Digital Compass as R.A. guide?

[philsail1]

Dear All,

I have a question. I've just read "Themos's" bit (if I've understood it correctly) about using this

http://nightskies.net/scopetest/accessories/wixey/wixey.html

to obtain a very accurate "declination" setting with a Dobsonian scope.

If this little instrument would give a very accurate setting of the Declination angle of an object, could one use a digital compass to obtain the Right ascension setting? (would such a compass would have to be aligned to match the mount in facing the Pole Star).

This is intriguing me, as if it is possible it might make finding deep sky objects a lot easier than using setting circles?

Does anyone have any thoughts on this?

regards,

philsail1

[daz]

Phil

Yes it most certainly could, except it would be Alt-Az coordinates, not RA. But any of the planetarium software gives the coordinates for both.

The difference (for those that need to know ) is the RA and DEC coordinates are 'fixed' on the celestial sphere. Alt-Az coordinates move because they are relative to the viewer.

I'm sure someone has a better explanation that this, but hopefully you get what I mean!!

[philsail1]

Thanks for that Daz - I'll be off down to "Maplins" on Saturday morning to price up these items!

Seriously though, I appreciate that you are saying a digital compass could be used in conjunction with the digital "inclinometer," but I don't quite understand the difference between the RA and Dec and "Alt Azimuth" systems? (do I need to? I would like to know!!). I know that the "Stellarium" program that I use does indeed give both systems.

Regards,

philsail1

[Kaptain Klevtsov]

The RA/Dec. system is fixed to the stars, imagine the stars had a grid drawn on them like longitude and latitude scales, that's what the RA and Dec. refer to. As you look up, the lines will go past slowly, like if you fly your lego spaceship over a globe on your desk. The lines stay with the globe or stars depending on if you are looking down or up. The north pole of this system where all the RA lines converge, is the North Celestial Pole near Polaris.

Because the stars move over time, you need to calculate where the star is from ground level, even though it doesn't move significantly relative to the other stars. Vega is on the way down when it gets dark, whereas a few months ago it was just coming into view. Allowing for this is the reason that you need to set your RA circle each time you start looking.

Wit the Dob. system, you need to be able to calculate Alt.Az. (left-right, up-down) co-ordinates in real time (or predict when you will want to find stuff) as they go out of date quickly.

Kaptain Klevstov

[philsail1]

Many thanks Kaptain K! that's a good clear explanation - which I'll print out and digest slowly until it eventually finds its way through all the fat in my brain!

(You've got a lot of confidence in me Kaptain when you say a Lego Spaceship! - I could only manage to build those square houses with green tiles for a roof, with my lego!)

Seriously though, I do appreciate your answer, and I'm getting quite enthused by the notion of using a digital inclinometer and compass to find things with.

Regards,

philsail1

[themos]

I've looked at digital compasses and they are far too expensive (compared to the Wixey). However, a cheap incremental encoder should suffice. Somebody with the right skills needs to try and make a device for a Skyliner, say, with an encoder, a numerical display and some logic circuits and a couple of microswitches.

http://www.usdigital.com/products/rotary-kit-encoders.shtml

[Kaptain Klevtsov]

Or alternatively,...

http://stargazerslounge.com/index.php/topic,4100.0.html

Kaptain Klevtsov

[philsail1]

"Themos", thanks for your reply, but I've seen some digital compasses on e-bay which seem to indicate they have a "dual" mode which encompasses a DEC angle as well as a N,E,S,and West compass.

Kaptain Klevstov! Yes I get your point with the mechanical inclinometer - but would it be as accurate as the "Wixey?"

I'll continue to make a few enquiries with the digital compasses.

Regards

philsail1

[Kaptain Klevtsov]

No mate, use the wixey and get the big protractor drawn out for the azimuth bit. You should be able to find stuff with that info.

Kaptain klevtsov

[philsail1]

Ah! thanks Kaptain K! I think I will get the "Wixey" I'm sure I've seen them going on e-bay for about £14.00. I'll explore making a bigger setting circle for the "azimuth" (RA) setting then.

Thanks for your help,

Regards,

philsail1

[themos]

Phil, just a quick note about your usage of "azimuth/RA" terminology. As you know, azimuth is the angle around the horizon, in the horizontal plane. RA, right ascension, is an angle measured on a plane different to the horizontal one, it is measured on the equatorial plane. To see the difference, think of Polaris: In 12 hours, its azimuth will have changed hardly at all (still very near the North direction). Its RA coordinate is fixed and hasn't changed but the two positions in the sky where it was (at the beginning and the end of the 12 hours) differ in RA coordinate by precisely "12 hours".

[philsail1]

Thanks "Themos" Am I to understand that Polaris (in Azimuth terms) would continue to lie vertually directly at right angles to the Earth's surface, so that there would indeed be little or no change in it's "Azimuth" co-ordinates?

Whereas in Right Ascension terms Even though Polaris would remain in its fixed position in the Polar scope, its R.A. co-ordinates would alter throughout the 24 hours in a day?

Regards,

philsail1

[themos]

Not quite! Polaris is not at "right angles to the Earth's surface", certainly not in the UK. That point, precisely overhead, is called the zenith and different objects occupy it every hour and every day. People on the same Earth latitude will see the same things at the zenith but at different times.

Polaris is at around 50 degrees above the horizon, above the North horizon point. If you start spinning round while standing up, that's a sweep in azimuth. The Dobsonian mount is an Altitude Azimuth mount because we get to spin it around, on the horizontal plane, and then raise it or lower it in Altitude. Neither the Azimuth nor the Altitude of Polaris will change appreciably during the night. That's because Polaris is very close to the celestial norh pole and so moves in a very small circle during the night.

In Right Ascension terms, every star is at a fixed RA. The RA, Declination coorsinate system is a sky-fixed system, unlike the Alt-Az which is earth-based. Polaris has a fixed Declination and Right Ascension and so do all the other stars. Well, not quite, because stars do move relative to other stars in the sky but this motion is only visible over a long period. So Polaris RA/DEC coordinates in 2007 are slightly different to 2000. That's the Epoch difference, if you've come across it.

The RA is measured in hours, there's 24 hours around the circle. The difference in the RA coordinate of two stars tells you how far behind a star is from the other, as they go round and round in the sky, circling aroung the celestial pole.

[philsail1]

Thanks Again "Themos." So whatever is directly above (at 90 dgrees) to your Dobsonian's base is at the Zenith. If your "Dob" is pointing to that object at the zenith and you do a complete sweep of 360 degrees (an Azimuth sweep), that object will still remain above, but not quite at the 90 degrees, as it will have moved a bit. Whereas, when setting the scope's Polar axis to the North Star, you are then locking that axis on a fixed point, and only have to make and initial adjustment for the object's "Declination." Once locked on to the object, your scope follows the object accurately.

I still don't quite understand What exactly "Right Ascension" is. (is it the position of a star in the sky at a given time - if so, where is the "Datum" line to enable the RA system to begin finding that fixed RA position?). Also how and why do "Right Ascension" co-ordinates remain the same for all astronomical objects, when they are continually moving across the sky?

My brain must be atrophying! (Result of being forced to watch too many episodes of Emmerdale, Corrie, and Eastenders!)

Regards,

Philsail1.

[BWBlackett]

I have the Wixey Gauge on my Dob but haven't found it that useful....

One of the problems is that it uses decimal degrees e.g. 45.76Deg where as Stellarium used deg,min,sec eg 45.50.45 deg

and converting between the two on the fly is a pain as they obviously change fairly quickly.

I use it to get in the right sort of area then use a wideangle eyepiece to hunt for my target.

I have had a problem with batteries on this gauge especially in the current cold weather - they don't seem to last very long.

Brian.

[themos]

If your "Dob" is pointing to that object at the zenith and you do a complete sweep of 360 degrees (an Azimuth sweep), that object will still remain above, but not quite at the 90 degrees, as it will have moved a bit.

A tiny bit, though. I can turn the Dob around in a few seconds and I don't expect to be able to see the difference unless I have a 600x eyepiece on!

Whereas, when setting the scope's Polar axis to the North Star, you are then locking that axis on a fixed point, and only have to make and initial adjustment for the object's "Declination." Once locked on to the object, your scope follows the object accurately.

I agree with that. With a Dobsonian coordinate system, both Az and Alt change during the night, so you need to move in both axes to keep the star in the centre. With an Equatorial (RA motor off), you only have to move it in the RA axis, not the Declination (if it's "polar aligned"). With the RA motor on, you don't have to do any more adjustment (provided you have perfectly smooth RA motion).

I still don't quite understand What exactly "Right Ascension" is. (is it the position of a star in the sky at a given time - if so, where is the "Datum" line to enable the RA system to begin finding that fixed RA position?). Also how and why do "Right Ascension" co-ordinates remain the same for all astronomical objects, when they are continually moving across the sky?

.

The Datum is somewhere in Aries, I think. It's exactly where the ecliptic meets the celestial equator, in March time. That marks a special point on the celestial equator and the star at that point has an RA of precisely zero, along with all the stars on the same celestial great circle that goes through the poles (the same celestial meridian).

The RA/Dec coordinate system is a sky-system. It doesn't care about the rotation of the earth, so coordinates for space objects in that system do not change with a 24 hour period. We are on the rotating earth, on the other hand, and what we see is most naturally described in the Alt/Az coordinate system.

Suppose you went star-gazing on the Space Station. Alt/Az coordinates would be quite useless to you, but RA/Dec are still useful.

[themos]

I have the Wixey Gauge on my Dob but haven't found it that useful....

One of the problems is that it uses decimal degrees e.g. 45.76Deg where as Stellarium used deg,min,sec eg 45.50.45 deg

and converting between the two on the fly is a pain as they obviously change fairly quickly.

I use it to get in the right sort of area then use a wideangle eyepiece to hunt for my target.

I have had a problem with batteries on this gauge especially in the current cold weather - they don't seem to last very long.

Brian.

Well, I was able to find many difficult objects in a very short space of time with it so I am pleased. I put up a report on SGL. I convert from arcminutes to decimal degrees by 0.1 deg = 6 arcminutes. So 45d50m becomes (50/6 = 8ish) 45.8 degrees. I don't have to be too accurate since I see a half or whole degree in the eyepiece. I haven't changed the battery yet. But I do turn it off as soon as I get a reading. It remembers the horizontal setting.

[philsail1]

Thanks for the info on the "Wixey" Brian - having to convert Decimal readings into degree readings is a bit offputting, but -

along comes "Themos" with the conversion calculation!.

What Batteries does the "Wixey" take?

(I think I'm pretty clear on the "RA" and "Dec" front now - the "datum" line being where the "ecliptic" meets the "Celestial Equator").

I'm making all these enquiries - and receiving great amounts of help and good advice from you all, but at the end of the day do you think it might be better for me to simply save up and buy the "GOTO" pgrade for the HEQ5 mount?

Regards,

philsail1

[themos]

Battery: 3.0V CR2032 (spare included)

To answer #2: Yes, but who's in a hurry to reach the end of the day!?

[Vega]

I recently got a wixey, fantastic little thing and very accurate.

Reading up... my rather crude and simple explanation of alt/azm and R.A./Dec goes like this...

alt/azm - 'Earth Stuff' grid

R.A./Dec - 'Sky Stuff' grid

Is there such a thing that exists that can give out readings just like the wixey only in azm where you just stick it on the side of the rocker box and hey presto you get accurate readings ? Or am I living in fairy land ?

Matt

[Astroman]

There is a conversion from alt/az to RA/Dec. It's called the "Hour angle" of the object. You have to know precisely your lat/long and the time of day. The calculation is messy, but there's a JAVA app that does it, here:

http://www.roman-britain.org/astronomy/astro.htm