Slipping RC8 focuser.

[SteveBz]

Hi People,

I've just acquired a used RC8 and the focuser is a bit slippy.  Originally, it was all over the place, but I took it apart and tightened the screws and now all the tilt/slop is gone, however, the focuser movement has directional difficulties.

The load is relatively high with an SX camera, filterwheel and AO unit.  If I'm pointing overhead (maximum load), focusing manually and I support the load lightly with one hand the inward travel and outward travel work fine.  If I'm focusing remotely with a motor and belt, the same is not true.  Outward travel is fine, but inward travel just slips.  There seem to be two screws to portentially tighten or loosen.  What's the correct process to get inward and outward focus working smoothly?

Kind regards,

Steve.

[vlaiv]

20 minutes ago, SteveBz said:

What's the correct process to get inward and outward focus working smoothly?

For me, it was to replace focuser with better 2.5" R&P unit (that also has threaded connection instead of clamping one).

[SteveBz]

6 minutes ago, vlaiv said:

For me, it was to replace focuser with better 2.5" R&P unit (that also has threaded connection instead of clamping one).

Hi Vlaiv,

Nice to hear from you.

Which one did you use and how much did it cost? And maybe a link if you have one.

Have a nice weekend.

Kind regards,

Steve.

[vlaiv]

3 minutes ago, SteveBz said:

Which one did you use and how much did it cost? And maybe a link if you have one.

I purchased this one:

https://www.teleskop-express.de/shop/product_info.php/info/p5769_TS-Optics-2-5--Rack-and-Pinion-Focuser---supports-up-to-6-kg---travel-95-mm.html

It is direct replacement (M90) no adapters needed.

It has rotation which is good for threaded applications as you don't need separate rotator. It also has M63 thread.

At the time I purchased it - it was about 25% cheaper than now (prices are of course going crazy at the moment).

I remember deciding between that and this one:

https://www.teleskop-express.de/shop/product_info.php/info/p6421_TS-Optics-PHOTOLINE-2-5-inch-Deluxe-Rack-and-Pinion-Focuser-for-Astro-Imagers.html

But I decided to go with first one as specs were almost the same (it can even handle more weight) - and price was smaller (as is at the moment).

[SteveBz]

Thanks for that.  It looks very nice, if a little expensive.  As I'm quite overdarwn at the moment, I think it'll have to wait

Thanks

Steve

[ollypenrice]

In my head only, I conceived of a sliding counterweight system to reduce the gravitational pull on the focuser to as little as zero if that's what you wanted. It would not be all that difficult to make.

- Start with a tube mounted parallel with the optical axis of the scope and behind the focuser's knobs.

- In this tube is a cylindrical weight of the right diameter to slide up and down in it like a piston in a cylinder.

- A cable (fishing line?) goes from this weight forwards to a pulley on the back of the scope, which it loops around, and then back to the camera.

That's it. Pointing at the zenith, the camera is pulling down, a counterweight equivalent to the camera's weight is also pulling down by the same amount, but the direction of the counterweight's pull is reversed by the pulley so it neutralises the camera's pull.  Both pulls diminish proportionally (bar minor differing effects of friction) as the scope approaches the horizontal (and the problem ceases to exist anyway.)

An industry capable of producing so many excruciatingly bad focusers should have no trouble making a tube, a pulley and weight. 

Olly

Edited October 30, 2022 by ollypenrice

[wvrfish]

I recently had this same problem myself. I tightened the pressure screw (on the bottom) quite a bit and it helped but I found it would still slip when moving inwards whilst pointing near to the zenith.

I was looking at getting either the R&P TS focuser or a Baader steeltrack, but thought I would give fixing it one more go.

I realised that it was actually the fine focus knob/mechanism that was slipping in my case - it would still move if I used the coarse focus knobs. I tried tightening the grub screw on the fine focus knob itself but that didn't help. I then dismantled it again and tightened a nut on the fine focus gearing a little and reassembled.

The nut in question can be seen on https://agenaastro.com/articles/guides/optimizing-your-gso-crayford-focuser.html, step 7. The focuser in that guide is slightly different to mine but the fine focus looked the same.

Once re-assembled I found that the operation of the focuser overall was a bit stiffer and less smooth, but the slipping issue was resolved. I was concerned that my cheap little 28BYJ-48 based DIY electronic focuser wouldn't have enough torque to move it, but it's been working just fine.

Now I just need to sort the collimation 🤦‍♂️

[Clarkey]

7 hours ago, wvrfish said:

Now I just need to sort the collimation

Don't sweat it. It's not as hard as it is hyped up to be.

[wvrfish]

15 hours ago, Clarkey said:

Don't sweat it. It's not as hard as it is hyped up to be.

What method would you recommend? Feels like I've read dozens of subtly different approaches, but I think it was David Davies(?) that sounded most logical to me. I also don't fancy spending hundreds on a Howie Glatter

 

Thanks

[Clarkey]

My method is relatively simple and works for me. I have a Reego collimator which I use to collimate the secondary mirror. (You can use a collimation cap or Cheshire but the LED in the Reego does make it a bit easier. However - do not buy the Reego for the RC8 as they are ridiculously expensive for what they are). I then get the primary correct using a star test. If you want to check the distance for the correct FL you can use a Ronchi grating, but I have never felt the need. It is loosely based on this method:

A Procedure for Collimating Ritchey-Chrétien and Other Cassegrain Telescopes (deepskyinstruments.com)

I can get away with the simple secondary collimation as there is no evidence of tilt in the mirror / focuser interface. If you have tilt here, it is more difficult. In this case you could use the method David suggests - he kindly sent me a copy too. It is certainly a thorough method, but I would not be dismantling my scope without the need to.

 

[SteveBz]

On 30/10/2022 at 18:02, ollypenrice said:

In my head only, I conceived of a sliding counterweight system to reduce the gravitational pull on the focuser to as little as zero if that's what you wanted. It would not be all that difficult to make.

- Start with a tube mounted parallel with the optical axis of the scope and behind the focuser's knobs.

- In this tube is a cylindrical weight of the right diameter to slide up and down in it like a piston in a cylinder.

- A cable (fishing line?) goes from this weight forwards to a pulley on the back of the scope, which it loops around, and then back to the camera.

That's it. Pointing at the zenith, the camera is pulling down, a counterweight equivalent to the camera's weight is also pulling down by the same amount, but the direction of the counterweight's pull is reversed by the pulley so it neutralises the camera's pull.  Both pulls diminish proportionally (bar minor differing effects of friction) as the scope approaches the horizontal (and the problem ceases to exist anyway.)

An industry capable of producing so many excruciatingly bad focusers should have no trouble making a tube, a pulley and weight. 

Olly

Hi Olly,

I'm sort of up for a version of this.  A simpler version might be to mount the pully on the main Losmandy plate. Dangle a vertical cable straight down parallel to the pier and with a weight suspended from it then attache the othe end of the cable to the focuser by a spare screw or something.  The only issue is that there might be some torque on the focuser making optical train less than square-on to the scope.

Not sure how to make this work.

Kind regards

Steve.