Skymax 90 focuser backlash.

[Merlin]

Does anyone know how to fix the backlash in this 'scope's focuser?

[JamesF]

Is this particularly when you're changing the direction of travel of the focuser?

James

[Merlin]

Yes James. I wondered if there was anything on Youtube to show a fix. I can live with the backlash, but it would be better without it.

[JamesF]

I think you might have to live with it I'm afraid

If the design is similar to my 127 Mak (and I imagine that it would be), the primary mirror is held on a carrier that slides up and down the outside of the baffle tube. There's a plate sticking out on the focuser side of the carrier behind the mirror. The focuser knob turns a threaded rod connected to the plate which pushes or pulls the carrier and mirror along the baffle tube.

Obviously the carrier can't be an interference fit on the baffle tube. There has to be some play to allow it to slide. The main source of backlash is, as far as I can tell, due to that play. Effectively when the focuser changes direction it "rocks" the mirror and carrier slightly on the baffle tube before it starts to move. This appears very obviously for me when I'm imaging at a focal length of four or five metres. The rocking changes the mirror alignment and the image shifts across the camera sensor by several millimetres before the focus starts to change. It's actually noticeable when imaging the Moon or Sun at the native focal length too, but less of a pain as I use the DSLR for that and don't tend to lose the target as a result

I don't know that there's any viable solution other than a better design for the focuser or a better designed mirror carrier that perhaps runs on Teflon or roller bearings. Even making the mirror carrier longer might help as the mirror doesn't move that far and the baffle tube is quite long. Should I get to the stage when I'm not using the Mak very much I might be tempted to rebuild it with a better design purely out of intellectual curiosity, but for the time being I've learned to live with it.

James

[Macavity]

It is indeed a problem with e.g. the "Mk.I" (Gold) MAK150 - But I always

intended to add a Crayford-style fine focusser to that one.

With smaller MAKs, rather (a heavy) overkill - And likely will not fit within the standard knob! If you want to do e.g. webcam (planetary) imaging, a small Helical focusser can be used to advantage on these. Do the rough focussing with the standard system and the fine focus with the helical focusser. If your mount height allows you to dispense with the star diagonal, can be used visually too! As ever, "more expense", but an effective solution to the problem in some specific instances.

http://www.firstligh...ser-s-7315.html

Now sold by FLO I see. Thread adaptors are available too.

As ever, these depend on the version of MAK you have...

[Merlin]

Interesting posts, thanks chaps. As with a helical focuser, a zoom eyepiece is handy for reaching final focus