3 Star Alignment with SynScan

[marineBLU28]

Hi

I was wondering when you're doing your 3 star alignment can you use 3 stars from the same constellation or should you use stars from different parts of the sky?

cheers

[Kai]

I always used to use one from the North , East and West in that order

HTH

[Karen]

Hello

I think the instruction book says the alignment objects should not be less than 60 degrees apart. But I normally use Betelgeuse with Pollux or Polaris .That usually works

[Skylook123]

Three star alignments with SynScan are a bit unusual in that they are not like other systems' routines. With most systems, the third star enhances the night sky model. With SynScan, the alignment routine uses the third star to correct for cone error. If you have a well-constructed setup, with the optical axis and rotational axis not divergent, then the third star in the mix can make the pointing on GOTO less accurate than a two star alignment; the system might be trying to cure a problem that does not exist.

My setup just loves a good leveling and polar alignment. If your polar alignment is well established, then even a 1 star alignment works well. If the polar alignment is off a bit, but there is no cone error, then a 2 star alignment seems to work well.

There are several algorithms for choosing stars for alignment. For a two star alignment, SynScan gives me one on each side of the meridian. With an SCT, that can sometimes introduce error due to the mirror flop passing through overhead. But usually, the mount will take a low path to the second star so it seems to work well. Fot the 3 star, the SynScan will pick two stars on one side of the meridian, then the third will be on the opposite side in order to do the cone error adjustment. I've had to stop doing 3 star alignments, since my setup has little or no cone error so the correction of the non-problem introduces error.

As much as the experienced equatorial mount uses say that leveling and precise polar alignment are not necessary, this mount seems to love the extra care taken at the begining. If I have a good Home position to start, and start level, and do a good polar alignment (I use an iterative polar alignment to get Polaris into the polar scope), then 1- or 2- star alignments work very well. If I cut corners on the setup, I end up chasing targets the whole session.

[Skylook123]

Hello

I think the instruction book says the alignment objects should not be less than 60 degrees apart. But I normally use Betelgeuse with Pollux or Polaris .That usually works

Polaris is a great alignment star for the AZ SynScan; I always use Polaris on my dob mount. It won't work for the EQ version since that's too close to where one axis is pointing; it won't give a unique solution to the sky model.

60 degrees of separation, or more, is really a good idea. It removes some of the human error effects. Large differences in RA/DEC, and/or AZ/EL, are helpful in smoothing out the human or equipment errors.

[barkis]

This link may be of use to Skywatcher owners.

Ron.

http://www.skywatchertelescope.net/swtinc/customer_support.php?class1=2&class2=201

[James]

That's a useful link Ron. For some reason Synscan prefers the first star to be to the south/meridian but it's not something you find out unless you trawl SW's site...

James

[Donkeiller]

3 star alignment?!

I'm running a Synscan AZ goto mount (firmware V 3.07) and only have 2 options-

Brightest star and 2 star align.

I guess I need a more expensive mount, or have I missed something?

[Karen]

3 star alignment?!

I'm running a Synscan AZ goto mount (firmware V 3.07) and only have 2 options-

Brightest star and 2 star align.

I guess I need a more expensive mount, or have I missed something?

I don't think so. Two star always works for me.

[James]

3 star alignment?!

I'm running a Synscan AZ goto mount (firmware V 3.07) and only have 2 options-

Brightest star and 2 star align.

I guess I need a more expensive mount, or have I missed something?

Not at all - German Equatorial Mounts normally have an option for 3 star alignment (or equivalent) - yours doesn't need it

James

[johnb]

3 stars, not for want of trying. I have noticed that if I select 3 star alignment as I chose a star then the next list for star number 2 is shorted and by the time i have to select the 3rd star the list is awful. Having said that if I level and do a decent polar alignment a subsequent 2 star works a treat

Kit in sig

Regards

John B

[Astro_Baby]

Thats interesting Jim - I often find that despite the theory that polar alignment and levelling dont need to be too accurate if I do them well the scope finds targets better.

Usually I do level the mount to pretty close to accurate with a bubble level and then I polar align quite accurately as well and generally find that a two star alignment process works very well and that after the firts star is set the second is usually very, very close on the next slew to second alignment star.

Your explanation sounds right on to me because sometimes I have found 3 star alignment just wont run no matter what.

I also use the finderscopes cross hairs to set the alignment stars exactly centre (obviously aligning the finder to the main scope before I do that).

I also have a white marker on my HEQ5 which is the exact centreline in RA so the mount can be set up consistently in its home position each time.

[reddoss]

My understanding is that with a 3 -star alignment synscan likes to chose the first two stars either side of the East/West meridian at about 60 degrees apart and lie not to far from the celesital equator, the third star is circum poalr (to the north for us) - usally one in Ursa Major/Minor or Casiopea.

I have used one and two star alignment before, but find accuracey of a three star alignment last longer through the night.

[Skylook123]

I think that the first rule of alignment processes is to try them all, with all the fidelity you want to do or have time to do in the setup, and use what works. My Atlas is such a bulky mount that aligning it to True North by eyeball is impossible for me, so I use a compass followed by an iterative polar alignment. I know other Atlas/EQ6 users who are almost gifted in their ability to get very close by eyeball setup, so they are able to get right to the star alignment process. Plus, we all have hidden/unkown features of our individual processes, and while I strive to be precise and scientific in my setups, if I found that sacrificing a chicken to Aman-Ra makes my GOTOs spot on, then we shall have fowl for late night snacking.

IF I am able to invest the time to do a precise setup (bulls-eye level on the tripod, use an inclinometer to validate the latitude angle of the head, set a good home position by first rolling the mount head in RA until the CW shaft is perfectly level using the bulls-eye on the body of the mount, locking the clutch, setting the RA setting circle to 6, then leveling the OTA with the same level and setting the DEC setting circle to the latitude, then unlocking both clutches and rolling the mount to 0 on both setting circles, then an iterative polar alignment), a 1-star alignment is all I need for a good night. If we have insufficient setup time due to late access to the observing site and the public is waiting, and especially if I have somehow altered the configuration of the mount head while stowing it the previous use, then just using a compass and level are not enough and it's a painful night. But, that is of course my experience; if the mount head is still holding the last setup's latitude mechanically, and I haven't slipped one of the axes against the clutches putting it away or taking it out, and I do a good compass alignment and level on the tripod, a two star seems to work fine without a precise polar alignment although the object will then drift over time. I have had one great night, and a dozen poor nights, with a three star alignment. I'm guessing that the great night with a three star alignment was due to mirror flop on my SCT causing an effective cone error for the three star to take out.

It is important to note that all of my comments are appropriate for the EQ version of SynScan; the AZ version is a different animal and has its own features, although a good starting point is a big help for success.

Finally, the latest versions of SynScan EQ firmware seem to have the ability under the alignment stars menu to tweek whether the mount will provide the suggested alignment stars or allow the user to pick. If the suggested star option is set, then the number of stars available to the user is signigicantly reduced as the algorithm tries to display only the second and third stars that match the "best combination" rules. At least, this is the reporting from some users.

Meanwhile, I have ordered a pen of chickens for next month's public sessions. Can't hurt.

[marineBLU28]

Thanks to all for the comments/ help

I have only done the three star alignment and so far makes finding things pretty accurate it was just a thought that maybe you could 'cheat' by using say three stars in Orion as it does allow you to choose your own alignment stars as well but I've probably got a lot to learn

Cheers