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250PX GOTO help please!


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Hi All,

I have a Skywatcher 250px Flextube GOTO, which I have had for about a year. Recently however I have had issues with the GOTO missing targets, sometimes quite badly. This isn’t a major issue as I do like finding objects rather than being shown them by the GOTO.

I have had pretty good results with the GOTO but recently it has been off by at least half of the (9x50) finderscope. I have recently tightened the ALT axis clutch a little after the tube nosedived whilst imaging the moon with my DSLR. I tightened the Alt axis so it wouldn’t slip with the DSLR mounted, maybe ¼ turn at the most.

My normal setup routine is:

1; Level base with a bubble level on the base (I Have a levelled slab in the middle of the lawn for the dob)

2; Collimate and align finderscope using Polaris

3; Switch on (Scope is powered by a Skywatcher 17AH powertank)

4; Date + time, daylight saving ON

5; Input co-ordinates (which don’t change as I always observe from my back garden)

6; 2 star align (I have been using Polaris and Capella ) After centring Polaris, Capella was pretty much centre of the eyepiece so I assume this is okay?

Last night after this alignment I slewed to M3 and it was out about half the diameter of the finderscope so I slewed back to Polaris, it was slightly off centre but not the same error as M3 was. I realigned the encoder on Polaris and tried again but the error was almost exactly the same.

I feel I’m doing something wrong but I can’t put my finger on it. Maybe the clutch(es) are too tight although it doesn’t feel like it when manually slewing the scope. They certainly are NOT too loose to cause slippage.

Poor choice of alignment stars maybe?

Perhaps I should just let it slew to objects rather than manually pointing it in the rough area then use the GOTO for the last bit?

I haven’t set anything in the backlash settings, could this be the problem?

I hope some more seasoned users of this scope can help me!!

Regards

Greg

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Yes, I wonder what that backlash setting does? I also have terrible "looseness" on the gearing. I can push the scope about a full FoV before encountering any resistance. Its nothing to do with the clutch torque in my case. Even if I tighten it so that it wont move I still have this "play". Utter useless. The saving factor is that the optics are wonderful.

Like you, I like to hunt out the objects, surprising how enjoyable this can be. But even that is difficult since it is not smooth enough and I need to counter the clutch play. In my case even makes it useless as a manual Dob as well!

I am very seriously considering removing the Goto and motors completely. I am going to have a look one last time tonight if there is anything I can do to keep it as a Goto.

On the basis that there wont be anything I can do I am going to contact Optical Vision Limited for a mounting bracket circle that goes onto the scope to mount it (the Goto ones have a different one on the motor side to the non motorised side) and the two mounting bushes.

Steve

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I am giving up with the goto on this particular scope - its pants. Going to turn it into a manual Dob. I need two mounting bearings and a side mounting ear for the OTA to replace the current bit on the motorised side.. On the lookout.... Going to call OVL today.

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ok this is going to sound a bit odd but bear with me... on my 250px it 100% depends on which stars you use and i think its reasonably safe to say with mine that the further apart the 2 stars are the better and more accurate.

i went through 4 or 5 sessions of not being able to align (basically the same problem as you are experiencing) anyway in the end it came down to stars - for mine i always now use either Arcturus OR Vega and then let the scope slew to another star appx 150 - 180 degrees behind you in either direction

if like me you need help identifying stars names, and you have an iphone / pad use something like NightSky to help you identify the bright ones

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Ok will give that a go later, thanks for sharing your experiences. It still does not take away the issue of the slop in the gearing in my example which means usign the scope manually is impossible because of two FoV's slack before you encounter any resistance.

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Hi all,

I did some testing last night and I found that the alignment stars play a huge part in the accuracy of the goto. I had previously been using Polaris and Capella with varying results but last night I used Arcturus in Bootes and Regulus in Leo. After the alignment I went back to Arcturus and it was bang on using the 10mm supplied eyepiece, I then manually moved the scope ( I should also point out that I have centred the alignment stars finished with up and right directions), I went into named objects and it went back to Arcturus, still centred in the 10mm.

I tried various other objects in different parts of the sky and all seemed to be within the 10mm eyepiece. Polaris seemed the most out but was still fairly central

So to conclude I think the clutch friction setting does pay a large part in the accuracy, but not as much as good alignment stars.

However to dampen the mood, when I attempted to view Saturn the scope nosedived again, so as I couldn't be bothered faffing with the clutch again I simply took off the finderscope and it seemed fine. This obviously is not a permanent fix but researching online I came across a good idea of using a 30cm long Magnetic knife holder (a kitchen devil one seems good) attached lengthways to the lower tube using some no more nails double sided tape so the magnetic side is facing away from the tube. Then using a steel weight about 0.6kg I can slide the weight up and down the magnet to achieve balance at lower alitutude observing, I may need to use more/bigger weights to balance my different eyepieces and DLSR.

I emphasise the Kitchen devil as the few I looked at a few in a local store and the ktichen devil one had a solid back face whereas the others had a hollow back so there is more area to stick the double sided tape on the kitchen devil one! (and it was black so matches the OTA)

Can't wait to get home from work now and try the magnet and weights out. I will post my findings here.

Regards

Greg

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well ive put the magnetic knife holder on, looks okay. i put it slightly off centre so the top truss doesnt interfere when the flextube is collapsed. it works really well it seems, 1 x 0.6kg weight prevents the front dropping at low altitude.

i should clarify on the front end i had a raci finder and an es 82 degree 11mm eyepiece in the standard 1.25" holder in the standard focusser. i am confident this will solve my issues with this scope now, but the only way to know for sure is to try it out once the stars come out next.

i have taken some pics but i will add them tomorrow at work. ( using xbox 360 atm)

i will add my findings here when i have tried it out.

regards

Greg.

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IMAG0561

here are the parts I used

IMAG0562

This shows the rear of the magnet knife holder with the double sided tape stuck on

IMAG0563

Here is the finished article complete with a counterweight made at work
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Very ingenious :) Al I did was saw off four inches of broom handle and drill a hole in the tube. Broom handles are a very good fit for certain 1kg and 2.5kg weights for weightraining..... Shown below. I need to paint this black. Also the weights are rubber coated so do not scratch the tube.

I like your method for fine tuning :)

post-16295-0-01139800-1367497815_thumb.j

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