Collimating a William Optics Star 71

[Pompey Monkey]

It wasn't possible to get the scope apart to inspect the focuser by hand, so I had to use a pair of strap wrenches:

 

[Pompey Monkey]

End result was that the focuser guide assembly did not impress me much: just a geared pinion and three strips of teflon, one of which has a modicum of adjustment via two flat brass screws:

[Pompey Monkey]

There are, however, two plastic grubscrews in the assembly that were not in contact with anything other than the outer tube. I don't have a close up picture of these, sorry

These grubs are very short but it was just about possible to wind them in far enough to push against the drawtube. So that's what I did. The end result was that I could correct a small amount of filed tilt, but that the acorn stars remained.

First (with Bahtinov mask) is the tilted field:

 

[Pompey Monkey]

Next (again with the mask - very useful!) is a much flatter field: 

 

[Pompey Monkey]

And finally, even with the flatter field, I still get the acorn stars: 

 

[Davey-T]

I think the plastic screws are just blanking off the finderscope fixing holes.

Dave

[Pompey Monkey]

So I conclude that there were two issues:

1. Tilted field;

2. Some other issue.

The first is easily fixable, indeed WO and TS fit tilt adjusters as standard on the later models. The second is beyond me. I strongly suspect that there is a clue to the issue in the directions of the diffraction spikes when using the Bahtinov mask, but I have no idea what!

If any of you lot can shed some light on this, then I'll be more than interested to hear from you

[gnomus]

I cannot remember whether you have already commented on this, but how tight are the brass screws on the camera angle adjuster?  Could there be some play there?  Have you tried the shooting-at-the-zenith-shortish-exposure star test?

[Pompey Monkey]

2 minutes ago, Davey-T said:

I think the plastic screws are just blanking off the finderscope fixing holes.

Dave

Afraid not. There are two positions for the finder shoes and these have round-head stainless (or chromed) screw fitted.

[Pompey Monkey]

Just now, gnomus said:

I cannot remember whether you have already commented on this, but how tight are the brass screws on the camera angle adjuster?

There is not one fitted....

[gnomus]

2 minutes ago, Pompey Monkey said:

There is not one fitted....

I guess that rules that out then!

[Pompey Monkey]

6 minutes ago, gnomus said:

I cannot remember whether you have already commented on this, but how tight are the brass screws on the camera angle adjuster?  Could there be some play there?  Have you tried the shooting-at-the-zenith-shortish-exposure star test?

The distorted stars are aiming independent and also move as the camera/tube orientation rotates, so I am forced to conclude that it's "in the system" and not sag

[Davey-T]

15 hours ago, Pompey Monkey said:

Afraid not. There are two positions for the finder shoes and these have round-head stainless (or chromed) screw fitted.

Not  according to WO, see attached.

Dave

[Pompey Monkey]

2 hours ago, Davey-T said:

Not  according to WO, see attached.

Dave

I suppose it could go there: my finder shoe is attached by the two screws immediately behind it. Back to the fiddly tilt adjuster it is then!

But it still doesn't fix the acorn stars

Steve has made another suggestion for a fix which I will try. I think I'm going to have to make an artificial star first: Waiting for clear skies for each test is a right PITA.

[Davey-T]

12 minutes ago, Pompey Monkey said:

I suppose it could go there: my finder shoe is attached by the two screws immediately behind it. Back to the fiddly tilt adjuster it is then!

But it still doesn't fix the acorn stars

Steve has made another suggestion for a fix which I will try. I think I'm going to have to make an artificial star first: Waiting for clear skies for each test is a right PITA.

WO says on no account use those screws to fit the finder but only having one screw to fix it doesn't seem a good idea.

Clear tonight hopefully

Dave

[Sleepy Kiki]

Do those two screws (behind that single finder screw) come in contact with the draw tube at all? I had a finder / guidescope attached using those two screws as well because the spacing was perfect

[Pompey Monkey]

2 hours ago, Sleepy Kiki said:

Do those two screws (behind that single finder screw) come in contact with the draw tube at all? I had a finder / guidescope attached using those two screws as well because the spacing was perfect

You can see the ends of the two screws in the photo above. They don't come near to the draw tube and if they are not for a finder then I'm stumped if I know what they're for. I'm losing confidence in WO by the day....

You can also just about make out the end of the grub screw at the back, in line with the other two.

[Jonk]

Is it possible that the centre of the lenses is perfectly aligned but you have a slightly mishaped lens?

Do the stars all align under different filters when registered , or can you see any CA on the stars visually?

I still think a proper star test will hopefully show you where the error is.

It's clear here so should be the same up where you are.

 

[Pompey Monkey]

22 minutes ago, Jonk said:

Is it possible that the centre of the lenses is perfectly aligned but you have a slightly mishaped lens?

Do the stars all align under different filters when registered , or can you see any CA on the stars visually?

I still think a proper star test will hopefully show you where the error is.

It's clear here so should be the same up where you are.

 

Well, my quick and dirty (no fllats) stack of 12*180s Luminance looks better. It's not perfect, but it's A LOT better.

What do you lot think?

 

[Davey-T]

Yes looks better, zooming in stars look a bit elongated on the bottom left corner.

Did you alter the tilter or just stick it back together ?

Dave

[ianaiken]

My Hubble artificial star just came yesterday so I was out in the OBS testing my TS Imaging Star71 (which is a clone of the WO) because I have similar star issues. I managed to get it near enough, providing I don't change the camera angle it should be better but I need some clear sky to test it. The issues I found were:

1. The scope did need collimation, which was reasonably possible using the two brass screws on top.
2. My Atik 460EXM has CCD Sensor Tilt. This hasn't really shown up before but it more evident I guess due to the scope being f4.9. I think critical focus for this scope is around 50-70 microns depending on wavelength so the slightest of misalignment will cause issues.

I don't know if you've read about the issues of the Iron Cross? This could also cause bad star shapes I guess.

[Pompey Monkey]

34 minutes ago, ianaiken said:

My Hubble artificial star just came yesterday so I was out in the OBS testing my TS Imaging Star71 (which is a clone of the WO) because I have similar star issues. I managed to get it near enough, providing I don't change the camera angle it should be better but I need some clear sky to test it. The issues I found were:

1. The scope did need collimation, which was reasonably possible using the two brass screws on top.
2. My Atik 460EXM has CCD Sensor Tilt. This hasn't really shown up before but it more evident I guess due to the scope being f4.9. I think critical focus for this scope is around 50-70 microns depending on wavelength so the slightest of misalignment will cause issues.

I don't know if you've read about the issues of the Iron Cross? This could also cause bad star shapes I guess.

11 hours ago, Davey-T said:

Yes looks better, zooming in stars look a bit elongated on the bottom left corner.

Did you alter the tilter or just stick it back together ?

Dave

I have read of the iron-cross issue but, funnily enough, it did not manifest itself until my latest attempt at a fix  - you can see it on one side of the stars to the left and above Capella.

I tried the draw-tube adjustment with the brass screws, but this did not fix the issue. Yesterday I removed the dew-shield and objective cover ring. I then loosened the objective retaining ring until there was just some movement in the lenses and gave the scope a gentle shake while upright to re-seat the optics*. I re-assembled and presto! the result was much better stars.

It's still not perfect, but I have not yet attempted to make the field properly flat after that shaking adjustment - Always, always, one adjustment at a time! I'll put the tilt adjuster back in the train tonight and next clear night I'll try to see how good I can get it. I might even get lucky and have a couple of clear hours tonight.

The danger is that I'll never be happy with it because I'll always be on the look out for defective stars, but I'll wait and see.

f4.9 is not too bad - I used the same camera on my Skywatcher 150 pds at f4.5 (with the coma corrector) and focusing was pretty easy, although I did have to tweak the focuser somewhat to eliminate sag.

*As advised by a professional!

[ianaiken]

The Iron Cross doesnt show on mine either, but that's probably due to being out of collimation too!

I think the objective retaining ring on the TS is glued in place. Did you use a special tool to grip it?

[Pompey Monkey]

I don't have a special tool - I improvised by putting two bicycle cone spanners the correct distance apart in a vice* and (carefully!) locating the notches in the ring onto them. It did feel a little bit like it was glued, but it also felt a little too tight after the initial "give".

*Accompanied by appropriate level of smugness for being so clever

[ianaiken]

What a brilliant idea! Of course could be bad if the lens was to be scratched. I may go investigate this method.