Jump to content


Flextube 350 mirror cell - help

Recommended Posts

A new 14" Flextube GOTO arrived at my door a few days ago. I built the base in an hour on Monday evening (simple, easy to follow instructions, but an error in the parts list), but have been out the last two evenings.

This evening my wife and I mounted the mirror cell in the OTA, collimated the 'scope and tried it out on high flats about half a mile away.

Mounting the mirror cell was pretty straight forward (despite no instructions). I had asked for advice on SGL on Monday evening and Brantuk kindly posted some pictures of the assembly mounting on his 10".

Basically you put a small spring over each of 3 male screw bolts that jut out from the back of the mirror cell assembly. These then go through 3 holes in the base of the OTA and you then screw female bolts onto this. So the mirror cell is held securely and you can adjust the cell position using the 3 female bolts, that push against the back of the mirror cell mounting, sprung by the 3 springs. All sits securely and looks correct.

I then collimated the secondary and then the primary mirrors. That took about 5 minutes to get perfect.

So far so good - only 20 minutes in total.

I put the OTA on the base, added a 16mm Nagler and pointed the 'scope to the roofline of the 20 story flats half a mile away. I then started to rack out the Crayford focuser - this has only about 1.5 inches back focus movement.

I couldn't achieve focus!!! - I need a further 1/3rd of an inch back focus....hmmm. I tried another EP, a 20mm Meade 4000 plossl - exactly the same.

So, I addded the only extender I have, a 2" 80mm WO extender. Now I didn't have enough forward focus.

I really only need an inch more back focus (that would give me more back focus than I need, but allow for slightly different focus points for different EPs.

I tried screwing the mirror down more tightly - no way I could get more than a few mm and then the mirror assembly was really tight against the base flanges on the OTA - that isn't the answer.

So, either I have mounted the mirror cell an inch too far from the rear of the OTA (I can't see how I could do that) or the focal length of the mirror is slightly longer than the light path to the EP.

I guess the easy route is getting a 2" 25mm extender tube (if such a thing is made), but I wonder if there is anything else I could do, apart from 'phone FLO tomorrow and tell him my woes?

Any ideas?


Link to comment
Share on other sites

Hi Tom,

Half a mile is probably too close to focus the same as it will on distant celestial objects. Once you get it under the skies you should find the focus point comes back to the ordinary position.

I was very surprised to find recently that objects have to be very far away indeed for focus position to act as infinity. Particularly with large fast scopes like your 14".


Link to comment
Share on other sites

I hope you are right Andrew. I never had any problem achieving focus on those Flats with my 12" f5 Flextube. The 14" is F4.5 and I would never have thought this would make such a difference - but I am no expert on optics.

There is 100% cloud cover here so I have nothing at infinity to focus on tonight. I will try again the next night it is clear, but will still speak to FLO tomorrow.

Here's hoping you are right.

Tom (on tenterhooks)

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.