12 pounds of telescope as counterweight (the 8-lb Mak and 4-lb ST-80) extended out on the shaft to balance out the 20 pounds of SCT on the other side.  The accuracy is flat-out poor, about 2 degrees off when I go outside of the two alignment stars.  My so-so little SLT mount does better than that at less than a third the price.

So, for example, I aligned on Betelgeuse and Pollux.  I then slewed to M31, basically on the other side of the sky, and it was about 2 degrees off.  Unlike the SLT mount, I couldn’t figure out how to realign as you go.  On the SLT handset (which is the same exterior handset as on the Sirius Pro, but with different software inside), there is literally an “Align” button.  No such button on the Sirius Pro.  This Align button lets you realign when you go to another object that’s outside of the field between the objects you’ve aligned on to improve accuracy.  You’re literally telling the mount, “No, it’s over here,” and the mount is responding by saying, “Okay, got it.”  On the SLT, it replaces one of your previous alignment stars with the new star or DSO.  You’re still working with just two points on the sky.

But how to do this on the Sirius Pro?  There is no obvious way to realign just from looking at the handset.  When I asked about this problem back in January, someone pointed out to me that there is a way, but it is completely non-intuitive.  You have to press the “ESC” button and hold it for 2 seconds.  Yeah, sure, okay.  Like I would have ever figured that out on my own.  “Well, it’s in the manual.”  Oops, gotta read the manual!

Pressing and holding ESC this time out, even that didn’t really work.  Sometimes it would; sometimes it would back out of whatever submenu you were in.  There is an alternate way to do it, where you press the Utilities button and then get to the “Pointing Accuracy Enhancement” (PAE) option.  That did work.  The manual says it divides the sky up into 85 zones; ostensibly, it remembers all of these and then can move off of the nearest one to your object.  But even when I realigned on M31, giving me three points of alignment, and then slewed to M33, right nearby, it was still off by a degree.

After retreating to and warming up in the warming hut for a while, I came back outside and realigned on Rigel and Regulus.  It found and centered M42 just fine (which looked pretty glorious, but was vibrating badly), which is between the two stars.  I then slewed to the Leo Triplet, M65/66, which is outside of the field between the two stars.  Although it was in the field of view, it was about half a degree off.

Now, this is annoying for such an expensive mount; annoying to the point where I’m going to contact Orion about it.  (I’m still under warranty.)  Practically speaking, it shouldn’t matter much – now that I’ve got this PAE thing down, I will definitely be realigning as I go.  I also plan to generally use the mount in a triple telescope set up.  Obviously, that includes the ST-80, which has more than a 3-degree field of view with the 25mm Plossl that lives in it (and over 4 degrees with the 24mm Explore Scientific 68).  So, as long as the mount gets me to within 3 degrees, I’ll be fine.

As for an actual observing report, I can’t really say much.  I was observing for less than an hour.  I took a nice look at the 3-day old moon, at M31.  Observing it through the C9.25, it looked the best I’ve ever seen it – there definitely was some structure in that big gray smudge.  Orion also looked pretty dang glorious.  But I couldn’t really get a good look at any of these with the wind causing the scope to vibrate.

Hopefully next time I go out to the dark site, the wind won’t be such a problem.  We’ll see!  And then we’ll observe!  (Sorry for the bad pun.)

* Banner photo of the DAS Dark Site from the Denver Astronomical Society.  That’s the outhouse in the foreground on the right, the observatory in the middle, housing a C14.  In between are the concrete pads for observing.  In the back is one of those construction site trailers, rigged up to be used as a nice warming hut.