Autoguider corrections.

[RobH]

Hello folks.

Just trying to get my polar alignment a bit better.

RA is now as close as I can get it, no drift at all when drift aligning after 8 minutes. Close enough I think.

I still have a little drift in DEC I think.

If my autoguider comands in DEC are normally North, does this mean that my polar axis is too far south.

Am I correct or have I got my axes in a twist?

Cheers

Rob

[roundycat]

Rob, the mantra goes like this:

If the star drifts North you are too far West.

If the star drifts South you are too far East.

ergo, if your guider moves the mount North you need an azimuth adjustment to move your RA axis a bit to the East.

Dennis

PS; don't get it all done and then whack it with the roof when you shut it or you will have to do it again!!

[psjshep]

PS; don't get it all done and then whack it with the roof when you shut it or you will have to do it again!!

Mine is... "Don't walk into the weight shaft, almost knocking yourself out, else you will have to re-do the polar align"

I tell people the bruises are from the wife !

Phil

[RobH]

Not done the roof trick, but the counterweight shaft.....done that trick!!!

So, drift to the south, i.e corrections to the north, applies whatever direction the scope is pointing?

Cheers

Rob

[centroid]

Rob

Star near the Meridian: If Star drifts North then RA set too far west.

If star drifts South, then RA set too far East.

Star near Eastern Horizon, and near Equator: If Star Drifts North, then DEC is set too high.

If Star drifts South, the DEC is set too low.

HTH.

Dave

[roundycat]

in essence you would be using a star near the equator/meridian or Eastern horizon because that gives you the best angle from the pole. The thing you are actually changing is the alignment of the RA axis which is pointing north.

Dennis

[RobH]

Hello folks,

If I select a star on, or very near the meridian, and watch its drift unguided, it stays bisected by my crosshairs for over 8 minutes with a DSI pro at 1200mm FL.

I believe this means my RA is pretty well spot-on. This is also confirmed by WCS.

However, and this is the reason for my query, when I autoguide, I notice that the DEC corrections are generally North.

The scopes altitude adjustment can only be either perfect (unlikely), or too far north or south. As the corrections are North (very small corrections), this is why I was wondering if the mount was pointing too far south. Playing around with another small GEM, and visualising the effects of errors either north or south, It seems to me that an error in the Altitude adjustment would show up in this way.

Of course, it could be an error in the mount too. Or easily an error in my brain!

I've done plenty of drift alignment in the past, so this wasn't the question really.

Visualise a GEM and have a think.....spacial rotation and all that

Cheers

Rob

[psjshep]

Rob,

Have you tweaked the backlash settings yet? - I can't remember...

I found this had quite an effect, as you may imagine, on guiding...

Phil.

[themos]

If it's always North, then your optical axis droops in time.

sorry!

[RobH]

If it's always North, then your optical axis droops in time.

sorry!

Must be an age thing ;-)

Phil, I have messed around with backlash a bit.

I found that anything above 7 or 8 on DEC caused some pretty bad oscillations.

I have it at zero on DEC and 5 on RA at the moment.

What settings are you using?

Rob

[centroid]

Rob

If you are only guiding in one Dec direction, and the balance of the scope is such that weight is always keeping the dec drive gears meshed against the direction of correction, then you don't need to have any Dec backlash correction applied.

Dave

[MartinB]

RA doesn't come into it Rob it's altitude and az.

Rough alignment. Point to southern star on meridian near celestial equator. Star drifts N in EP star is too far east so adjust azimuth. Point to a star in the east, star drifts north axis is too low so increase the alt angle.

Polar misalignment always causes drift in dec not RA.

[psjshep]

Phil, I have messed around with backlash a bit.

I found that anything above 7 or 8 on DEC caused some pretty bad oscillations.

I have it at zero on DEC and 5 on RA at the moment.

What settings are you using?

Rob

Rob...

The meshing in the Dec gears is not as tight as it should be, but I am determined NOT to do any adjustments until I am at home with this beast...

The manual states that values between 20 & 50 are good for visual and even HIGHER possibly for guiding!

My DEC is set at 50 atm (for both +ve & -ve) - the guiding is doing well with those settings. Nipping up the Dec mesh is straight forward enough, but there is no need at the moment. RA is much better - using 15 for both values.

Phil.

[RobH]

RA doesn't come into it Rob it's altitude and az.

Rough alignment. Point to southern star on meridian near celestial equator. Star drifts N in EP star is too far east so adjust azimuth. Point to a star in the east, star drifts north axis is too low so increase the alt angle.

Polar misalignment always causes drift in dec not RA.

Thanks Martin,

My use of 'RA' was confusing, I meant alt/az.

As there is no N/S drift when using a star in the South, my az must be correct. I will drift align again to an E/W star to check alt.

Phil,

I started with my DEC backlash set at 50, and ended up with a guide graph that looked like the sort of siesmograph reading you'd get from an earthquake!

It looks like all of these mounts are set up slightly different out of the box.

Do you have a link for the DEC gear adjustment.

What sort of time can you currently guide for and still get round stars, and at what FL, and what guide program do you use.

Dave, That makes sense, I think I'll weight the OTA slightly to keep it meshed to the north and see how that works.

I hope I can get this sorted, currently the guiding performance isn't a patch on my old fork mounted meade 14!

Cheers

Rob

[MartinB]

Rob, I used to use a Vixen GPDX which had some backlash. It caused problems when calibrating as well as guiding. Backlash compensation was never satisfactory largely because the backlash tended to vary over time (unlikely to be such a problem with your mount). When I did use it the thing that totally messes things up was any over compensation, better to be conservative and under compensate.

Offsetting the dec balance transformed things and more or less made the problems go away. The only problem with offsetting is that it is a lot more of a faff with side by side mounting arrangements.

[psjshep]

Rob,

I have used Astroart & Maxim to guide. I am using an Orion SPX 250mm with fl of 1200mm. I have had best success, obviously, at shorter fl with the 8" Skywatcher (1000mm) which I used to train the PEC (therefore > 8 minutes in total.) With the SPX, I have also managed > 8 mins cumulative (i.e. no guide reset after the worm cycle). The most images so far are 12 x 5 mins - M76 :-

(gradients are due to the Moon, + I have stretched to show stars... this was supposed to be a quick test)

Also, when I find that article, I will post

[RobH]

Thanks Phil.

I'm doing 5 minutes reasonably well on targets near the meridian and the equator, and longer as I get nearer the poles, but I would like to be able to do 10-15 minutes.

I suppose it's that or lots more shorter subs. Some of the discussions on the forum, and UKAI, are saying that this will give the same results. Despite the science, I am yet to be convinced!

Cheers

Rob

[psjshep]

Rob,

I can't find the article I originally found with pictures but a guy on Cloudy Nights, Joe Cipriano, posted the following procedure :-

btw, for the DEC adjustment, you need to remove the RA shaft (take off the counter weight shaft first) by removing the 4 hex bolts on top - you can't get a socket on the 2 bolts holding the motor otherwise.

To gap the RA worm:

1. Remove scope & counterweights.

2. Turn the RA axis horizontal, and tighten the clutches.

3. Using a 5/64" hex key, remove the 4 motor cover screws, and lift the motor cover off.

4. Using an 11mm box-end wrench or shallow socket (don't use an open-end unless you have no choice; you can scratch the finish something silly if you slip), loosen the 2 bolts on the underside of the motor housing; these hold the motor/worm assembly in place, and are the only bolts under there. Don't remove them - you want them just loose enough to be able to move the worm block.

5. Push the worm block against the worm wheel. Make sure you're sliding the worm block, and not just cocking the top forward (easy to do). There's not alot of space in the housing, so you'll need small fingers or some imagination to get this done. If you do angle the top forward, tightening the motor mount bolts will pull the block back flat to the bottom of the housing, pulling the worm away from the worm wheel, and making the backlash problem worse. Also ensure that you're vertically square (put equal pressure on both left and right sides of the worm block); each side of the block butts up against the housing - self-evident once you get the cover off. There will be shims between the worm block and the housing - don't lose them or allow them to shift. Don't start with a lot of pressure against the block; use maybe a couple of pounds of force at first.

6. When you've certain you've got everything square, tighten the 11mm worm block bolts while holding the worm block in place.

7. Power up, and run the RA. Make sure the motor is running without laboring or stalling (believe me - you'll hear it if it does). Lock the Dec clutch, grab the saddle, and check for play (don't use the counterweight shaft - a loose shaft can feel like loads of RA play). There should be almost no backlash.

8. Replace the motor cover, mount a scope up, and test.

9. Be prepared to do this all over again if it isn't right.

The CGE was not built for easy mechanical worm adjustment; Celestron wants you to use the software's anti-backlash control, which IMHO ain't worth the time it took to write the script. This is frustrating, and unfortunately has to be performed seasonally (backlash increases when the temperature drops).

[MartinB]

Thanks Phil.

I'm doing 5 minutes reasonably well on targets near the meridian and the equator, and longer as I get nearer the poles, but I would like to be able to do 10-15 minutes.

I suppose it's that or lots more shorter subs. Some of the discussions on the forum, and UKAI, are saying that this will give the same results. Despite the science, I am yet to be convinced!

Cheers

Rob

You don't lay down big money on a mount and expect to put up with limited exposure times!!

Manually tweaking the backlash is route one to misery. I've been there with a couple of Vixen mounts which are designed to be tweaked, once you start you are never happy and there is nothing worse than over doing it and hearing those finely engineered gears binding! If you don't have play in the mount I would leave my allen key in the tool box and if you do have play I would be on the phone to the supplier.

[psjshep]

You don't lay down big money on a mount and expect to put up with limited exposure times!!

Manually tweaking the backlash is route one to misery. I've been there with a couple of Vixen mounts which are designed to be tweaked, once you start you are never happy and there is nothing worse than over doing it and hearing those finely engineered gears binding! If you don't have play in the mount I would leave my allen key in the tool box and if you do have play I would be on the phone to the supplier.

In the scheme of things, even the CGE is not big money. It's only when you start splashing out for A.P's or Paramounts that you should be expecting very good "out of the box" results - just my opinion.

As for manually tweaking the backlash, as so long as you are referring to mechanically tweaking, I totally agree. I only included the adjustment quote as I had made a ref to it earlier.

You have to be VERY careful that you get it very right else all sorts of other things come into play. Compared with other mounts, in my experience, the CGE has good backlash compensation settings which let you play with positive & negative "taking up of the slack" which should be explored first. I am only just finishing that route & I am happy with the results so far. To date, with guiding, the best I have achieved is an hour in 5 minute subs (one-shot colour + sky pollution = less than 5 min exposures) - The guiding is so good that I need to dither between exposures... not for resolution but so the chip imperfections don't become part of the image!

As Rob said earlier, each mount can be different even for the same models. Sods law?

Phil.

[RobH]

I think I'll leave the mechanical side of things alone for the time being! Sounds like a right can of worms.

I'm going to experiment with weighting the mount slightly differently, and also play with barlows and focal reducers on the guidescope and see what effect that has. At the moment the guidescope has a greater magnification than the imaging scope (meade DSI = 6mm EP) and I may be running into some problems there.

My whole reason for going for the TMB was greater image quality as opposed to the 14 inch SCT, but I realised of course that I would have to increase my sub length to compensate for the loss in aperture. If I can't do this due to mount issues it seems to me I've taken a step backwards to some extent, and I really have no way to afford an AP or Paramount for the forseeable future.

Rob

[MartinB]

You don't lay down big money on a mount and expect to put up with limited exposure times!!

In the scheme of things, even the CGE is not big money. It's only when you start splashing out for A.P's or Paramounts that you should be expecting very good "out of the box" results - just my opinion.

Yes I agree Phil although you can add Taks to the above list. The CGE by all accounts (I've never used one) is a very good mount but like G11s it may well need a tweak or two. My point is it should be possible to do as long exposures as you like with a CGE, albeit with a few tweaks along the lines that you have done.

Maxim also offers excellent backlash compensation. You can use the move command under the guide tab to finely calibrate the adjustments whether you are doing it with the hand set or with Maxim. The trouble with the Vixen mounts was that I found I needed to make the adjustment every session and it is quite time consuming. The CGE will be a lot more stable than the GPDX though.

Don't be downhearted Rob, I'm certain that offsetting the dec balance will solve things. It might sound crude but consider it as an "automatic dec backlash compensation setting".

[psjshep]

Rob,

Don't be disheartened - I am sure you have not taken a step backwards.

Until I started to see how all this new "stuff" fitted together, I did wonder if I had done the right thing.... I was, after all, getting very good results with pulse guiding the EQ6... Just was at (or probably beyond) the weight limit of it.

You imply that you are guiding at a greater fl than imaging? I certainly would not do this; the word is you can get away with up to at least half if not more that half the fl of the imaging setup.

As for weighting, only a small imbalance (east, iirc) is enough for RA... if, like me, the DEC has the worst "slop" then an imbalance either way is ok.

If you are imaging @ 1200mm fl (beginning to lose the plot, sorry) things do work - just need to find the right combination...

Phil.

[psjshep]

Yes I agree Phil although you can add Taks to the above list. The CGE by all accounts (I've never used one) is a very good mount but like G11s it may well need a tweak or two. My point is it should be possible to do as long exposures as you like with a CGE, albeit with a few tweaks along the lines that you have done.

I keep forgetting about Taks...

I agree with your point... as I said, I am getting exposures as long as I like...

Don't be downhearted Rob,

Spooky!

Phil.

[MartinB]