From: chris1011@a... Date: Fri Sep 28, 2001 11:45 pm Subject: Polar aligning Mark Jenkin's 900 Hi Folks, Last night I went over to Trent's observatory to help polar align Mark Jenkin's 900 mount and try out the 9" Mak-Newt with my ST10E CCD camera. Mark had his 900 all permanently aligned, but then removed it from the permanent pier so he could bring it and the 9" Mak to Astrofest 2 weeks ago. So when he put it back, it was all misaligned, and he figured he would have to spend the next 2 days drift aligning it again with a crosshair eyepiece. So I volunteered to come over and use the power of the keypad and servo drive to polar align his mount. Here are the steps I used: Step 1) I powered up the mount, and entered the setup mode to the Park functions. I entered Park1 and waited for the mount to finish slewing. Then I manually moved the RA axis so that the counterweight shaft was level (I used a small Carpenter's level). I then exited the screen, and entered Park2. Now the scope slewed to the eastern horizon, and with the bubble level I manually leveled the scope's tube assembly. Exited the screen again, and did a final park to Park1 position. I manually leveled the tube as it was pointing to the northern horizon. The mount was now pretty well aligned in altitude, and I could find objects like Vega in the finder, even with the sky still too bright to see any stars naked eye. Step2) next level of precision was to touch up the altitude axis. With the scope on the west side of the mount, I used a 9mm crosshair eyepiece in the 9", aligned the finder crosshairs also, and centered Vega exactly in both scopes, then pressed #9 Rcal. I then advanced the meridian delay by 1 hour to the west, and entered Vega again. The scope slewed to the east side of the mount (fortunately, Vega was pretty much overhead at 7pm). Now Vega was about 1 -2 degrees too far in dec from the center in the finder. I moved it about half way toward the center with the altitude knob, the rest of the way with the keypad buttons, and pressed #9 Rcal. Two more iterations back and forth (with the meridian delay at 0W and 1W) and the star remained on the crosshairs at low and high powers. I then slewed the scope to Deneb, centered it at high power, hit #9Rcal, and slewed to Altair in the south. Altair was off only a tiny amount in the 9mm crosshair, so I made the adjustment 1/2 way with the azimuth knob and half with the keypad buttons, hit #9Rcal and slewed back to Deneb. This time the star was dead on the crosshairs, so we figured it was time to go to step3 in the alignment. Step3) precision drift alignment using the ST10 CCD camera and CCDOPS (You can do this with the STV also). We chose a starfield near the Andromeda Galaxy to do our CCD drift alignment. Using CCDOPS, we went to Track& Accumulate, chose a star to track on, and started a series of 5 exposures of 30 seconds each, 2x2 binned. The T&A function in CCDOPS has a neat little chart that shows you how far the star has drifted after each 30 second exposure. After two exposures, we could already see that the dec was tracking pretty good, but the RA was drifting up on the chart. First we lowered the polar axis about 1/8 turn on the big altitude knob, but the star drift was now twice as large. Back to the start position, and 1/16 turn at a time, we went from 3 pixel drift to an average of 0.8 pixels RA, and 0.2 pixels Dec per 30 second exposure. The 0.8 pixels drifted up and down along the axis, which meant that the main component now was periodic error (approx +- 2.2 arc sec. with the 38.8" FL). Step4) Final precision adjustment of the polar axis. To get the scope to track accurately for many minutes meant we had to remove the periodic error, and make our final altitude and azimuth adjustments so that the average drift was as low as possible on the T&A chart. By going into the selfguide mode, choosing a 1second guide time, we were able to program the PEM in the servo drive. The program takes about 7 minutes to record one worm cycle, and the ST10 self-guide mode with 1 second intervals was perfect to get a really accurate periodic error function. During guiding, the guide star moved less than 0.1 pixels because we had quite good seeing. Now with the servo PEM programmed, we were able to get the RA drift down to an incredible 0.08 pixels average in a 30 second time period, and a total drift of around 0.25 pixels in both axes for a 5 minute time period, good enough for totally unguided exposures. I followed up by taking some quick unguided exposures of M110 to see how sensitive the 9"F4.3 and ST10E combination is. In 2x2 binned mode, I saturated the detector in 30 seconds, so I did 4 more exposures at 20 seconds, and averaged them in Maxim DL. The image will be posted in the picture section. Keep in mind that the Moon was up and very near full. The whole sky was awash in glow, so the image was buried in background light and had very low contrast. If I get a chance, I will shoot this little gem again in better conditions. I will also shoot it unbinned to get the most out of the fast optical system. At 2x2 binning, the fainter stars are a bit square. After I left, Mark took a bunch of 3 minute unguided exposures of the Veil with nice round stars. Why not use the built-in autoguider? Well, there are times when you cannot find a suitable guide star, or you are using filters that block starlight to such a degree that no stars appear on the guiding chip. Also, there are CCD cameras that don't have built-in guide chips, so one either guides off-axis, or takes unguided exposures if the mount and alignment are up to the job. I hope this post is not too long and cumbersome, and that it will help those who want to try this method of aligning their mountings. Roland Christen From: chris1011@a... Date: Sat Sep 29, 2001 3:53 pm Subject: Re: [ap-gto] Re: Polar aligning Mark Jenkin's 900 In a message dated 9/28/2001 11:10:17 PM Pacific Daylight Time, chuck_olson@a... writes: > I wonder if in step 1) you could pin down which of the manual > adjustments were performed using the mount's altitude knob. > > Yes, I should have made it more clear. First slew the mount into Park1, and move the RA axis until the counterweight shaft is level. Tighten the RA clutch. Second, slew the scope to park 2 and manually move the Dec axis until the scope is parallel to the ground. Tighten the Dec clutch. Third, slew the scope back to Park1, and turn the altitude knob until the telescope tube is parallel tot he ground. This process takes only a few minutes. Step 2 where you slew to 1 star overhead on both sides of the mount and 2 stars north and south to adjust the mount's azimuth axis takes maybe 15 minutes (more, if you are way off on the north direction). If you just want to do some visual, this is precise enough to get objects into a medium power ocular (assuming your scope has reasonable orthogonality with respect to the mount's axes). Steps 3 and 4, where you do drift alignment and tweaking of the PE may take 1/2 to 2 hours, depending on the level of accuracy you want to achieve. How long did it take us to go thru the entire alignment? I started the alignment around 7pm, and according to my laptop, I took my first exposure of M110 at 8:24 pm. Between yacking with Mark and Trent, and looking at some recent images that Trent had captured, and talking about various image processing software and all kinds of other cool distractions when three guys get together at an observatory, I would say I wasted about half that time. So, theoretically, the whole deal could have been done in 45 minutes, the amount of time a real champeen would have taken. By the way, a lot of this alignment can be done in daylight, and I have been known to set up my scope at noon and polar aligned it using the bubble level/Park1,2 approach, using the Sun as my celestial reference. After that, you can easily pick up bright stars in your scope at low power for final tweaking. Roland Christen