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questions about my alignment with CGEMii mount

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#1 pete4www

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Posted 23 September 2019 - 03:14 AM

So I have some dumb questions about my CGEMii mount / EdgeHD 8 combo, with StarSense. I have gotten partial answers to my questions by looking at lots of other posts on CN, but not enough to make it fully click in my brain, so I will try to explain the concepts I don't understand. The first night I took my scope out, I did the whole StarSense Auto alignment, did the StarSense calibration, did an All Star Polar Align, then another StarSense Auto Align. It centered everything really good in the eyepiece. I am only doing visual for now, no astrophotography yet. I've taken my scope out a few more times, and I have noticed that sometimes it does not center the object quite as well in the eyepiece. For example, the other night, as a test I repeatedly did a GOTO to Rigel, Betelgeuse, and Sirius over and over again. I used the stock 40mm eyepiece the scope came with, and I removed the diagonal, using only the eyepiece inserted into the visual back. All 3 stars appeared in the eyepiece, but they were all off-center. Best way I can describe is they were to the right of center, at the 3 o clock position in the eyepiece. I went to some other stars in different parts of the sky, and same thing. The star showed up in the eyepiece, but off center by the same amount. Here are the questions I have about this whole scenario:

 

#1. When I am using my scope with a low power 40mm eyepiece, should I expect stars to be almost dead center if my alignment was successful, or is this asking too much and it's perfectly acceptable/normal for the stars to be in the eyepiece, just not dead center? What about a high powered eyepiece like 9mm? Do expect to have to center the star in a 40mm piece, before you pop in the 9mm eyepiece?

 

#2. If I am doing visual ONLY, is it good enough to point the alignment peg to North when setting up the tripod, and just do the StarSense auto align, and not worry about doing an ASPA after, followed by another auto align? How are GOTO's and tracking affected if I am not perfectly polar aligned for visual only? I have read that a "rough" polar alignment is sufficient for visual, but what does that mean? To those who do visual on a CGEM, and only looking for accurate GOTO and tracking, how far do you go to polar alight your mount? I am just trying to get a sense of how accurate my polar alignment must be, if I am only doing visual. 

 

#3. Is it possible my GOTO's behave this way because my polar alignment is off, or do you think it has more to do with the StarSense needing to be re-calibrated? 

 

#4. I understand that by adding additional alignment stars in the hand controller, it makes the GOTO's more accurate. That part I understand, but what I am still confused about, is does that help my GOTO's for JUST that alignment session until I power off the mount, or does this also affect the GOTO accuracy of future alignment sessions? I might be wrong, but it seems like it saves these additional alignment stars in the hand controller. Is there ever an instance where you would actually want to delete all the alignment stars and start over, or do you just leave your additional alignment stars alone once you have them saved?

 

#5. If doing visual only, and your polar alignment is not perfect, does the mount compensate for it somehow? Like does it track in RA and DEC, or RA only? I noticed there is a setting in the hand controller to have it track in RA, and also RA and DEC. The default is RA. Do you guys ever ENABLE the RA/DEC tracking in the hand controller, and if so, why?

 

One other comment I want to make is I understand so many other people's frustration with the azimuth adjustment on the CGEM, it is NOT smooth at all. When I do an All Star Polar Align, it is extremely difficult to tighten down the azimuth screws and get the star in just the right spot. The altitude adjustment isn't so bad. One thing that really, really **** me off above all though, is after I get my az and alt adjusted, and I go to tighten down the CENTER bolt underneath the tripod, the WHOLE THING MOVES and throws off my perfect, dead-center alignment!!! So, I have taken matters into my own hands, and I followed the suggestion of another CN user, and I am sanding off the paint from the surface of the mount, adding a layer of PTFE teflon I ordered off amazon to both sides, and see if that helps. Whole solution will cost under $30 by the time I am done with it. If this does not work well enough, I am going to spend the money on that expensive Starizona Landing Pad for the CGEM. I really don't WANT to spend the money, but if the teflon does not work, I will do it. If my polar alignment does not need to be super accurate for visual use though, I might not worry about it until I try my hand at astrophotography. Again, sorry for all the dumb questions, but I am just trying to get these basic concepts explained to me in a way I understand. 



#2 sg6

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Posted 23 September 2019 - 06:10 AM

1: They are not perfect, so to an extent central is good just do not expect center. I think a lot comes from the real computing power of a goto. They are FEEBLE. Processor will be 8 bit, memory will be Kbytes. I suspect we are too familiar with PC's, tablets etc to realise.

 

2: Might be but what happens is the software has to determine the corrections that needs to be applied, the more it has to correct the worse the general goto will be. Eventually it will be garbage. If you want accurate you do your bit of the deal and set it up well.

 

3: As 2 the better the polar alignment the better the goto. Unsure of Starsense. But all these are the same the better the set up is the better the results are.

 

4: Cannot comment as I have no idea what the extra and later alignment stars do. I get the idea they may be more of a Sync action then an alignment one. Maybe there is some feed back, but unsure how. Just cannot see how after 15 minutes that making star 12 an alignment star feeds back to the RA/Dec correction calculation.

 

Performing a Sync and saying start from here as this is where you really are makes sense, but not alignment. Once you power off everything is lost, you start all over again.

 

5: If visual then would have expected RA+Dec otherwise any Dec correction required is not applied.

 

My approach is that You set up the mount, you level the mount and orientated it, you supply the data, you center the required stars. And if you do all that fairly well the mount might/should point the scope at the requested target.

 

That is a lot of manual action for a computerised scope - 4 parts manual 1 part computer.

What makes anyone describe them as computerised or automatic is questionable.



#3 kathyastro

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Posted 23 September 2019 - 06:52 AM

1. Your gotos are affected by the accuracy (or lack thereof) of your home position.  Start with an accurate home position and your gotos will improve.

 

2. Just pointing the north peg to north-ish is not really a polar alignment.  A rough polar alignment suitable for visual use would be using a compass, corrected for magnetic variation, to find north, and a level and/or inclinometer to set the elevation.

 

3. A poor polar alignment will prevent the goto calculations from being accurate.

 

4. Adding additional goto reference stars works only for that session.  If you park the mount and do not move it, you can begin the next session by unparking, and the extra reference points will still be valid.  However, if the mount is moved, all goto information is lost and you have to start over for the next session.

 

5. The mount has no way of knowing that the polar alignment is not perfect, so it cannot compensate.  It assumes that it is.  It is up to you to make it so.



#4 pete4www

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Posted 23 September 2019 - 03:54 PM

Kathy, yeah I forgot to mention I always level my tripod before I place the mount on, and I check it again once it's on there. I also start in the home position with both index marks lined up. I really like the feature in the hand controller that lets you save the home position, and return to it on command. I use it a lot when I need to redo an alignment, and when I tear down for the night. It looks like what I need to work on most at this point, is make sure I get the polar alignment done properly as best as I can. I also think my expectations of dead-center GOTOs in general are too high, and I should be satisfied that it's actually moving to the correct spot and it shows up in the eyepiece somewhere. Once I get my azimuth movement smoothed out, I think it will make polar alignment less of a PITA, because that's my biggest problem right now. I will get much more enjoyment out of my setup, if I can get polar alignment issues ironed out. Thank you for the replies!




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