Focus problem

[Jm1973]

Hi everyone. I have a skywatcher 200p with the standard focuser. I have an issue where I stick my dslr on, take photos while slowly adjusting the focus wheel until it is in focus. Then when I tighten the lock screw, to lock the focus wheel, it knocks the focus off a bit. And I have to untighten the lock screw and start again. It's very hit and miss and it can take me half the evening to get focus locked, and sometimes I can;t do it at all.

 

Is there anything I can do to adjmust it so that when I tighten the lock screw, it doesn't affect the current focus?

 

Thanks!

[Alan64]

These telescopes, and most all others, are made in China.  You get only just so much finish and precision as a result.  If it were before me, I would remove the focusser, disassemble it, and correct where lacking, if possible.  Failing that, I would replace the focusser with one of better quality.

Oh, incidentally, a focusser for imaging usually requires more precision, compared to that for visual-use with eyepieces.  The 200P will have a visual focusser, as the 200P-DS is configured overall, the focusser included, for imaging primarily.  But that's not to say its focusser would be any better in so far as the shifting when locked, although I would expect so.

Sometimes a disassembly can help fix certain isuues.  Where there is factory-grease, that can be cleaned away and replaced with that of better quality, like Super Lube, a PTFE(Teflon)-based grease.  Who knows, perhaps it was not assembled properly at the factory, a mass-production facility.  A careful disassembling and cleaning can work wonders for that mechanical.  If you decide on that, take photographs every step of the way, as a guide when reassembling.

Edited December 17, 2020 by Alan64

[Jm1973]

That sounds like good advice. Thank you.

[wulfrun]

I have a couple of suggestions, which may or may not be useful. First one, do you actually need to lock the focus or is there enough friction to leave it unlocked? Second, try and figure if the lock moves focus by a consistent amount and which way. Mis-focus to that extent then lock to bring it back to correct. I agree further action is needed but this might give you a working stop-gap.

[tooth_dr]

I’m a big advocate of the standard focusers on my SW scopes.

I wouldn’t recommend removing the focuser, rather just remove the drawtube and check for even wear along the flat part. Then tighten up the screws to minimise movement of the drawtube. I do mine up really quite tight.  I have 1.8kg of kit on mine, so a DSLR shouldn’t be an issue.

As wulfrun suggests, you might not need to lock it, provided the tension is good.

 

 

[tooth_dr]

1 hour ago, Alan64 said:

Where there is factory-grease, that can be cleaned away and replaced with that of better quality, like Super Lube, a PTFE(Teflon)-based grease. 

I think this scope uses a crayford, in which I wouldnt expect any grease

Edited December 17, 2020 by tooth_dr

[Jm1973]

10 minutes ago, wulfrun said:

I have a couple of suggestions, which may or may not be useful. First one, do you actually need to lock the focus or is there enough friction to leave it unlocked? Second, try and figure if the lock moves focus by a consistent amount and which way. Mis-focus to that extent then lock to bring it back to correct. I agree further action is needed but this might give you a working stop-gap.

Yeah, it defintely needs locking. I attach a DSLR and if I don;t lock it goes out of focus during the session.

I think it might be a consistent thing. I need to run some tests when I'm not imaging and so not under pressure.

Thanks.

[Jm1973]

12 minutes ago, tooth_dr said:

I’m a big advocate of the standard focusers on my SW scopes.

I wouldn’t recommend removing the focuser, rather just remove the drawtube and check for even wear along the flat part. Then tighten up the screws to minimise movement of the drawtube. I do mine up really quite tight.  I have 1.8kg of kit on mine, so a DSLR shouldn’t be an issue.

As wulfrun suggests, you might not need to lock it, provided the tension is good.

 

 

To be honest I haven't even tried tightening the screws yet. It may be that that would be enough.

Thanks.

[tooth_dr]

1 minute ago, Jm1973 said:

To be honest I haven't even tried tightening the screws yet. It may be that that would be enough.

Thanks.

100% tighten then enough so that it moves with the knob but not too easily.

 

Good info here:

 

[Jm1973]

2 minutes ago, tooth_dr said:

100% tighten then enough so that it moves with the knob but not too easily.

 

Good info here:

 

Great. I'll take a look.

[tonylumley]

13 minutes ago, tooth_dr said:

100% tighten then enough so that it moves with the knob but not too easily.

 

Good info here:

 

Cheers tooth_dr, great pdf, don't need it myself(yet😉) but at least I have it saved if and when the need arises, Lum.

[wookie1965]

I did this with my 150p and 200p makes hell of a difference.

 

[tooth_dr]

7 minutes ago, tonylumley said:

Cheers tooth_dr, great pdf, don't need it myself(yet😉) but at least I have it saved if and when the need arises, Lum.

No problem, all credit goes to Monsieur Fozzybear @fozzybear for the link

[tonylumley]

8 minutes ago, tooth_dr said:

No problem, all credit goes to Monsieur Fozzybear @fozzybear for the link

OK, thanks Fozzybear  too!!😉

Edited December 17, 2020 by tonylumley
Spell check altered name!

[Alan64]

1 hour ago, tooth_dr said:

I think this scope uses a crayford, in which I wouldnt expect any grease

Yes it does have a crayford-type focusser.  My JMI crayford is without grease, but I know nothing about the Synta crayfords.