ETX105

[astrofox]

Has anybody got the outside diameter of a ETX105 tube. Need to see what sort of rings I need to mount one.

Thanks

[Psychobilly]

Just dug mine out and measured it the Front cell is 123.7mm and it is flush with the tube ...

So 5" rings should fit IIRC I did demount mine once and use it in some 5" guidescope rings and there was next to no adjustment...

Peter. ..

Badly typed on my Galaxay S4 in Tapatalk4

[ronin]

Get the same 124mm in effect, depends on the generosity of whoever makes 5" rings.

[astrofox]

Many thanks. The rings I have are adjustable with 3 screws.

[Psychobilly]

That type are normally known as Guidescope rings... they would normally be bought oversized and the screws used to adjust what the scopes pointing at... This used to be important in the days of manual guiding using a reticule EP but I cant rember ever having to adjust the guidescope these days when using a guide camera...

Peter...

Badly typed on my Galaxay S4 in Tapatalk4

[RT65CB-SWL]

Hi, mine measured in at 123.7mm too.

Just be careful when tightening up as you may distort the meniscus cell. This is especially important when using tube rings rather than guide scope rings as I have found out. Collimationof an ETX can be a 'headache' though. Would be interesting to see some photo's of it. I am currently editing a post of my modified ETX105. It will be posted in the 'DIY astronomer' section when completed.

[astrofox]

Thanks

I hadn't thought of distortion affecting collimation. I have the guidescope rings already and wanted to know what size of ETX would fit on top of my scope for guiding.

Frederick

[RT65CB-SWL]

Hi, mine measured in at 123.7mm too.Just be careful when tightening up as you may distort the meniscus cell. This is especially important when using tube rings rather than guide scope rings as I have found out. Collimationof an ETX can be a 'headache' though. Would be interesting to see some photo's of it. I am currently editing a post of my modified ETX105. It will be posted in the 'DIY astronomer' section when completed.

I was using the office PC at work when I added my post yesterday, so here is a little more info.

I have previously used two sets of tube rings. I was not a happy chap :-(

First set was a 125mm Vixen rings, with a strip of 0.5mm rubber on the inside to take up the slack and prevent any slippage. Second set was a set of bespoke rings made by Raymaq Engineering/Ray's Rings. Both sets I suspect were deforming the OTA slightly oval and may not have been a perfect circle as I was getting a slight ghosting/flare of about half the size of what I was observing, (ie Juipter, Saturn stars upto +1.5 to 2.0, though none or not as bad for lunar or terrestial observing). Maybe I had been over-tightening both sets and this was "pushing out" the meniscus cell slightly and I was not able to reattach the supplied threaded end cap ever again! (and I have now lost/mislaid it during a temporary house move).

My ETX105 is now attached to a dovetail bar with 2x No:6 Jubilee clips/rings.

No ghosting/flare and no collimation. I am now a happy chap :-)

[astrofox]

Many thanks to all who helped.

I already had the guidesope rings and was hopefull I could use an ETX105 in them. However as the rings are 128mm id it's a very tight squeeze to fit the OTA in them. It looks like I'll have to goto plan B which is the good old fashioned standby of using s ST80 or clone as guidescope.

Once again thanks a lot o all who contributed, you've saved me buying the wrong kit.

[Psychobilly]

The ST 80 being 400mm FL @ f5 will give you a much better chance of finding a suitable guide star than the ETX105 which is 1470mm FL at f14.

You probably won't need to use guide rings as  computer based guiding will allow you to pic a star pretty much anywhere in the FOV.

I only tried using my ETX105 as a guide cope once that was when I was using the CPC800 at f30.

Peter...