Sunday, November 6, 2016

#72 - Sunday, November 6, 2016 - The Mount Issues are Mounting

Another gorgeous night!  It was a little warmer, although I was still fighting dew, just not as much.  Because of the end of Daylight Savings, I got to get started at 6 instead of 7, which was awesome.  Also, Orion comes up an hour earlier now.  Since the first quarter moon was so high, I decided to grab a video on that before I got started, and I also grabbed a video of it on the guide scope with the QHY5, but it didn’t have good enough contrast to get a nice shot, even after messing with the gain, exposure, and gamma as much as I could.  I guess I could’ve thrown my neutral density filter on, and it wouldn’t have changed the focus because it screws onto the end of the extension tube, which just slides inside the guidescope, but oh well. 
Daylight Savings Time seemed to be causing issues with the telescope as well; it wasn’t quite getting on stars quite right.  So I replaced the alignment stars and calibration stars, and that seemed to work, so I started imaging Stephan’s Quintet.  I got two images in when I needed to do a meridian flip, so I did that, but then PHD started having fits.  During calibration, it was giving me non-orthogonality errors, so I ran the guiding assistant to see if it would help.  It didn’t, and the manual said it might be due to bad polar alignment.  RA was the main problem.  I figured maybe the time change would affect the polar alignment somehow, so I started re-doing the polar alignment, but then I accidentally touched the RA cable, which caused it to jump when it shorted.  So then I had to re-align completely.  I left the camera attached so that I could do fine alignment using BackyardNikon’s Frame & Focus feature, which uses live view.  At ISO-3200, I can see not only the stars I usually use for alignment, but even the precise goto stars show up no problem, since the D5300 will adjust the shutter speed & ISO to see something.  But its guesses as to where Vega and Altair were terrible; I actually had to go get the Telrad since it was not even close.  When this happens, it’ll usually correct itself out as I add stars, but it was consistently guessing terribly, which it shouldn’t have been because I was polar-aligned still from the previous nights.  So then I shut it down and restarted completely, and it was getting even worse, not even making it close to Vega.  I even tried putting it an hour ahead and back in Daylight time, but the result was the same.  Basically, it looked like RA wasn’t going far enough; dec was pretty close.  It was either stopping too soon, or not spinning the gears as fast as it thought it was.  It might have been my imagination, but sounded less aggressive than usual when it was moving.  After trying various things, power cycling, unplugging and replugging, changing the time, etc, it was still not working.  So I went inside to warm up and have some dinner (I brought leftovers, but Bob cooked some brats that he shared), and then I packed it up and left by 11:30.  It’s weird that it worked in the first part of the evening; it went to the moon from the home position, all the way in the south, with no issues, and also went to Stephan’s Quintet on the east side of the meridian with no issues.  It even went over to Fomalhaut, the polar alignment star I’m using, just fine.  It wasn’t until I hit the cable and made it jump.  The cable also got hit a few more times as I was re-situating it.  Oh, also, it seemed to take longer than normal to find the switch position in RA.  Bob suggested that maybe it’s time to send it out to get hypertuned, and maybe they can do the Bennett mod while they’re at it.  He said he knows someone who had their Celestron mount tuned with a guy named Dr. Clay.  It’ll cost some money, but will be cheaper than getting a new mount – the CGE mount’s original price is $3,000.  A quick look online shows a site called Deep Space Products that does it, including the Bennett mod (although you have to supply the kit), for $505 (so $855 total with the Bennett mod kit).  That sounds pretty good to me if it’ll save me the headache of the cabling issue and these other issues I’ve been having.  I can also get a dual Losmandy-Vixen ADM dovetail attachment put on there for another $110, which I was talking to Bob about last night – he thinks I should get a refractor sooner rather than later, like before I get a deep sky camera.  Seeing the results from just my guidescope, I’m inclined to agree.  There are a lot of issues shooting with the SCT, and another one is possibly cropping up – mirror shift.  It’s not a big deal to re-focus for each new target and after a meridian flip – it only takes a minute, especially now with BackyardNikon – but it might be the cause of the drift I still see.  Even with guiding, the image slowly moves across the frame, and that movement shows up in pretty much every other frame.  My RMS error isn’t too bad in PHD (about 2 arcsecs), although sometimes when I’m moving the scope at slow speed, it is a little sticky in dec, I think.  (Although, it’s my RA error that is the larger of the two – I do have some backlash).  I haven’t done the mental geometry, but it seems to be moving in the same direction each time – not the same camera direction, but the same sky direction.  This might indicate the mirror shifting as it tracks across the sky in RA.  There are two fixes I could do: use off-axis guiding (in which case, I’ll probably need to get a more sensitive CCD, but I might not, I’ll have to test), or send the scope off too for a hypertune (I could get a spring put behind the primary to keep it from shifting, since I can’t lock the mirror on this C11). 

So, a disappointing end to an otherwise spectacular weekend.  But it was a great weekend!  And I got some sleep last night.

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