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2 star alignment question synscan az mount


adder001
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Hi Folks,

I have been happily using my new skywatcher skymax 127 on a synscan az goto mount and a question occured to me when going through the alignment procedure.

Does the time between centering the first star in the eyepiece and aligning the second star matter.

The reason that I ask is that sometimes the accuracy of the goto differs slightly and I am wondering could it be due to me being too slow to find the next star. I am good with constellation identifying but the star names I only know a few of the major ones such as vega, betelgeuse etc., so I have to rely on a star chart to pick a suitable star which takes maybe about an extra minute. I am wondering if the mount does not keep the 1st star centred due to setup not being fully complete at this point and obviously the earth is still rotating so the star will drift and then the reference point of this star is now not centered when aligning to the 2nd star.

Everything else is always the same as I have already marked the ground where I put the tripod so it goes in the same place every time and levelled it with a spirit level. The power is supplied with a skywatcher powertank so I know its not a battery power problem and all the information is correct such as time/date, elevation, lat/long etc.

It isnt massively out, so far everything I have requested has been in the FOV of my 25mm eyepiece but its just something that I wondered and also whether I could improve things.

cheers

 

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I've checked in the manual and it doesn't give any information on that, and doing it with mine I can't see that it matters much as long as it's within a couple of minutes. What I have noticed is that there is a little play in the gearing system, so when slewing from left to right and back again at very low speeds it doesn't always move the scope immediately. I think the variation is something like half the field of view with the stock 25mm eyepiece with mine, but some of that could be age related (it's pre-owned and I don't know how old it is, nor how much usage it had). For that reason when I set up I always approach the alignment stars from the same direction for both. e.g. get the star into the bottom left quadrant, and then bring it into position properly. That seems to help. Another thing I do is to ignore the built in spirit level and use a short spirit level across the top of the tripod before mounting the drive unit and scope.

Picking out the stars before using them for alignment helps too. They need to be a distance apart and you don't need to know a large number of them, just the bright ones like Vega, Capella and Sirius, or easy to find like Betelguese, Caph and Deneb

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When aligning with my Nexus, because it doesn’t move I use Polaris as my first alignment star, although I’m told I shouldn’t need to. Could you also use Polaris, you wouldn’t need to rush then if it did make a difference? 

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I use Synscan's "Brightest Star" alignment, and have also wondered if the time duration to do the manual slew to the first star, and also between aligning the 2 stars, can affect GoTo accuracy. "Mognet" has covered the basics, and I also try to start with the OTA pointing North, and both mount and OTA level. Whilst slewing to the required azimuth, I can save time by setting the altitude scale pointer to the value shown on the handset. Choosing a second star whose name is at the beginning or end of the alphabet, saves time when stepping through the handset-suggested names.

I found that, having aligned on the first star, and accepting the Synscan-suggested second star, it was often behind a roof, fence, or tree; requiring a restart of the alignment process :homework:. I sometimes use Polaris as a second alignment star, but only if it differs, significantly, in both azimuth and altitude, from the first star. If the automatic slew to the second star does not place it in the FOV of my finder, then I tend to start alignment again, as the algorithm does not have a "useful" fix on the first star.

I decided to do a bit of homework to speed things up. I used Stellarium and produced a table of 24 bright stars (rows), setting the date for the middle of the month, for each of the 12 months (columns). I advanced time at dusk, with Stellarium adding the names of the stars as they would become visible to the naked eye. For each month, I selected 3, 4 or 5 of the 24 stars, (avoiding any "hidden" ones), that would give a good Alt/Az separation, and noting, in the table, their general direction (N, NW etc.) that month. Printed out on an A4 sheet, this makes alignment easier. For me, Sirius is only good in February, but Arcturus features from April to September. I did not include Polaris in the table, as I know where it is at a gap in the roof line.

Geoff

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Hi Adder,

Having the same setup as you, I wondered the same thing when I started out aligning.  As you say, it doesn't 'hold' the first star once you've confirmed it, and seems to drift away, especially if you've stepped up to a higher magnification on the first star.  I don't think it makes a big difference, but as Mognet mentioned, if you can wrap up the second star (and third, if you're using 'brightest star' method) in a couple of minutes or thereabouts, you should be fine. 

I time I found I could improve the accuracy of my alignment by getting the chosen star central in the 25mm eyepiece, then checking with a 10mm (and for when i was imaging planets, eventually a 2x or 3x barlowed 10mm) that it was still bang in the centre.  Clearly it takes longer to change out the eyepieces, but once you've done it a few times and have everything to hand when you set up, you can soon whip through it.

Like Geoff, I lost count of the number of times I'd slew to the second star only to find it was out of sight behind trees or houses!  As has been mentioned, the software Stellarium is great to have at hand so you can quickly find out where a particular star is, and you can also get apps for your phone that help you to find your way around the stars too.  Getting familiar with that AZ goto alignment procedure was a great step for me in learning the brighter stars!

All the best,

Derek

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Hi,

Yes, timing does matter. The slower you are between getting the 2 stars centered and aligned then the bigger error will be introduced into the goto accuracy.

A top tip to speed things up is to defocus the stars so they become "large circles", it's much easier to centre a large circle than a tiny dot :) 

You need to use high power EP for more accuracy. Stick to the 10mm for alignment process. You can even buy alignment EPs that display a centre cross! (Not really needed though)

As for the scope drifting on the first star, this is because the motors (tracking) are not engaged at this pre-alignment completed stage. Only once you complete the alignment procedure, do the motors start (tracking the earths rotation) and stars will appear to "hold position"

Alan

Edited by alanjgreen
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