GOTO mount - Polar alignment, when you can't see Polaris

[BigBoot]

Having recently tried to 3-star align my mount without a polar alignment, and being disappointed with the accuracy, I finally had a go at polar alignment last night.

However, I quickly realised that I can't see Polaris from my my back garden (due to a line of trees).

I'm not entirely sure if the lack of accuracy is down to the lack of polar alignment, or that I am mis-identifying the alignment stars (might have misidentified Phad last night).

So, I have a couple of questions that I am hoping you folks could help me with.

1) Is accurate polar alignment needed for the SynScan/Goto functionality?

2) Is there a workaround for sites where there is no view of Polaris?

Any thoughts would be much appreciated.

[jambouk]

1. Depends on ones definition of "accurate" and on the scope in question, and on the duration of the observing session. Ideally try and get Polaris in the view of the polar scope (or that degree of accuracy even if you can't see Polaris).

2. Yes:

a. the handset Polar Alignment routine (pretty easy, but only do it after a two star alignment using two starrs on the same side of the meridian)

b. drift aligning (painful if setting up each night)

c. using a compass and working out the correct elevation somewhere else first.

James

[jambouk]

In reference to "c" above. Take you mount to a place you can see Polaris near to your chosen observing location (ideally the front of the house if observing from the back). Level the mount roughly. The get Polaris in the polar scope using the altitude and azimuth adjusters; anywhere will do but if you want to get it bang in the middle of the cross hairs then do so.

Take the mount back to your chosen location and level it, then make sure the RA axis is pointing as accurately north as possible using a compass, but don't stand too near the mount as the metal will effect the compass. Don't alter the altitude knob. Then you will be very roughly Polar Aligned.

James

[jambouk]

With reference to "b" above, do something along the lines of "c" but maybe not as accurately, so you get the mount's RA axis roughly pointing towards the North Celestial Pole.

Set up the handset as normal. Do a two star alignment using two stars on the same half of the sky (either east half or west half). Then once you;'ve done that it will tell you the Mel and Maz errors; this is the amount of error the mount thinks your Polar Alignment is out by; Mel is elevation, Maz is azimuth. Note these numbers down or take a picture with your phone. Then select Polar Alignment from the handset. It will ask you to pick a star, slew to it, then it will ask you to centre the star with the handset. Do this, then it will slew the scope away from the star and ask you to centre it as well as possible by ONLY using the elevation knob. Get the star as near the centre of the FoV as you can (it might be a long way off the first time). Then press OK and it will then re-centre the star, and then slew again away, and ask you to only centre the star using ONLY the azimuth knobs. Do this. Once you've done it, do a new two star alignment, again with just two stars on the same side of the meridian. It will give you new Mel and Maz errors; comapre thes the the first set and the numbers should be smaller - in theory values of 0 for Mel and Maz mean you are exactly Polar Aligned. Repeat the Polar Alignment routine two of three times, each time doing a two star alignment in between; I always use the same two stars to align with.

Easy.

James

[BigBoot]

1. Depends on ones definition of "accurate" and on the scope in question, and on the duration of the observing session. Ideally try and get Polaris in the view of the polar scope (or that degree of accuracy even if you can't see Polaris).

The scope is a Skywatcher Explorer 200P with an EQ5 mount, and I am really hoping to be able to use the scope to find and track DSOs. I would be hoping to get the accuracy to find DSOs, for a 2-3 hour observing session.

I also hoped that I would be able to do some photography with the mount, planetary and, hopefully, some DSOs too. I realise that the HEQ models are preferred for photography, but hoped that I could get decent enough results with an EQ5.

Thanks for the very helpful workaround advice that you gave in the later posts. I really appreciate you taking the time to post. I am going to need to re-read it a few times to digest it but then, the first chance I get, I will have a go at using the methods that you describe.

Am I correct in thinking that the whole of post #4 describes the "drift aligning"? Would that give the best accuracy of the 3 methods?

[jambouk]

No, when i said "b", i meant "a"!!! The method in post 4 describes using the handset polar aligent routine, which will give you pretty accurate polar alignment, certainly good enough for observing DSOs. It will also probably give you good enough polar alignment for 30-60 second exposures, but you will have to experiment as the limitations of the mount will also influence the length of exposure you can achieve.

James

[jambouk]

Drift alignment would probably give you the best polar alignment, but i'm not convinced you would notice much difference between the handset routine PA and drift aligning as the limitations of the mount will probably start to be greater than the potential benefits of the one PA method over another.

Very much a case of trial and error. Most of these things are.

If you are going to use the handset routine, i would look at investigating how much cone error you have and attempt to reduce this if it is easy to do so.

I should have also said above, if you are doing a handset PA routine, always end woth a Three star alignment. The three star alignment should be the last thing you do before you start observing and using goto. The three star alignment will compensate [in part] for any residual cone error. The skywatcher synscan manual (i think) describes cone error, else if not in that one, it is in the manuals for the mounts which use synscan handsets (heq5, neq6, azeq6, eq8).

James

[BigBoot]

Drift alignment would probably give you the best polar alignment, but i'm not convinced you would notice much difference between the handset routine PA and drift aligning as the limitations of the mount will probably start to be greater than the potential benefits of the one PA method over another.

Very much a case of trial and error. Most of these things are.

OK, I think I will start with the handset PA and see how that goes. Will report back. Thanks again.

[Sirius Starwatcher]

Well gentlemen I have been considering putting my 200 p OTA from the Dob base onto a tracking mount and tripod. Having just read the long complicated threads above I Estimate I will be losing about 1 and a half hours observing every time I set the system up as I have the same problem of not being able to see Polaris from my observing position in the backyard. I May have to re consider and stick with the Dob.

[jambouk]

It may well take you an hour and a half the first time, half an hour the second, but once you know what you are doing and understand what it is trying to do, you can complete this in a few minutes, under ten; I can do it in under five.

But that is five minutes on top of the set up. Again, setting up, balancing, whatever all takes time. It gets quicker, but it does take time. One of the drawbacks of setting up an accurately aligned equatorial system each time you want to observe or image. 

James

[Dave In Vermont]

To those out there who are thinking of using a compass the help you align on true-north, I wish to tell you - briefly - about 'Magnetic Declination.'

Magnetic Declination is the number of degrees you will be OFF from True-North when using a compass from your location. This is simply because there is a difference between Magnetic-North and True-North due to our (the Earht's) Van Allen Radiation-Belts* which serve to protect the Earth from being bathed in the cosmic-radiation the Sun fires off into space.

Here are some maps you can download and put on your wall:

https://www.google.com/search?q=Magnetic+Declination+map&tbm=isch&imgil=ssVw6ToKy4AypM%253A%253B8e_aS4cfQDjtgM%253Bhttp%25253A%25252F%25252Fgeokov.com%25252Feducation%25252Fmagnetic-declination-inclination.aspx&source=iu&pf=m&fir=ssVw6ToKy4AypM%253A%252C8e_aS4cfQDjtgM%252C_&usg=__MCPI3alnVv9EESGaYkVZ98pbvD0%3D&biw=1600&bih=742&ved=0CCgQyjc&ei=6qcyVeCnF4yrgwTnsoCIDQ#imgrc=W1Z3yZAzNdOIfM%253A%3BSPGOYtOIx93TxM%3Bhttp%253A%252F%252Fupload.wikimedia.org%252Fwikipedia%252Fcommons%252Fthumb%252Fd%252Fd4%252FWorld_Magnetic_Declination_2010.pdf%252Fpage1-1280px-World_Magnetic_Declination_2010.pdf.jpg%3Bhttp%253A%252F%252Fen.wikipedia.org%252Fwiki%252FMagnetic_declination%3B1280%3B853

And here are some explanations:

http://www.ngdc.noaa.gov/geomag/declination.shtml

Those of you in the UK - you are lucky enough to live in a region where the difference is very close to nil.

Clear Skies & Spinning-Needles,

Dave

* http://en.wikipedia.org/wiki/Van_Allen_radiation_belt

[BigBoot]

Had a very frustrating attempt at handset polar alignment last night.

After centering the star (Spica) the handset gave me a Mel of 6 degrees. After doing the manual altitude and azimuth adjustments it gave me a Mel of 3 minutes. I then did a 3-star alignment, but didn't re-check the Mel and Maz values! The alignment was clearly way out, but it wasn't until later that I tried another 3-star alignment and saw that the Maz was 31 degrees!

I have a feeling that I did the azimuth re-alignment against the wrong star

Anyway, the good news is that I think I have a better feel for what the handset is telling me, and how it corresponds to the manual adjustments, so I'll try again tonight (clear skies permitting) and just be a bit more methodical.

Oh, I became well acquainted with Mizar (and Alcor), during the 3-star alignments too...which was nice.