A Really Dumb Question

[Screwdriverone]

Hi All,

Please help me.

For the life of me I cannot figure out what I am doing wrong with my RA setting circle axis and I have got myself into a horrible confidence mess with this now.

This is what happens on my GEM EQ2.

1) I align my scope to the SCP. (easy)
2) Scope's Dec setting is showing 90 degrees.
3) Swing the scope West to line up Sirius (for example) in the eyepiece and then using Astronomy 2008 details on page 137, set the RA setting circle (bottom scale for Southern Hemisphere) to value 06 45.
4) I then swing my scope to say, Acrux in Crux and check the RA scale to see if its reading 12 26 etc. NO IT ISNT! Last night when I checked this Acrux was showing up as 00 15???

Do I need a brain transplant? or just another kick in the head to figure this out?

I have tried setting off Betelgeuse, Canopus, Spica, Aldebaran, Rigel and Rigil Kent and NOTHING seems to work. Whatever I set the circle to (and yes, I have tried the other scale too) seems right for the object targeted, but then I can never swing the scope to something else and look at the scale and go....Bam, right RA number as its not even CLOSE to what it should read!

Ummm, Help???? :help:

Chris

[Jeff]

Hi Chris,

I'm a Dob user with digital setting circles, but think I have a basic idea how to use manual setting circles on an Equatorial mount. Here goes...

1. You need to remember that RA is a static measure for the position of an astronomical object (ie. from Greenwich london, at the point in the year when the March Equinox occurs). The is a good description on Wikipedia.
http://en.wikipedia.org/wiki/Right_ascension

2. Sounds like you've done al the tricky stuff already. Only think you need to do is twirl your RA circle to match the "Star Atlas" RA when you line up on a known object. As an example, point at Sirius then adjust your RA circle to read 6 hrs, 45 min.

3. Of course, you may need to re-adjust on Sirius a few times through the night if you want to locate some more distant fuzzies using RA/DEC coordinates

Hope this helps.

[omnivorr]

you are leaving the polar axis pointed at the SCP aren'tya?? that "swing" it to this n "swing" it to that, kinda sounds like ya not... sorry ifn my answer "dumber" dan da question was.
Cheers
Russ

[Screwdriverone]

Hi Jeff and Russ,

Yes, I am leaving the polar axis in line with the SCP (NOT moving the tripod legs in any way) , loosening the screws to slew the scope in both RA and DEC axis to get the star lined up, tighten screws to enable slow motion controls, fine tune star until its in the middle of the eyepiece, then setting the RA axis to the star's coordinates.

What happens then is that if I turn the scope to a new target such as Regulus and then check the RA axis, - it's not at the right RA hh mm ss.? I have made sure the setting circle is tight before moving the scope off Sirius first.

Am I right to assume that if I set the RA axis correctly and then replace my scope in the EXACT same place and direction (lined up to SCP) each time I view, then I should never have to change the RA setting circle EVER? Also, is there a star that is at 0 hms Right Ascension so I can check it that way, or is this just a stupid idea?

Or is there something I just dont get?

Chris :mad2:

[bizarro]

Hi Chris,

Been a while since I used setting circles (currently spoilt with a goto scope) but I recall two scales for RA on mine. One was for the northern hemisphere and one for the southern.

Since RA is fixed to the objects in the sky, as the sky rotates you will have to recalibrate the RA circle before hunting for some DSO but it shouldn't be out that much when you move it immediately. I'd say you're looking at the northern hemisphere scale. Sorry - I can't remember which one was which on my mount (inner or outer) but I used to do just as you're doing until I figured out which one was which each time I used it.

If you've only got one set of numbers then it's probably northern hemisphere. I'll have to defer to someone cleverer to come up with a hack if that's the case.

Cheers,
Greg

[Jeff]

Hi Chris,

My understanding is that (after initial setting), for every hour that passes you need to either:

1. Shift your RA circle by another hour to the West ... or

2. Subtract an hour from your target RA value for an object (eg. Sirius will then be at 5 hrs, 45 min if you leave your RA circle unchanged)

Best way to visualise is that:
- RA coordinate is fixed for any given object (eg. Sirius)
- if your RA circle is to remain accurate, you need to adjust it as time passes and the earth rotates

Hope this helps.

[Screwdriverone]

Hi Greg and Jeff,

Ahhh, this may be the problem that is getting me confused to the point of zero confidence namely that I set the RA circle at the beginning of the session and then it gradually gets whackier and whackier as the night goes on, usually I only start to reference the RA circle again about 2-3 hours in after I have checked out the usual bright objects such as Eta Carinae and M42 etc.

I also checked out a forum thread somewhere that suggests that you set RA circle off a close star where you need to be, and then RA / DEC hop to the DSO by adding or subtracting the targets coordinates from the reference point, this way, the margin for error is reduced and my brain spasms can leave me alone once and for all.

That said, I will have to work out the difference in coords to hop to the target, but at least I think I have found out where the problem is, finally, its in my perception of the truth, or what is actual, based on my scrambled idea of reality. Mulder? are you there Mulder?

Thanks guys..;)

Chris

[rider]

I have never realy understood why the RA setting circles on mounts are not "clockwork" based so that once set, they rotate with the time.

there's a project for someone..

Rider

[Astro78]

Ah yes, recall the stressful week 9 months ago myself in your position.

If you use software (I use Voyager 4, which is free and highly recommended) you can easily just freeze time. That way everything matches up with your mounts RA axis. Then just revert back to live every couple of hours and set your
mounts RA back to another star you know of.

Personally I'm glad not be able to afford a goto, otherwise would not have learnt
the basics.....or so i keep trying to tell myself!

[Jen]

:hi:Chris This is not a dumb question at all i need to know the same thing, at the moment all i do is pick up the whole scope and turn it around to where i wanna go :scared3: (please dont yell at me) :whistle:and it is getting kinda heavy after a few cans in the middle of the nite i wouldnt know where to start using all that RA stuff :tasdevil:
:shrug:

[AstralTraveller]

Chris,

If you move straight from Sirius to Acrux the reading should be pretty close to right; the stars don't move far in 5 minutes. {As an aside to 'rider', on the scope I used to use the pointer moved with the rotation of the RA shaft. That way it was always right once you set it up at the beginning of the night. Don't modern mounts have some equivalent system?}

I note that when you move from Sirius to Acrux you travel 12 26 - 6 45 = 5h 41'. Now 00 15 - 6 45 = -6 30. So it may be that you have some scale a**e about. However I still can't explain the 00 49 discrepancy. Perhaps something grips or slips when it isn't supposed to.

You do have it aligned pretty right don't you? You can follow starts for 5-10 minutes just by moving the RA shaft?

If all else fails you can join us trogolites and just learn to star-hop :thumbsup:.

[Screwdriverone]

Hi Astral Traveller,

Yes, I do have a pointer that travels around with the RA circle, I have attached a photo of the mount showing the two markers that I am using. The R/A mark in white is what I am setting the circle to and it stays fixed while the setting circle rotates past it to the next location and that is what I am reading it off.

While taking the pictures, I have noticed that the difference in hh mm ss between the silver pointer in photo 3 and the R/A marker in photo 2 is 22 - 15 10 = 6 hours 50 mins?? could this be the phantom 49 mins you mentioned???

Am I doing this all wrong? should I be setting the R/A circle from the white marker dial or spinning the R/A marker to line up with the metal pointer? Or is the metal pointer deliberately supposed to be 90 degrees (or 6 hours) away from where the R/A mark should be?

I have locked down the allen keys holding down the outer ring that has the R/A mark on it and this can move by about +/- 1 hour so perhaps this is supposed to be lined up at 6 hours west of the silver pointer?

Sorry, this has now got me all confused all over again, can someone please put me out of my misery?

NB I am reading the scales at the bottom of the RA ring as per the instructions which said the Southern Hemisphere is the bottom.

Chris

[Jen]

Hey Chris, when you have worked it out let me know ok :thumbsup:
:stupid:

[AstralTraveller]

Chris,

Sorry to have gone quiet on this, I hadn't forgotten you. I just got busy and then got the flu. So today I was too sick for work but well enough to pull out my EQ5 and have a look see. Now I'm confused:ashamed:. I'm also very annoyed that there are no instructions with the scope nor on the Celestron web site :mad2:

On mine there are two independent scale rings. The main one has two scales running in the opposite directions (for N and S hemisphere) that go from 0 to 24hrs. The second one goes from 0 to 12 and then 0-12 a second time (ie has two lots of 12hrs) and I'm buggered if I can see what that is about.

The main ring works for pointing in that if a set it to 0 and rotate the RA shaft to the east the reading on the lower scale increases. However when I let it track the reading also increases!! The RA of an object does not vary and so the reading should remain constant when tracking. There is a small locking screw that didn't seem to do anything useful, but now I have a theory. Perhaps I'm meant to lock it when I engage the RA lock, so that the ring doesn't move as the RA shaft turns. Then one unlocks it before moving in RA. That's clunky - to say the least!! I'll test the theory when I have the chance but it won't be tonight.

Meanwhile, have you had any success? I do want to get this sorted so anything you've found would be great :thumbsup:.

cheers,
Dave

[Screwdriverone]

Ok, I think I may have figured out this thing and what I am doing wrong.

1) I am reading the wrong part of the RA circle. I should be reading the "inner" circle of numbers not the outer one.
2) When I go and set the RA circle on a star and then immediately point to another star, I am pushing the scope in the wrong direction. That is, I am setting the RA circle on Sirius in the western sky, then swinging the scope LEFT to point it back at Acrux and checking the numbers which are then wrong. I should be rotating the scope "clockwise" when looking down on it (so that the open end points west, then at the ground then east to get back around to the eastern sky).
3) When I do set the scope at the beginning of the night (say at Regulus), I engage the RA motor (but I should then KEEP the RA motor ON and use the controller to fine tune the star in RA) What I am actually doing is setting the RA circle, swinging around to the other side of the sky and then disengaging the RA motor while I line everything up and more often than not, forgetting to re-engage the drive again. When I mean other side of the sky, think of a tennis ball cut in half from North to South with the Western and Eastern halves as a result.)

What happens then is there is little spurts of NO synchronisation with the earth's rotation that all add up to more and more errors as the night goes on and I couldnt figure out where these were being induced.

In a nutshell, I CAN set the RA Circle on a known star at the beginning of the night and engage the RA drive motor to keep the RA circle in tune with the earth (and therefore sky), but when moving the scope, ONLY rotate it in a clockwise direction on the RA circle and all should be fine.

IF I want to swap sides between western and eastern "halves" I can disengage the drive, set the RA circle again (as a NEW starting/setting point) off a star in that half of the sky and then re-engage the drive motor. Undoing the bolts and making large slewing movements with the scope in RA will NOT affect the RA motor engagement of the drive as it only affects the slow motion RA cable for fine adjustment, and I can do that with the motor I have.

Phew! - Finally, the marbles all stopped rolling around in my head and all lined up and I had a spark of understanding. I hope this makes sense to Jen and anybody else who may be interested in this problem.

If not, please post a reply and I can see if I can summarise this more succinctly for you. (that means clearer and shorter) :)

Cheers

Chris

[Kokatha man]

Screwie, I just don't believe that bit in brackets - it's like saying the Pope is Jewish!

Gunter wanted to add his 2 deutschemarks worth; but he's too busy breaking in a new pair of lederhosen. (he puts them on wearing nothing else, stands under the shower - cold - for 10 minutes, and then jogs around the block constantly between the hours of 8pm and 2am. By then they're dry and with a quick going over with a wire-wheel in his drill chuck whilst still wearing them, the suede surface comes back up as good as new.)

Cheers, Darryl.

[ausastronomer]

Now I remember why I threw my Equatorial mount in the lake 30 + years ago :)

Cheers,
John B

[Ian Robinson]

Good setting circles (analog style) have a vernier scale.

Takes a little while to get your head around looking for a lining up of the outer and inner graduations to get the precision RA or DEC measurement.

Some setting circles on cheap and nasty GEMS are useless except as a rough guide that you might pointing the scope in the right general direction.

[renormalised]

Glad you've managed to clear this one up, Screwie:)

Hey, if you thought this was a dumb question, how about this. Some time ago at a star party, I heard someone from the public who was having a look through a SC scope say this..."Which end do I look through'!!!!:eyepop::screwy::rofl:

[Screwdriverone]

Yes well, Darryl has got me pegged. I couldnt be brief if I tried.

Hang on, I just was! :)

Chris

[Jen]

:screwy::screwy::screwy::screwy: ok Chris now u made me all dizzy and stuff :lol:
when i read things (like your posts:P) it goes in one ear and out the other hahahaa so it looks like i need to go outside and spend a few hours trying to this this RA stuff :screwy::sadeyes: i need to be shown how to do it i think instead of reading about it :help:
i still think that picking up my scope and turning it around anyway i like and pointing up in the sky is much easier :scared3: even though it is bloody heavy :thumbsup: thanks Chris i will give it a try though cheers
(keep it clockwise yeah :D)