A little dispondent :( Help with RA?

[parallaxerr]

I'm having a little problem with my first scope (Celestron Astromaster 90 EQMD). I though I was getting reasonably competent at polar alignment but I can't figure out the RA ring. With Polris in the centre of the EP, the dec ring was reading 89 deg. Now where should the RA scale be? I checked on stellarium but I'm not sure I'm using the right figures. Do I take readings from RA/DE (J2000), RA/DE (of date) or Hour angle/DE? DE appears to be constant across all three but I must be doing something wrong with RA because when I swung the scope to point at Betelgeuse I was nowhere near...

Another problem I was having was tracking with the Celestron DC motor. Stars were racing through the EP untill I set the motor to the southern hemisphere setting!?!?! It's as if the motor is wired up in reverse!...

Finally I attempted to put what I thought was Saturn in the EP but I must've been mistaken. It didn't get any bigger regardless of magnification, like a star. At what mag should I start to see the shape of the rings?...

Thanks for reading and hopefully someone can shed some light on my issues. I'm not giving up yet!

P.S. Anyone here a member of Cardiff Astronomical Society? I'm thinking of going along to their observing session on Friday night as it's only a few miles from my home

[steppenwolf]

Certainly don't give up, your approach is just right so hopefully you will be able to resolve the issues.

I tend to take my readings on the very rare occasions when I do so from J2000 but I am not really sure why you are using the setting circles on Polaris? For setting your circles, it would be much more accurate to use a known star nowhere near Polaris as Polaris is right by the north celestial pole (of course) so setting accurately to it is very difficult as it is so subjective. You would, however, use Polaris to align a Polarscope if your mount has one.

The motor reversal is a strange one and I would cross reference this with your supplier although you do seem to have resolved the issue anyway, it is simply that it shouldn't work that way!

Saturn will resolve into something that is clearly not just a star at quite low magnification - I would say 25 x would be more than enough for this to become very apparent so I guess you must have missed it!

If I have misunderstood your use of Polaris, my apologies.

[parallaxerr]

Steppenwolf,

Thanks for your reply. I figured setting the circles on polaris wouldn't be the best, just went for it as I was aligned there at the time. I will try a star out to the East or West next time to get best accuracy. I'll have to experiment with which readings in Stellarium work best, I just wasn't feeling it last night I guess...

I have a spare motor so will compare rotation tonight. Being made in China it wouldn't suprise me if there was some bad wiring in there!...

I must definitely have missed Saturn then, I went upto 200X and nothing, that's a relief in some ways. Will have to try again but I'm thinking I need to travel to a darker site, my back yard has too much light pollution, pesky street lamps!

[steppenwolf]

I will try a star out to the East or West next time to get best accuracy.

That would be my advice.

I must definitely have missed Saturn then, I went upto 200X and nothing, that's a relief in some ways.

Yup, I agree - better luck next time

[Iris]

Setting circles on consumer scopes are notoriously tricky it seems and many folks even hereabouts will probably tell you that they are pretty much cosmetic and to go with star hopping instead.

It's true that they have generally low precision so they, at best are only going to get you near to your object of desire.

I decided to have a play with the circles on my CG5 - just for fun really to see what I could get - I've only used them a couple of times and had mixed results really. At best with bright stars I was able to get my target just in the edge of the FOV of my finder scope a couple of times and managed to stumble across a couple of the Auriga clusters (M36-M38) another - but they are quite big really. What I found was that the DEC circle on my mount wasn't too bad and seemed to be quite close to the DEC co-ords I took from Stellarium for my target but that the RA circle was much poorer - which is no surprise given the precision of the circles.

On the CG5 the DEC is marked in divisions of 2 degrees with a vernier to give 0.25 Deg. The RA is marked in 10 minute divisions with a Vernier to give 1 minute.

One thing that I also noticed that the verniers, especially the RA one, is pretty tricky to read off at night in red light!

And I am pretty sure that I was not able to read them sufficiently well to get to the 0.25/1 'capability'

After I tried and pretty much failed on my first attempt, but noticing at least that the DEC circle wasnt too far off I noticed another post in here by Astro Baby who made the clever suggestion of setting the DEC as accurately as possible and then swinging the scope in RA until one magically sees the target which ought to swing through the FOV. I refined the idea and used the setting circle to position the scope just to one side of the desired co-ordinates and then slewed in RA - for reasosn that hopefully would be obvious. That didn't actually turn out to be discernably better than my previous attempts, if I am honest.

But further even if one is reading the circles with a high level of accuracy (lets for argument's sake say perfection) that would only find your target perfectly if your mount was also perfectly polar aligned.

So there are now two components of four measurements to worry about :

1) How precise are your measuring devices i.e the Setting circles, the polar alignment scope or whatever other means you use to polar align the mount.

2) How accurately you can use those devices to arrive at a measurement.

Having got this far I realise that it was pretty good going to manage to get a target star into the edge of my finder to be honest when one considers that I only roughly polar aligned the mount by looking through the boresight of the mount (no polar scope) and could probably only read the DEC circle with an accuracy of +/- 0.5 deg and the RA circle to perhaps +/- 2 Deg.

I wil persevere though just to see how well I can put the money on - or rather near -the button.

Don't give up but I'd suggest not to expect fantastic things either!

edit: fix error in units for RA graduations - should have said minutes rather than degrees

[parallaxerr]

Iris hi,

thanks for your input. I think you're right, I'm expecting too much from a starter scope. The more I look at the rings the more I realise that they are never going to be particularly acurate. I'm starting to realise now that if they point me in the general direction then that will suffice and I can star hop to my target. To be honest I think the scope is only really going to be any good for the moon and low mag images of Saturn maybe but we'll see what happens when i get to a dark site on a good night...

I'm already contemplating moving up to a larger scope, maybe a Meade SC. I quite like the look of the ETX125 but I feel like I'm cheating and perhaps not learning by using a goto scope. I like the fact that once I'm aligned I can track objects properly along the equatorial axis. I must say though that already I can see me going down the route of astrophotography so the less time spent aligning the better I suppose. On that note, do the Meades track by motoring both axis or is a wedge needed?

[Iris]

The ETXs will track in both alt and az but that isnt terribly good for astrophotography except for short exposure moon and planets etc.

The field tripod that comes with the 125 has a cheapo built in tilt plate but I dont know if it is any good. I had one but it only saw action once before the focus spindle detached from the mirror and I uptraded to the Celestron I have now.

[parallaxerr]

Yeah I see the wedge is for long exposure photography to prevent image rotation. Just had a look at the celestron stuff, I do like the look of Nextar SC's. The 5" with neximage remote control software would be nice so I could do all my imaging from indoors! This looks like a nice packege Celestron Nexstar 5 SE CCD Nex Image PC Package although a wedge to suit is another £200:o.

Which Celestron did you go for Iris?

[parallaxerr]

Just found this ...

Celestial-Equatorial (RA/Dec) Demonstrator

I think it's answered my question in one!

It appears the hour angle in stellarium must be used with an equatorial mount. At 0 (Zero) hours a celestial object is on the prime meridian (GMT) formed by the vernal equinox (the point at which the sun crosses the celestial equator in March), which, luckily for us in the UK is due South (give or take a few minutes depending on your location).

So once polar aligned, it's possible to set the RA ring to zero with the scope level and pointing due south. I'm about 5 minutes off the prime meridian so with the OTA pointing due south I simply set the RA ring to 5 mins. I tried this tonight and sure enough when pointing West the RA read 6hrs, N read 12hrs and so on (Easy numbers to remember in the UK ). This of course can be refined by locating a celestial object and fine tuning the RA ring in accordance with stellarium....