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  #1  
Old 22-05-2018, 10:01 AM
Startrek (Martin)
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2 Star Alignment Accuracy - Synscan

I would like to know what Stars IIS members are using this month for their 2 Star alignment procedure using the Synscan hand controller ( I assume some people are still using the hand controller not a Laptop with alignment software )



I'm getting various results using Hadar and Acrux early in the evening before they cross the meridian ( between 55 arc sec and 4 arc minutes of error )


I'm using both a 20mm and 12.5mm illuminated reticle eye pieces with and without a 2 x powermate ( the 12.5mm is so difficult to use due the narrow FOV , crossing back and forth from the finderscope to the eye piece)


My HEQ5 mount has a specified pointing accuracy of up to 5 arc minutes, so I assume that if my Star alignment error is below 30 arc sec then my pointing accuracy should be close to 5 arc minutes ?


I welcome any advice , comments and information of your Star selection and / or procedures etc..


Thanks in advance
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  #2  
Old 22-05-2018, 12:25 PM
gaseous (Patrick)
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Hi Martin,

I believe when using the synscan hand controller, it's advisable to use stars of a reasonably similar altitude and at least 60 difference in azimuth, so the Hadar/Acrux pairing are possibly a little close together to give the best alignment possible. I've been using the Synscan Wifi adapter and Synscan App, and have generally been using Rigil Kent / Canopus, Rigil Kent / Regulus, or Rigil Kent / Sirius as my two alignment stars, depending where in the yard I've set my scope up.
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  #3  
Old 22-05-2018, 01:43 PM
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billdan (Bill)
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Hi Martin,

Like Patrick said Hadar and Acrux are too close together to get a decent pointing alignment. I would use Acrux and Canopus (if its not low down in elevation) or Miaplacidus as your two stars.
If you have a permanent setup in an Obs you really only need one star to align on (which is what I do).

Cheers
Bill
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  #4  
Old 22-05-2018, 02:32 PM
Startrek (Martin)
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Thanks Patrick and Bill,


I set up in the same place in my yard on a paved area every time, so I have marked true south with a line and also the tripod leg positions


I have no south view below 60deg, no west view below 60deg, east view is ok above 30deg and north view ok above 30deg . So I have a limited selection of alignment stars with enough magnitude ( say > mag 2 ) to use.



I usually look at Stellarium an hour before I start the nights session and pick 2 stars for alignment and a list of 3 or 4 objects to image at mag 6 or lower



Tonight the sky in Sydney should be relatively clear , maybe some minor high level cloud so I will try Rigel Kent and Spica for my alignment stars ( at 8.00pm they are 55deg and 65deg Alt and 90deg apart in Az )


I read somewhere to keep clear of the SCP and Zenith areas as this could put your alignment out and also keep the 2 stars on the same side of the meridian



I have never tried just a 1 star alignment as I was told the minimum is 2 star and 3 star for good pointing accuracy



Welcome any advice or comments
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  #5  
Old 22-05-2018, 03:05 PM
gaseous (Patrick)
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Hi Martin,

I'm unsure of the intricacies of aligning an EQ mount, but avoiding stars near the zenith is a good idea, as I find my dobs don't like aligning/tracking things directly overhead.

I've never done a 1 star alignment with either the hand controller or the synscan app - always go for a minimum of 2. If you've entered all the time/date info correctly and you've chosen the right stars accurately, you should be pretty close.

The meridian thing must be an EQ idea too - I regularly use stars that are on either side of the meridian for alignment, with no issues. I guess if you're line of sight is limited you'll need to make do however you can - good luck!
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  #6  
Old 22-05-2018, 07:49 PM
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billdan (Bill)
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The method to use one star alignment is not to use the hand controller.

Tell the Synscan controller to move to your chosen star, say Acrux, and then loosen the clutches and move the scope by hand so the Telrad inner circle lines up with Acrux. Once its close lock the clutches and you can fine tune with the hand controller.
As I mentioned this only works if the mount/scope is permanently mounted in an Obs and is already polar aligned.

Cheers
Bill

Last edited by billdan; 22-05-2018 at 08:35 PM.
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  #7  
Old 22-05-2018, 08:30 PM
Wavytone (Nick)
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As one who did the maths ages ago to write a two-star alignment routine, the maths suggests the alignment works best with:

a) one star as low to the horizon as possible due east or west, this is effectively the azimuth reference; and
b) one star near the meridian, ideally at 90 degrees angle from the first star; this is effectively the altitude reference.

Personal experience with the Synscan handsets (I have an AZEQ6) suggests it is valid.

It's not particularly critical though and it can work with the second star near the pole as long as the other is close to the equator. If the stars are closer together the accuracy is poor.
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  #8  
Old 22-05-2018, 10:43 PM
Startrek (Martin)
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Thanks all

Just finished some imaging of Jupiter and the Moon tonight

Chose Rigel Kent and Spica for the 2 Star alignment ( about 90 deg apart in Az ) only required 2 iterations to get alignment error down to 2 arc minutes with a 20mm illuminated reticle and 2x powermate.

After imaging Jupiter and the moon, tried finding a few DSOs. Pointing accuracy very good , landed Omega Centauri just off centre of my DSLR display on live view.

I will now use alignment stars at least 60 deg apart as proven tonight by your advice and comments

Also polar alignment using the Synscan PA procedure only took 2 iterations with hand controller reading a PA error of 000.0000 Mel and 000.0000 Maz This usually gets you polar aligned to around 1 arc minute or so as I was starting to get elongated stars from 80 sec exposures onwards

It was a good night !!

Thanks again !
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  #9  
Old 23-05-2018, 01:32 PM
Oddity (Andrew)
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Thanks for the info, I've had some trouble aligning myself, using alt-az mode on azeq5 so this is good information.

I'm finding objects to be just out of view on Goto attempts on a wide field 30mm eyepiece... it might be star selection, which varies for me based on observing location.

How important is the home position in alt-az mode if doing 2-star alignment on Synscan mounts? There seems to be varying opinions on this. I usually am unable to have a permanent setup location and have been doing compass north rather than true north. The annoying thing about SynScan in Alt-Az mode in the southern hemisphere is that its home position requires true north rather than true south - which would be a heck of a lot easier to find, especially as the entire northern hemisphere is blocked by trees from my home, meaning I can't use SkySafari to pick a star that's approximately north at that moment in time.

I'm tempted next time to find a point close to celestial south pole eg. sig. octanis, lock the setting circle to 180*, move back to 0*, spirit level the OTA, and then power it on.

Quote:
Originally Posted by Startrek View Post
Chose Rigel Kent and Spica for the 2 Star alignment ( about 90 deg apart in Az ) only required 2 iterations to get alignment error down to 2 arc minutes with a 20mm illuminated reticle and 2x powermate.
Do SynScans actually get more accurate with repeated alignment iterations? I was, perhaps mistakenly, assuming that it wipes/resets its internally aligned map on each attempt and didn't have the capacity to update it the way SkySafari does.

Last edited by Oddity; 23-05-2018 at 05:18 PM.
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  #10  
Old 23-05-2018, 01:55 PM
Oddity (Andrew)
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I also have a potentially silly question. When entering latlong on powerup, if it's a high number of seconds, do you take simply the base number of minutes regardless of seconds (eg. if it was 5'55", treat it as 5'), or do you round up to the nearest minute ie. 6'? Or does it not really matter at that level?
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  #11  
Old 23-05-2018, 03:48 PM
gaseous (Patrick)
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I always round to the nearest minute, up or down. 5' 55" should be rounded up to 6'.
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  #12  
Old 23-05-2018, 04:04 PM
Startrek (Martin)
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Hi Andrew,

Cant help you out with Az Alt alignment as have a EQ mount but I can help you out with finding true north and true south or close enough

Forget compasses they never worked for me, I use the solar noon method as follows,

The ground must be relatively flat / level where you set up your mount, preferably paved , tiled or concrete etc

Buy a 1m long x 12mm dia piece of wooden dowel and use a hacksaw to cut a cross in one end about 3mm deep

Cut 2 lengths of string 2 metres long and source 4 bricks or half kilo weights

Download Stellarium ( free ) to you laptop

Pick a sunny day to perform the method

Fast forward the time date clock on Stellarium to the sunny day you want to perform the method.Fast forward the clock to the point when the sun passes directly over the meridian line , this line also passes through the celestial poles ( check the data up in the top left corner, when the suns degrees click over from 359deg to 0 deg , that will be the solar noon time you will use later on, say for example 1.08pm )

On the day you have selected, about 1 hour before the solar noon time, set up your timber dowel 90 deg to the ground below your mount position

Use a good quality bubble level to ensure the dowels NSEW points are at 90 degrees to the ground ( this is important )

Use the 2 strings and weights as guy ropes ( NSEW ) to anchor the dowel in position

Exactly at the pre determined solar noon time ( I use my iPhone ) mark a thin line on the ground about 500mm long using a tile cutter or similar and a straight edge or ruler along the exact centre of the shadow cast by the timber dowel ( the more precise your are the more accurate your true north / south line will be )

I have used this method in my two locations ( Sydney and Narrawallee south coast ) and it has never let me down as long as the mount is positioned correctly with respect to north / south

I set my home position , azimuth centred on the mount head , altitude set to location and start a 2 Star alignment ( 1st Star always ends up in the FOV cross hairs of my 20mm illuminated eye piece )

Hope the above helps

Sorry to write such a long procedure but it should only take 5 to 10 minutes after you have all the bits and pieces ready

Martin

PS: there are numerous other ways to find true north / south but Im old school and the sun never lets you down !!
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  #13  
Old 23-05-2018, 04:23 PM
ausastronomer (John Bambury)
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Quote:
Originally Posted by Wavytone View Post
As one who did the maths ages ago to write a two-star alignment routine, the maths suggests the alignment works best with:

a) one star as low to the horizon as possible due east or west, this is effectively the azimuth reference; and
At an altitude of 5 degrees above the horizon the effects of atmospheric refraction will cause the target to be visually incorrectly positioned by about 10' (1/3rd the diameter of the full moon ) in relation to its true actual position.

How did you factor this into your mathematics and how will it affect the overall pointing accuracy of a computerised telescope pointing system, if one was to use a star situated 5 degrees above the horizon, as an alignment star?

Cheers
John B
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  #14  
Old 23-05-2018, 08:06 PM
Wavytone (Nick)
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Quote:
Originally Posted by ausastronomer View Post
At an altitude of 5 degrees above the horizon the effects of atmospheric refraction will cause the target to be visually incorrectly positioned by about 10' (1/3rd the diameter of the full moon ) in relation to its true actual position.
John

It is derived from a formula known as the "surveyors algorithm" for determining position on earth astrometrically in absolute coordinates, using a theodolite to observe the stars, and a precision time source. A theodolite is just a precision altaz telescope where the azimuth circle is accurately aligned to north using a terrestrial reference mark (from a map) and, with the base precisely levelled, the altitude measurements are also accurate.

In the surveyors case:

- the star positions are known (from an ephemeris);
- the mount is dead level, and
- the measurements from the circles are accurate, so

the problem is to determine the telescope's lat and long.

In the astronomical case, however:

- the star positions are known,
- the telescope position is known (thanks to GPS) and
- the orientation of the circles is unknown, so

the problem is to determine the orientation of the circles relative to the sky.

In both cases the algorithm is curiously the same apart from transposing some variables. The algorithm works by projecting the polar axis ( EQ mount) or azimuth axis (altaz setup) on the sky then:

a) for an EQ setup, it has to find the altitude of this point above the horizon and its azimuth; or
b) for an altaz setup, it determines the declination of the point on the sky, and the hour angle (east/west).

It then calculates the offset from the correct values, based on your longitude and latitude, assuming the mount is level, determines the errors and stores these for later coordinate conversions to find any object at some future time. This is why - if you enter incorrect date/time/lat/long data, the resulting alignment and GOTO's can be WAAY off. Levelling the mount/tripod head is also important - something many may not be aware of.

In the EQ setup, the star near the horizon primarily determines the azimuth of the polar axis relative to the star. Refraction affects the altitude, not the azimuth, and any error from refraction is thus negligible for this - well below the desired precision.

The star on the meridian is used to determine the altitude of the polar axis and hence it does need to be reasonably high to avoid an error due to refraction, andn ideally on the meridian because during the actual observation, as the star crosses the meridian its altitude is constant, to a first approximation.

When surveyors use this to calculate an astrometric position they do include a correction for refraction in the measure altitude of a star, using the site altitude, barometric pressure and humidity and a lookup table from the USNO Almanac. But they are trying to achieve sub - minute of arc accuracy, whereas I’m content with being within 15 arc min, so there’s no point worrying about it - especially when a telescope mount has greater mechanical errors between the axes.

There are cases where the algorithm is vulnerable to errors:

a) two stars close together;
b) two stars more-or less opposite due east and west; in this case the azimuth error is small but the altitude correction will be poor.
c) both stars at about 45 degrees elevation or more and neither near the meridian, in which case the both altitude and azimuth accuracy will be poor.

The latter case does seem to arise frequently in practice with a Synscan, due to the limited choices of bright stars.

Lastly there is a significant difference in the quality of an alignment done by just eyeballing the reference stars vs aligning with an eyepiece with crosshairs - the latter invariably achieves a much better result. The subsequent GOTO's are also sensitive to non-perpendicularity between the optical axis of the scope and the dec axis of the mount, and between the dec axis and RA axis. The Skywatcher EQ mounts complete with telescope+mount combinations may well have cone errors of the order of a degree.

Last edited by Wavytone; 23-05-2018 at 10:39 PM.
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  #15  
Old 02-06-2018, 10:36 AM
davo50 (Craig)
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Quote:
Originally Posted by Oddity View Post
How important is the home position in alt-az mode if doing 2-star alignment on Synscan mounts? There seems to be varying opinions on this. I usually am unable to have a permanent setup location and have been doing compass north rather than true north. The annoying thing about SynScan in Alt-Az mode in the southern hemisphere is that its home position requires true north rather than true south - which would be a heck of a lot easier to find, especially as the entire northern hemisphere is blocked by trees from my home, meaning I can't use SkySafari to pick a star that's approximately north at that moment in time.
Is starting with the scope aligned north a critical step for 2 star alignment an alt-az dob? I'm having a lot of trouble and would like to skip this step when making repeated attempts to get a working alignment.

Also, my synscan crashes "searching..." on selecting single star alignment, does anyone have any ideas how to solve that?
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  #16  
Old 04-06-2018, 02:17 PM
gaseous (Patrick)
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Unsure about Alt-Az tripod mounts, but the initial tube position of a Skywatcher Goto dob makes no difference to the alignment (that I've found) when you turn it on and begin the star alignment. This is when using the hand controller at any rate. I tried having the scope level/north facing for a while, then discovered it doesn't really matter.



I've found when using the Synscan app, the two star alignment lets you start with the tube in any location, then manually slew to the first star. With a 3 star alignment, you need the tube level and pointed north when you turn the mount on, as the scope will automatically slew to the first star under the assumption the tube was level/facing north. There were some very dicey moments trying a 3 star alignment before I discovered this, as the scope started to slew wildly all over the place, but now I only use the 2 star procedure anyway to avoid stuffing about with leveling the scope and getting an accurate north position.
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