Mirror Cell base diameter ?

[ronin]

Considering buying a small mirror, 150mm diameter, and attempting to build a small easiiy portable scope. Maybe a Christmas project.

I can buy circular ply bases in 2 diameters 150mm and 180mm. May enquire if they will do  custom option in both diameter and thickness but that is presently not a consideration.

With a 150mm mirror would I be best advised to get a 150mm base for the mirror cell or the 180mm diameter base. The presumption is that whatever I have to hold the mirror from moving laterally would need space so the 180mm base is the better. It would allow me 15mm around the mirror.

Equally I half suspect someone will comment that they presumed the same and found that a base matching the mirror diameter worked out better/easier.

[saac]

Ronin don't know if it would be of any use but there are presently a couple of 130mm mirrors including cells available on Astroboot. One set includes secondary mirror and spider assembly - may save a bit of work.

Jim

[Astrobits]

150mm. you don't need much extra diameter ( if any ) to fix side clips.

Personally, I would not use side clips nor a circular disc at all but a triangular one with the mirror bonded with Silicone at three points only, making sure that there was about 2-3mm gap between the mirror and base board. That gap prevents any strain transferring from the baseboard to the mirror and makes it easy to remove the mirror when the time comes to re-coat the mirror.

Even a triangular board is more than necessary, all that is needed is a Y shaped base with the arms about 1" wide. They will provide the points for the Silicone and the collimation bolts/screws. Such a shape will be the lightest possible and give the best airflow to keep the mirror temperature at ambient.

Nigel

[ronin]

34 minutes ago, saac said:

Ronin don't know if it would be of any use but there are presently a couple of 130mm mirrors including cells available on Astroboot. One set includes secondary mirror and spider assembly - may save a bit of work.

Jim

Haven't looked but the 130's on Asto boot tend to be f/5 and I want an "easier" f/8. Have checked astroboot in the past and never managed to bump into a 150 f/8 OTA. As you say it would have all the accessory bits I would require - very, very useful.

Have checked the buy amd sell options but missed one some time back (think it was someone straight down the A10 from me also) - about a year ago - and nothing really since. Didn't want a pristine one as I am likely to produce something worse, if I manage anything workable. Something I have thought about for some time and as the bandsaw runs again (2 new tyres from the US) I thought that clearing the bench, scribbling a design and getting started was overdue.

It is the classic "used" market dilema, wait for something to appear with no guarantee of when or just accept it and go buy the items. Next question will be how do I build a spider that works. Cos I have no idea about that bit.

[Starlight 1]

It may be 150 f8 in the shed you did not look in ,  I check on Sunday . 

[martinl]

How are you going to mount it? In a “Sonotube”-type cardboard tube? Then you probably need a 200mm base plate as I’ve never seen 180mm tubes. The mirror could rest either on a 150mm disk with  bent alu bar stock for edge support OR a 180mm disc with three M6 bolts with nylon nuts for edge supports and large washers on top to keep the mirror from falling out OR (best) a triangle as Nigel suggests.

If you choose to silicone the mirror you don’t need any edge support.

Place the mirror support points at 40% radius and make sure to cut holes for airflow in both discs.