Re: To User Cytan on Cloudy Nights

Cheng-Yang Tan

Hi Rolando,
   Thanks for the instructions. Just to confirm, the data that I took should *not* be used for correction since my mount is OFF right now. I'll need to retake the data and correct everything on the same evening?

   I plan to take photos tonight because it's moonless. Even without perfect PEC, the PE is so smooth, I believe guiding can easily take out the error already.

   I'll do this exercise again on a Moonfull night.


On Saturday, August 11, 2018, 5:20:35 PM CDT, chris1011@... [ap-gto] wrote:


Just go ahead and create a PE curve. Mark all harmonics that have values above 0.1 and press Create PE Curve. Try every drift fitting, not just linear, until the remainder RMS value in upper right is smallest. This will give you the best corrections and will insure that the resultant PE - ON will drop the final value to around +-1 arc sec. Then load that curve and take another run with PE turned on. This all has to be done while the mount is up and running under the stars. Don't load the curve some other day after you have turned the mount off.

The key here is that every cycle lies on top of every other one, which means that you will indeed cancel the PE almost completely and end up with a straight line for every worm period, not just the first one. Even if the peak value is a bit out of spec (probably due to the loading on the axis), you have an excellent chance to compensate and remove that error down to nothing, and that is really what you are after.

This is the one really important thing that almost every CN poster misses when they comment on a PE run. You will see this consistent curve in our gear drive mount because every spur reduction tooth always lands in the same place every worm period. You will not see this consistency in belt drive mounts because the pulleys wobble, they are mostly not proper multiples, and the belt teeth produce random wiggles that add and subtract randomly and cannot be removed with PE correction. That's why some mount manufacturers will only publish one PE curve, because if they showed 2, 3 or more, it would become instantly obvious that there is no consistency.

Hang in there, get your mount running and do some imaging. You are on the right track, don't go down blind alleys, it's a waste of time. Summer is short and skies right now are ripe for imaging. You have the setup, so go use it and don't obsess over an arc second.


-----Original Message-----
From: Cheng-Yang Tan cytan299@... [ap-gto]
To: ap-gto
Sent: Sat, Aug 11, 2018 4:41 pm
Subject: Re: [ap-gto] To User Cytan on Cloudy Nights

Hi Rolando,
   Thanks for the encouragement! 

    I've sent my files to Howard for him to have a look. 

   I had not intended to post the latest measurements to the the thread that I had started on cloudynights. In fact, it had already dropped off the front page. But it seems that there was a lot of traffic yesterday and I couldn't resist :)

   The scaling of 2.1 arcsecond/pixel is correct.


On Saturday, August 11, 2018, 4:23:20 PM CDT, chris1011@... [ap-gto] gto@...> wrote:

Hello Cytan,

In your post on Cloudy Nights today you showed a PEMPro run and you made the following observation (post #21 on

 "I did a second run with PemPro last night (in fact, the 2nd time I've used the mount) and I have come to the conclusion that using my FSQ106 setup is not the right way to measure the PE of my Mach1GTO. Here's the graph:"

No no no, you did it right! Your FSQ is just fine and your results are also fine and what you should see. You say there's Dec drift, BUT Dec drift has no bearing on a PEMPro run. Forget Dec, it's not part of PE analysis. The drift of the 5 lines is strictly RA drift due to slight polar misalignment. That also has no bearing on the PE analysis. All you have to do is click on the little boxes next to these: "Adjust X for X drift" and "Adjust Y for Y drift". That will bring all the lines together so they lay on top of each other and cancels out any drift. Then just go to the next stage and press "Create a PEC curve" and follow my instructions that I posted months ago on this user group.

In order to make sure that all your values are correct, you must go thru the Mount Wizard routine, and then all your data will have the right scaling. That is the most important thing when using PEMPro to make a PE curve.

Remember also, based on what I read on typical CN threads, it is more like the blind leading the blind, and if you want real info, please post here - we have very knowledgeable people that really do know what we're doing! Don't get led astray by well meaning people who maybe don't really know their stuff.


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