Polaris in Transit

[JamesM]

Hi everyone,

Now this is my first question on the forum so be gentle.

I was reading Steppenwolf's excellent guide to polar aligning and I noticed that he refers to a program called "Polar Finder" which is needed in order to help calculate when Polaris will be in transit for my local time and date.

The thing is, I only have a Mac computer and when I attempted to download it (...and there is a serious chance that I probably didn't do it right) I didn't seem to receive anything. I have a hunch that maybe this program is only for use with PC's - Have I got that right or does this confirm that I'm hopeless with computers? Are there any other programs I could you try?

Any help would be grateful as I intend to buy this type of mount and obviously if I need a PC I shall have to budget for this as well.

[Psychobilly]

I didnt bother with polar finder instead i generated a transit table for the next month from the USNO website...

http://www.usno.navy.mil/USNO/astronomical-applications/data-services/mrst-world

I was also pointed in the direction of a you tube video....

Which with Steve and Astro Babys tutorials finally made it click...

Peter...

[JamesM]

Thanks for the info Peter,

I had a feeling that there was a different way to get hold of this info and I certainly like the idea of constructing my own table if only to help me understand a little more about the subject. This is one (...of many) reasons I've held off from buying anything so far until I can get my head around all the other things that might be needed down the line. I suppose it's an attempt to sort of future proof my choices so as to keep as many options open as possible e.g EQ6 mount, although very heavy, will allow different set up options for observing or even imaging should I change my kit in the future.

Again thanks for getting back to me.

James

[FraserClarke]

Just found this useful little Mac widget, which will give you a sidereal clock for your dashboard;

Mac Software

(scroll down to LSTclock).

You need to set your longitude (Greenwich will be close for UK people) in the preferences, and then it will give you the Local Sidereal Time. LST gives you the right ascension of objects transiting now.

Polaris' right ascension (RA) is 2h 31m 49s -- so when your sidereal clock say 2:31:49, polaris is transiting

[great_bear]

Actually, you don't need any software at all to calibrate a Polar Scope.

The phrase "when Polaris is in transit for my local time and date" is a bit of a non-sensical statement, because our time-of-day is itself defined by the position of the sky above our heads (OK, specifically the Sun, but the descrepancy is only about 4mins per 24hr period - that's more than close enough for Polar Alignment). The only consideration required, is your location's longitude meridian offset relative to your local time zone - and Polar Scopes have little E-W correction markings that you reference during everyday use in order to account for that.

You need no software, no table of transit times, not even Polaris itself in order to calibrate the Polar Scope. You can do it in the comfort of your living room (at least once the reticule is centered).

All you do (as a once-off procedure) is to loosen the Index Ring (that's the black ring with a single white line on it), look through the Polar Scope at its markings on the reticule and rotate the scope around such that the Polaris target mark in the reticule is now located at the exact bottom* of the view. Then reposition the Date Ring so that November 1st** is at the top, and then rotate and align the Index Ring so that its white line aligns with meridian offset zero against the Date Ring. Then lock the Index Ring's set-screw - it should never need adjusting again***. The Date Ring is left free to rotate.

In everyday use then, with the mount set-up, just start by locking the RA Ring so 0:00 (midnight) is at the top, then twist the Date Ring so that your location's longitude meridian offset aligns with the white mark on the Index Ring on the Polar Scope, and then rotate the entire Polar Scope (normally by rotating the telescope's entire RA Axis) such that today's date (on the Date Ring) aligns with today's current time (on the RA Ring - but not including Daylight Saving Time). Lock off the RA axis, peer through the Polar Scope, and adjust azimuth and altitude to get Polaris into the marked place on the reticule.

- it sounds (as always) a lot more complicated than it really is - it's actually rather fun :-)

And let's not forget the relatively rough degree of accuracy that Polar Alignment provides: The HEQ5's RA Time Ring has ~2 mins of rotational slop each way about the set-screw when tightened, and the Date Ring itself is a very sloppy fitting around the Polar Scope body. It's like this because the lack of absolute precision really doesn't matter.

That's why using software to calibrate the Polar Scope is a waste of time: There's nowhere near enough precision in the Polar Scope's design to make it a worthwhile exercise. For precision alignment, you need to drift-align - and that's a totally different kettle of fish altogether...

Hope that helps.

Notes:

* "bottom" because Polar Scopes invert the image, so Polaris transits are at the bottom, not the top, of the view.

** November the 1st, at midnight, is a perfectly suitable Polaris transit time to calibrate to.

*** The only purpose that the Index Ring serves, is as a fixed external reference indicator for the internal orientation of the reticule.

[LukeSkywatcher]

The phrase "when Polaris is in transit for my local time and date" is a bit of a non-sensical statement

Tell me about it. I thought Polaris was in a fixed position.

Sorry i am not really helping. The title of this thread confused me.

[JamesM]

Sorry Paul about my heading, I'll make my questions a little clearer that they are coming from a newbie and the particular difficulty I'm having.

Tea Dwarf I like the idea of the widget - though I have to admit, that although my existing widgets are all space/astronomy related, I've got so many of the damn things , I feel I'm turning into a widget (....see the truth has come out now! Look what you made me say. ) For your efforts, I will just this once, attempt to reorganise my screen to squeeze just one more in though!

Great_Bear - great advice! I had to read your instructions a couple of times not because they weren't clear but simply to embed them all in my this wooden box I call my head In my original hello to you guys on the forum, I acknowledged the fact that I had witnessed many routes to a problem and your kind advice has reinforced this. (I'm saying this in part as relief, as I'm not that good with computers and so one less thing to think about!) Than you for your advice.

Kind regards

James

[great_bear]

Forgot to say - for anyone who isn't convinced that software is completely unnecessary to get Polaris transit times, try the following experiment.

Rotate the Date Ring so that November 1st aligns with 0:00 on the RA Ring.

Now - if you look - you'll see that every Polaris transit for the entire year is now shown isn't it? You don't even need to touch anything; simply read off what time is shown next to each day of the year around the edge of the Polar Scope.

It's that simple.

(double-check it with software if you don't believe me )

If you can't be bothered getting out of your armchair, you can pull the same stunt with a Planisphere if you've got one handy. Merely line-up 0:00 (midnight) with November 1st, and the Polaris transit times for the rest of the year are automatically aligned with every date around the edge of the Planisphere.

And they would have to be wouldn't they? After all, there is only one orientation that you can align your Planisphere such that Polaris is shown in transit (provided of course it doesn't have a rivet punched through it!)

- and when you realise that you can do this for any "fixed" celestial object - and that it is in fact the entire underlying principle of a Planisphere, then the penny should finally drop:

Your Polar Scope is a Planisphere!

rgds,

Bear.

[JamesM]

Bear, I am glad that you used the word 'Planisphere' as I actually HAVE one of those. It's the only astronomically specific thing I've bought so far. Cor, I actually feel like I've crossed a line there and have come over to you guys - as I've got some gear

Again many thanks for being so thorough!

James

[great_bear]

Made a couple of tweaks since the original post, so if you've printed this post off, you may want to reprint it.

In particular I noticed a moment ago that I referred to time as "GMT" when all I was meaning to convey was just "not summer time"

Updated original post.

(lost track of how many "minor" tweaks now I've made to it!)

...it's so difficult to get the right wording sometimes!

[JamesM]

Your efforts have been appreciated and it is important to be as precise as you can be due to the nature of this topic. Yes, of course this will now mean I'll have to replenish my printer inks and order more paper in! :D (.......not really)

Thanks again

James

[Psychobilly]

And let's not forget the relatively rough degree of accuracy that Polar Alignment provides: The HEQ5's RA Time Ring has ~2 mins of rotational slop each way about the set-screw when tightened, the Date Ring assumes exactly 30 days for every month (never 28 or 31) and the Date Ring itself is a very sloppy fitting around the Polar Scope body. It's like this because the lack of absolute precision really doesn't matter.

Thanks for the info and setup method

I had a closer look at the Date Scales...

Not all months are marked the same...

Peter...

[great_bear]

Not all months are marked the same...

After reading your post, I checked my nice SW HEQ5 mount and yes - you're absolutely right - the months are correctly marked with the right number of days, although it's not obvious at first glance. I suspect this wasn't the case on a cheapo mount I previously had.

Thanks for letting me know Peter - original post updated accordingly!

[jnc71106]

thanks for the guide will have a go at doing it this way tonight

[starstuck]

look through the Polar Scope at its markings on the reticule and rotate the scope around such that the Polaris target mark in the reticule is now located at the exact bottom* of the view............... hi! great simplification of an overly complicated subject! just one question, sorry if its a pedantic question!..(still trying to get my head round this clockwork universe)...in the above text," does rotate the scope" mean the polar scope or the telescope on ra axis?.... thanks in advance......paul

[Stargazer51N]

(still trying to get my head round this clockwork universe)...in the above text," does rotate the scope" mean the polar scope or the telescope on ra axis?.... thanks in advance......paul

Hi, this is my first post on SGL so apologies if I get something wrong. I've been very confused by my telescope manual whilst trying to set up my polar scope. Having read through all the posts in this topic it finally clicked and I am now feeling confident enough to answer starstruck .....

It means rotate the telescope on the RA axis which will in turn rotate the polar scope.

Thank you to everybody in this topic for your help.

Regards,

Stargazer51N

[great_bear]

I've been very confused by my telescope manual

Yes - the SkyWatcher manuals in the past have been confusing in some respects, and just plain wrong in others. Unfortunately, it appears that the person who wrote those manuals didn't entirely understand the subject.

[sublime_mick]

must try this

[RichardL]

This seems a good time to say hello. First post so here goes ... here's what I do when I can be bothered. I have a TAL 150mm Newtonian and a pole finder (which is basically just a long-ish tube) which sits on the equatorial mount before I attach the cradle rings. (I have to assemble from scratch each time.) The angular diameter of the far end of the pole-finder tube is the same (approx) as the angular diamter of Polaris's orbit around the pole. The attached table (if I managed to attach it) I made up some time ago. So using the sidereal time widget (or a stand-alone sidereal clock, which is what I use) one can figure out where abouts at the end of the tube Poloaris should be.

In practice, if I can even see Polaris from my light-polluted area (NE London) I am pleased.

Position of Polaris.doc

[Singlin]

1.My Longtitude is 7° 37" 29,66 E

Should i round off to 7° or 8° for my offset calculation?

2. What is the RA scale from 0-10 to the left of the time circle locking nut on the basic Eq5 mount?

3."and then rotate and align the Index Ring so that its white line aligns with meridian offset zero against the Date Ring" does this mean corrected offset Zero?

Regards,

Simon

[Dellis]

I've just used this guide to set my polar scope. Very helpful indeed. Thank you.

Dave

[biondi]

I wish I found this post about a year ago Very well explained, I think for the first time it clicked after reading this.

cheers,

Matt

[Ouroboros]

If the mount is aligned to where Polaris is now - looking at Stellarium say - and if one is using guiding, would this be sufficient in most situations to avoid star trailing during imaging?

[Daddystu]

Polaris is offset about 2/3 of a degree from the NCP so go-tos on a low power widefield rig would probably be fine. For imaging though I wouldn't want to give the software too much error to account for. I guess any trailing would be proportional to exposure length so thats a further consideration.

[Ouroboros]

Yes, I suppose guiding can't compensate for polar misalignment indefinitely.

What I'm asking though is do imagers with guided mounts get away with approximate polar alignment for typical exposures per sub of a few minutes say? Or is a rather more precise polar alignment absolutely necessary?

I'm interested because I'm thinking of up upgrading to a guide-able mount.