Single tracking motor - Dec drifting

[prbaxter]

I am having loads of success with my new RA single axis motor drive however I'm still experiencing a little drift in the DEC plane. Any ideas what would be causing this?

I aligned my scope with North last night and also levelled it off with a spirit level. Could it be that it is ever so slightly misaligned or is there something else I need to do?

[brantuk]

Did you polar align it or just magnetic north align? Magnetic is not as accurate as PA, but would be fine for observing only (not imaging) with odd tweaks in Dec

[prbaxter]

Magnetic, because I wanted to get the mount aligned just before the sun went down so I could get the moon at its highest point just after sunset. Hence, I couldn't make out the pole star.

[JamesF]

I think it will be a slight misalignment. I get the same thing and have assumed that the accuracy of the polar scope and the time/date scales just isn't good enough to get perfect alignment, not to mention that it's actually not that easy to get Polaris perfectly centred in the marker on the polar scope whilst shining a red torch down the end to make the reticle visible in the first place.

If you want no DEC drift at all I think you'll have to get close using polar alignment and then drift align to get it spot on.

James

[prbaxter]

...time/date scales...

am I missing something here?

[MikeP]

How did you set the altitude? That also needs to be correct to avoid drift. The scales on the mounts are not very accurate but close enough for observing.

Mike

[prbaxter]

lost me there? I've set something to about 54 degrees for my location when I first got the scope. Thats all I have set. May this explain what those other little dials and levers are for on the mount?

[The Stoat]

I also have an EQ3-2 mount and by far the best option is to purchase a Polar Scope if you don't have one allready. You should be able to get one for about £30.00.

[JamesF]

am I missing something here?

Let's hope I'm doing mine right now I'll assume that you have already correctly aligned the polar scope with the RA axis. I found that a bit of a pig on my (old) EQ3-2, but resolved the problem with the aid of a tap washer. AstroBaby's website covers this for the EQ5/6 mounts, but my EQ3-2 is a bit more primitive. If you haven't done it yet and have problems then I may be able to help.

Around the eyepiece end of the polar scope should be some scales, one for day/month and one for time (GMT). There's also a longitude adjustment scale, too. There should be a mark on the polar scope index ring that in the UK you'd align with zero on the longitude adjustment scale.

I have my polar scope set up so that the position of the Polaris alignment ring is at the bottom at midnight on Nov 1 (IIRC -- I'm doing this from memory and may be wrong). To set it up for any given night I remove the scope, release the RA clutch and spin the mount around the RA axis (in my case I have to turn it different ways for different times/dates because the mount obstructs itself at one point) so the current GMT time on the time ring lines up with the date on the date ring and lock the clutch off. Then do the alt/az alignment on Polaris. Once that's sorted, return the mount to its "normal" position and remount the telescope.

AstroBaby has a detailed description of polar alignment on her website, but I took my method from a posting by someone else on SGL who said (and I probably paraphrase poorly) "that all works, but here's a simpler way to do it". Unfortunately I can't recall who that was. He's welcome to stand up and take a bow if he recognises himself

If you need more detail let me know and I'll see what I can do. Assuming I'm doing all this correctly in the first place, of course...

James

[MikeP]

"that all works, but here's a simpler way to do it"

I'm not sure who you are referring to but I think the method you refer to is included in the following. It was written by Kaptain Klevtsov who does not post often these days and it refers to HEQ5's and EQ6's but is applicable to anything with a polar scope.

Mike

Polar alignment

In order to get good tracking of the stars in the telescope, it is vital that the axis of the right ascension movement is lined up with the earth's axis so that the movement of the mount can accurately counteract the rotation of the earth. On the HEQ5 and EQ6 this is easily accomplished to a remarkably high accuracy. The first step is to locate Polaris, the pole star as the alignment is based on using the polarscope to view this and align the mount accordingly. Place the tripod so that it will be the correct height for use with the counterweight bar in the direction of Polaris. Adjust the angle of the top of the mount so that the angle corresponds to your viewing latitude. Do not try to adjust this unless the 'scope and counterweights are not yet mounted, or the 'scope is fully balanced. There is a scale on the side of the mount to assist in this, although it is not necessary to know it accurately. The method is to locate Polaris in the polarscope which is fitted through the centre of the right ascension axis. The counterweight bar has to be out as it gets in the way otherwise and the declination axis also has to be oriented correctly. Looking down the top cover will quickly show you which way it should be turned. Once Polaris is located in the polarscope, you should be able to see markings which correspond to the pattern of bright stars around Polaris. The right ascension axis should be rotated so that these stars are in the same orientation in the sky and in the polarscope. If the markings are not visible, shine a torch across the top of the polarscope to illuminate them. Once the right ascension is turned so that the markings match the sky, use the altitude and azimuth adjustments to centre polaris in the markings. The position for Polaris is inside a tiny circle which is on the edge of a larger circle. The torch may need to be used again to find these. Once Polaris is inside the small circle, the mount is polar aligned and ready to go. Move the right ascension and declination axes back to where they should be to mount the 'scope, tighten the axis lock knobs, fit the counterweights and mount the 'scope. After balancing the 'scope you are all set.

[JamesF]

I'm not sure who you are referring to but I think the method you refer to is included in the following. It was written by Kaptain Klevtsov who does not post often these days and it refers to HEQ5's and EQ6's but is applicable to anything with a polar scope.

Mike

I think it may actually have been great_bear, though I've not immediately been able to find the post.

The polar scope in my EQ3-2 doesn't have the stars marked on it, just cross-hairs, a ring for Polaris's track around the CNP and a ring for the actual position of Polaris. That means it has to be correctly adjusted for the transit time of Polaris before you can even start.

James

[JamesF]

lost me there? I've set something to about 54 degrees for my location when I first got the scope. Thats all I have set. May this explain what those other little dials and levers are for on the mount?

If you have a polar scope then you can certainly get it a lot better.

Basically, pointing the scope north and setting the correct latitude will get you reasonably close, but probably not close enough that you'll not need to adjust the DEC alignment every so often. Better accuracy can be had if you have a polar scope and you use that to align the scope with the celestial north pole using Polaris (which actually appears to move around the CNP, but polar scopes allow you to correct for that). For me, that's sufficient for planetary imaging. I still have to correct the DEC axis occasionally, but probably only once every ten to fifteen minutes. Some polar scopes make life much easier than others. Mine is a fairly basic one that makes me do most of the work.

If you want perfect alignment, drift alignment is probably required, but I think that might be trying to run before you're quite steady on your own two feet

James

[pook]

With my polarscope, once calibrated (and they do need calibrating before use) I can get over 1 minute exposures without even being particularly **** about polar alignment. I just use Polar Finder to give me transit times, and roughly set it to where it is on the screen.

Before I got the polar scope, even 30 second exposures were a mess.

[edit] I wasn't swearing, i used the word "a n a l" in a perfectly acceptable context.

[glowjet]

The other day I came across the instructions for polar aligning on one of the American mounts, the only difference being the polar scope rotates in the holder, there is no setting of date or time scales, the process is very simple :- set the scope up so the polar axis and your telescope is pointing North, your mount should be leveled, loosen your Dec clutch and set the scope to the Latitude for your location, with the Alt or Az adjusting screws bring Polaris into the center of the field of view of your finder scope, I hope we are all singing from the same Hymn sheet so far, now comes the best bit, your polar scopes show the offset for Polaris as a little circle on the edge of a slightly larger circle in your field of view, other types may be different but should show the offset from the pole star, if you now look at the orientation of Ursa Major and Cassiopeia in the night sky just turn your polar scope so the view mimics the same orientation, with adjustment of your Alt and Az screws slot Polaris into the little circle, job done, if you have Jason Dale`s polar finder program you can look up your observing time then set your polar offset circle to match that which is given, all this is OK if you can rotate your polar scope in the holder of the mount, the series 5 mounts have a screw in type, so long as the optical axis has been adjusted so that the cross hair remains stationary on a fixed point some good distance away while the scope is revolved in some sort of fixed V support all should be well, if some lithium grease is used on the threads the movement should be quite smooth once the scope is threaded home, allowing at least one back revolution without a lot of play which is all that is needed. I have not tried this yet, but I see no reason that it should not work, both the threaded portion of the polar axis port in the mount and the outside thread of the scope should be reasonably accurate having the same parallel axis, if you have set the etched diagram of the polar scope already in the mount, following the recommended procedures, then it should be spinning on center.

Having said all this, accuracy will only be minimal, excellent for viewing and some short exp imaging/photographic work., but for longer exposures drift aligning, and the ultimate, a separate guiding set up, I am sure the ingenuity of some to make their polar scope revolve will now spring to light

[ronin]

You said you aligned it magnetically, if so that is the problem.

magnetic North is something like 4 or 6 degrees off of True North.

So yes it will drift off at a fair rate.

If you have set the scope to 54 deg I assume that you live somewhere like Bridlington-Ripon-Lancaster, basically a line a little north of York.

[Astro Imp]

Hi,

I would suggest you have a look at Astro- Baby

You will see that Astro-Baby gives a quick method suitable for observation only and a more accurate method.

FYI I have used the first method very successfully and suffer from very little drift when observing.

HTH

[prbaxter]

Lots of information there for me to read over the weekend.

Thanks poeple!