Choosing Best Stars For 3 Star Alignment

[PembrokeSteve]

Hi,

Is there some list by Month and Approximate Time available, that gives the best and most convenient stars for carrying out a " 3 star alignment".
Tonight I had the experience of Arcturus and Capella coming up on my Skywatcher SynScan handset, including Vega, which for me was not yet visible.
I had to skip Vega to choose another star.
Then my handset gave me loads of not well known (and probably feinter) stars, which was confusing, when you have not got a clue if these stars will be visible either!
Any advice here appreciated,

Regards,

Steve
 

[Ande]

I run Stellarium on my iPad, and then compare the sky against it. If a certain star is suggested that I’m unfamiliar with, I search for it on Stellarium, and then see if I can locate it in the sky with the naked eye.  If I can them I’m good to go ?

[Tim]

I feel your pain Steve, having been there myself.

In the end I used the technique Ande described, checking on Stellarium, as I didn't have a clue which star was which and if I was on the right one.

As long as your time and date settings are correct, the handset should give you appropriate stars, but will assume flat horizons.

I read this advice once: Choose three stars in a triangle around the object you wish to observe/photograph. This will make the system more accurate for that patch of sky, making it more likely your target will appear in the right place, centred in the eyepiece or camera.

That makes sense, most handsets will choose three widely separated stars if they can.

Anyway, stick in there, you will soon get the hang of it, and the list you need will be in your memory

[Louis D]

With my Sky Commander DSCs, I look up which bright stars will be visible from my location using Stellarium or star charts.  I then align on two of them and then locate an object, center it, and have the unit align on the object to refine the alignment.  When I slew to a far different part of the sky, I again realign on an object to refine the local accuracy.  I don't know if all pointing units have this align on object capability, though.  I find it to be very helpful.

[Buzzard75]

I usually look at the sky first and try to identify the brightest stars using a sky atlas, planisphere, mobile app, what have you, before picking them from the list on the hand controller. I try to look for ones that I already know before moving on to ones I don't, trying to find suitable stars. I also try to pick stars that are in complete opposite parts of the sky. Of course there's the old faithful Polaris that's easy for just about anyone to find, which means I only have to identify one or two more. I used to do Brightest Star alignment, but didn't always recognize the stars it was picking and sometimes I would end up with a failed alignment. So now I do Two Star and actually pick the star I want. Over time I've found that I've learned a considerable number of the brighter stars and can identify at least two or three easily enough regardless of what time of year it is. It's just something you learn through practice.

[Dazzyt66]

I tend to use 2 star instead cos I have a bit of light pollution and bright stars are a must. When I started out I used sky safari (basic is 99p) on my iphone - you can then ‘look’ at the sky through the phone and it will tell you the names of anything you are looking at. You can also use it to plan ahead if you know what time you will be out and I use Stellarium too for this.

Over time you will know which stars are best for you if you are observing from the same spot, and even if you change locations you get used to finding them in the sky visually. My favs for 2 star are any of: Vega, Dubhe, Pollux, Arcturus. All really easy to see and find ?

HTH!

[Geoff Lister]

Steve, I share your pain. I was also frustrated with Synscan suggesting (particularly for the second star), something behind a roof, fence or tree. Some bright stars are easy to identify, but some do not have an easily-identifiable constellation associated with them. I use Synscan with Az/Alt mounts, so EQ mounts may require slightly different target stars. I spent some time with Stellarium, the program adding the bright star names as you advance time around dusk. I produced a table for the middle of each of the months of the year; and 14 of the brightest stars. For each month I identified 3, 4 or 4 of these stars, not hidden by my surrounding "scenery", noting their direction (N, NW, W, etc.) and their altitude angle. I also avoided anything close to the zenith, as they are a pain in the neck (literally). Polaris is not a bright star, but is handy for a second star, as the alignment system should slew into its vicinity (not so helpful if you have an EQ mount). I used the same table to identify the best stars about 1 hour before dawn, for those early morning sessions.

This is how I understand the alignment system; others may have more knowledge of the finer points.

At power-up, the software sets the Az/Alt encoder settings to zero, so if you start with the mount level, and pointing towards celestial North, these settings can be used, together with the longitude/latitude/date/time inputs, to work out where all the objects in the database are to be found. In this configuration, there is no cross-component of altitude in the azimuth axis, and vice-versa. However, unless you used "Park" "Home" at the end of your previous session, and don't move the mount, the mechanical axes will not be at zero. The alignment process works out the errors in true level and North, and uses 3D trigonometry to add the errors to the expected position and cross-axis corrections for tracking. It is relatively easy to get a tripod/Dobsonian base level, particularly if you have checked any supplied mini bubble level's accuracy with a builder's spirit level. It is not so easy to get the OTA level and pointing North.

Now, this is where the choice of alignment stars is critical. If your base/mount/OTA was perfectly level, alignment would only need 1 star to fix any azimuth errors - any star (non-zenith) would do, with the best accuracy on stars close to the horizon. Your second star should confirm the azimuth and altitude angles. However, in the real world, there will be a (hopefully slight) altitude error, and by choosing 2 stars with close to 90 degrees azimuth difference, the software should be able to work out the North-South & East-West levelling errors. Also, by choosing stars with a reasonable altitude difference, the software should be able to resolve the second-order errors. You will get a good idea if you are getting close to a level setup, when the second star is close to the centre of your eyepiece after the auto-slew. If, after alignment, a GoTo to another bright star is way off, it is worth starting again, because the sums will probably be wrong for all other targets.

Geoff

[happy-kat]

I wish that I could pick stars within 90 degrees off the first, as I might have 180 degree vista but 3 street lights limit it down to 90 degrees to actually see a star) the second star I just have to accept as I can never see it. I do use a small three way level to make sure I am level, I don't like those bubble things.

Polaris is I gather not a good star to use as it doesn't actually move (enough).

[Buzzard75]

16 minutes ago, happy-kat said:

...

Polaris is I gather not a good star to use as it doesn't actually move (enough).

Shouldn't matter. You're just using it to set your encoders. Your mount doesn't care how fast your alignment stars are moving, only that they're centered when you tell it they are. I find that Polaris is easier to align on for this reason. It's easier to get it in the reticle of my eyepiece since it's moving so slowly. I also find that I have slightly reduced errors and track a bit longer without making adjustments when using Polaris as one of my stars. YMMV. I don't know if it has something to do with the encoders knowing with better accuracy where the pole is or what.

[Geoff Lister]

I agree with Buzzard75. It should be fine for an Az/Alt mount, but not so useful for an EQ (a bit like using a star at the zenith on an Az/Alt). I found that using Polaris as the first star, it was easy to centre on the wrong one - not a problem with Altair, Arcturus or Vega. Polaris, + another star at 20 or 70 degrees due East or West should work well.

I found that the bubble level on my Skymax tripod was fairly accurate, but on the Skyprodigy, I had to re-seat it, as it was way off. I added a bubble level to my Skyliner's Dob. base, and use a wedge under one of the feet to get it level, (turning the base so that the bubble is opposite one of the feet, and push the wedge in to centralise).

Geoff

[Buzzard75]

48 minutes ago, Geoff Lister said:

I agree with Buzzard75. It should be fine for an Az/Alt mount, but not so useful for an EQ (a bit like using a star at the zenith on an Az/Alt)....

Thanks for clarifying that. I should have classified that statement with the fact that I have a dob.

[Louis D]

Everyone keeps talking about leveling before alignment, but I can't find anything in my Sky Commander manual about needing to do that first.  I've always had good accuracy even on slopes with my SC equipped Dobs.

[Geoff Lister]

29 minutes ago, Louis D said:

I've always had good accuracy even on slopes with my SC equipped Dobs.

The alignment process should be able to compensate for most slopes. With Synscan, you have to make a manual slew to the first alignment star. If the mount is level, you can set the altitude angle on the mount, and then stand behind the OTA and slew until the azimuth angle is close enough to get the star in the field of the finder. The second slew is done automatically by the mount, where a level mount will assist the software to get it closer, with a reduced final, manual, adjustment.

If I start before dusk, with Sirius and Capella (+Venus, Jupiter or Saturn) just visible as the first star, I often find a slightly less bright, "Bright Star", e.g. Regulus, is just visible in a 9x50 or 6x30 finder, as the second star.

[Louis D]

5 hours ago, Geoff Lister said:

The alignment process should be able to compensate for most slopes. With Synscan, you have to make a manual slew to the first alignment star. If the mount is level, you can set the altitude angle on the mount, and then stand behind the OTA and slew until the azimuth angle is close enough to get the star in the field of the finder. The second slew is done automatically by the mount, where a level mount will assist the software to get it closer, with a reduced final, manual, adjustment.

Being a push-to, I slew manually to both stars.  I didn't realize these goto mounts will do the slewing to the second star.

[cloudsweeper]

I never use three star alignment, much preferring two named stars.  Stellarium can help with identification.  (Occasionally I use one star, or lunar/planetary.)  As for levelling, it seems logical that a levelled 'scope will be better at GoTo, and in any event, levelling is so easy anyway.

Doug.

[Louis D]

On 4/20/2018 at 15:25, cloudsweeper said:

As for levelling, it seems logical that a levelled 'scope will be better at GoTo, and in any event, levelling is so easy anyway.

Not on a dob without any way to adjust the height of individual feet under the ground board.

[Geoff Lister]

3 hours ago, Louis D said:

Not on a dob without any way to adjust the height of individual feet under the ground board.

The Virtuoso's Dob mount is fitted with 1 medium-length foot and 2 shorter feet with screw threads. These have enough adjustment to get the bubble level centred on a table sitting on a slightly sloping patio.

With my Skyliner, the 3 feet are fixed. I fitted a 35mm diameter bubble level to the rotating section of the base. When I place the base on the patio, I lift and rotate the whole assembly, until the bubble is opposite one of the feet. I then slip a "Fox Wedge" under that foot, and push it in until the bubble is central. I then fit the OTA, with it pointing roughly north (hiking compass), and level (altitude scale on mount), then power up. I have found that, by doing this, I get the most accurate GoTo and long-duration tracking.

Geoff

[cloudsweeper]

^^^^^ plus of course it's easier for a tripod to be way off level!

Doug.

 

[Louis D]

8 hours ago, cloudsweeper said:

^^^^^ plus of course it's easier for a tripod to be way off level!

Doug.

 

Absolutely.  I have no trouble leveling my tripod mounted alt-az grab and go setup (DSV-2B mount).  The bubble level is built into the tripod.  However, it has no electronics on it whatsoever.  The reason I level it carefully is it will swing wildly in azimuth if it is the least bit off with an offset mounted telescope.  With telescopes on both sides to balance it out, it's not quite as likely to swing around.

[david_taurus83]

I struggled at first with the handsets suggestions! Now I just look around and pre-choose the best alignment stars. Usually include Dubhe in the big dipper, Pollux in Gemini and Capella if visible. The Orion stars are good to choose from in winter.

[newbie alert]

Alignment should start in the West and end up in the east and the first alignment star will be out the most so I'd use a big bright star such as Capella ..alignment should also be away from the pole and meridian....id choose the biggest brightest stars and accuracy to centre them in the ep is better done with a illuminated crosshair one..not sure on the mount you'e using but on my celestron I can park in home position and hibernate, meaning all my star alignment is saved..just a case of waking up and then sync on a star near my target for the night..only ever need to re-do if the calibration is too far..which is normally 2-3 weeks

[Geoff Lister]

1 hour ago, newbie alert said:

on my Celestron I can park in home position and hibernate

The Skywatcher Synscan system has a similar feature; you just have to enter the correct date and time after power-up.

I used Stellarium, and by altering the date and time, I produced a table with an entry for the middle of each month of the year, and 14 of the brightest stars. For each month, I have identified 3, 4, or 5 of these stars, their rough direction (N, NW, W etc.) and their altitude angle, at dusk, and a similar table for an hour before dawn. This way, I can avoid any handset-suggested stars behind fences, roofs or trees, or any too close to the zenith or horizon.

Geoff

[Louis D]

3 hours ago, newbie alert said:

not sure on the mount you'e using but on my celestron I can park in home position and hibernate, meaning all my star alignment is saved..just a case of waking up and then sync on a star near my target for the night..only ever need to re-do if the calibration is too far..which is normally 2-3 weeks

Folks on here and CN have reported good luck by marking where each leg of the tripod was placed and putting it back on the next outing in the exact same spot, coming out of hibernate mode, and being very close to aligned again.