Clement Focuser

[Stub Mandrel]

3 minutes ago, Horwig said:

No it's for this one, 40 cm f3.5:

Bet you'll get really upset if your neighbours install a security light :-)

[Horwig]

Well, here it all is under test for backlash:

And the backlash WITHOUT software compensation was 10 microns, but I'm sure with a bit more fettling it could be even lower, FLI quote zero backlash for the Atlas in a similar test after all.

[Stub Mandrel]

If you feel you need to enclose it in a bellows, a helpful starting point might be here:

http://www.arceurotrade.co.uk/Catalogue/Machine-Spares/Machine-Bellows

[Horwig]

8 minutes ago, Stub Mandrel said:

If you feel you need to enclose it in a bellows, a helpful starting point might be here:

For imaging, the Wynne corrector protrudes through the focuser into the OTA, but for visual you are right, it would be useful.

[Stub Mandrel]

2 hours ago, Horwig said:

For imaging, the Wynne corrector protrudes through the focuser into the OTA, but for visual you are right, it would be useful.

I was thinking you might not want one anyway, as it's an open-frame scope inside an observatory so sheltered from stray light sources..

[Horwig]

3 hours ago, Stub Mandrel said:

inside an observatory so sheltered from stray light sources..

You'd be surprised! We are mostly surrounded by fields, the nearest street light is a half mile away. To the North, two fields away are farm holiday cottages, quite a few (urban?) visitors there can't cope with the darkness and insist on leaving ALL the outside lights on ALL NIGHT!!!!!

On a positive note, fiddling with belt tensions has brought the backlash down to about half, so roughly 5 microns. What is interesting is that there's more stiction than backlash (?), the motor does not respond to commands of under 4 steps or so (that's 8 microns).

I use a Lightphase controller which has backlash compensation, so 5 microns, if constant should not be a problem, trouble is a Newtonian imaging train can be either side of horizontal, that's a test for another day as I have not fixed the Wynne corrector to the focuser yet.

 

Huw

[Horwig]

Right, now my lathe has been sorted I've finished the metalwork and assembled the focuser, the camera should be orthogonal within about 20 microns and the backlash is under control, not zero, but small enough I hope.

Next stage will be mounting it on the scope and re colimating.

Would anybody be prepared to sell me some clear sky for a first light test?

Huw

[Horwig]

Eventually it all came together

1.67 pixel HFR at a scale of 1.28 "per pixel is I think 2.1", which for what is really first light I'm quite happy with, more work on colimation to follow.

One thing I've been trying to remove for such a long time is an annoying residual astigmatism which I hoped the focuser would cure, it did not.

It eventually came down to the safety strap araldited to the back of the secondary:

removed that, and all came good.

One thing to note in that picture is the excess of silicone everywhere, this was taken in 2012 before the scope was fist assembled, it's since been corrected.

It's only taken me seven years to get to here!!!!!!!!!!!!!!!!!

Huw

 

[Davey-T]

Any updates Huw ?

Dave

[Horwig]

5 hours ago, Davey-T said:

Any updates Huw ?

Dave

Well yes and no. Progress on the setup has been either Pandora's box, or a can of worms, depends on your point of view.

Firstly, trying to achieve better PA I've been forced to address some flex in the polar wedge of the mount, this is how it used to look during the mount's construction:

The bottom fixing of the wedge consists of two hefty steel hinges, the trouble with them is that when loaded with the fork and scope there is a significant upwards force on them, and some play as well, they are not precision parts, so the whole mount can sway east-west when the mount is swung, when measured with a dial gauge, it was close to a mm in total, the length of the wedge is about 400mm, so you can see the problem with doing PA.

The mount was strengthened with three 6.3mm ali plates, two triangles either side, and an end plate to hold everything rigid:

This has been a great success, I now measure about 10 microns deflection when swinging the OTA from West to East, still not great, but getting better. PA is now within about 1 minute of arc or so, and I'm tempted to leave it at that.

I've also had a go at eliminating backlash in the drives, there was considerable backlash in the Pittman motor gearboxes, but there's a routine in Sitech to eliminate it, I started with a croshair eyepiece and an electricity pole in the next field, but the resulting correction was too severe, causing the OTA to wobble on direction changes, so back to the dial gauge, and watching that whilst reducing the anti backlash gave a good compromise I hope.

Collimation now shows nice round stars when the OTA is below about 60 degree elevation, but for some reason I've still got strange star shapes when pointing at the zenith. More thinking required.

Don't think I will ever get this finished.

 

Huw

[Horwig]

Oh, by the way, I forgot to mention, the new focuser is SUPERB, I get constant textbook V curves!!!!! At least that was a success

 

Huw