Large ‘very’ thin mirror making Why you should use thick glass!

TRO Telescope 1998: 30 inch diameter 1 inch thick plate glass Edge ratio 30:1 Centre thickness ratio 60:1 !!! We were proud as punch and a lot younger.

The concept did not really get going until December 1996 when patience ran out Some Christmas funding just happened to be available. The length of my garage determined a size of 30" in aperture at F4.

Lots of hard work!!!!

The Telescope worked!!.....and was cheap (£400) It showed us stuff that was not possible in smaller telescopes. Colours in M42 and detail in objects like M1 that were breathtaking!! But it had a problem Bad astigmatism and under-correction. Only VERY low power was of any use.

The original TRO telescope was decommissioned in 2001 The new 30 inch TROK was built in 2001 (TRO bits used to complete). Re-work on the mirror started in 2008.

New TRO lightweight tube and mount was essentially completed in 2008

Key point! Testing the mirror: Standard Foucault tester and Star test.

In 2010 (delayed due to personal issues ) we have hit a big snag! Images are astigmatic and under-corrected? The star test did NOT compare with the Foucault test (Figure XP) data 1/2.43 PV 28 nm RMS according to Figure XP Fabric slings are just no goo on thin mirrors!

2013: Bath Interferometer now used. Fabric slings are just no good for thin mirrors! Foucault only tests across middle of mirror and not whole mirror! Flexing in strap 2500nm PV!!! 10 waves PV wavefront At 12 deg Altitude Flex orientation not repeatable!

Flexure in the Interferogram!!

Should look like this!!!

Wire sling now used. Located at the centre of gravity of the mirror. Flexing is now top to bottom and significantly more repeatable.

2013: TRO testing. Taking gravity away (symmetrical errors only) The mirror is greater than 1.5 waves PV on the wavefront!! NOT 1/2.43!

Corrective polishing work using error map from Interferometer. Rub where its RED, leave the Blue alone! Sub diameter tools 6 inch and 3 inch star laps.

Under correction has been significantly reduced. But Telescope still does not perform due to axial and lateral support. Mirror is just too thin.

Testing on sky Tested at Zenith. DOH!!!!...the mirror does not perform well. Better than the first time, but not good enough. Astatic support would probably not work properly. 60:1 ratio in the middle is not easy! Solution......Active mirror support. LOTS of £££. Bath Interferometer has been proved to work on number of optics. What is the sensible limit? 20:1 ratio (Dave Robertson's mirror 18nm RMS) What now? Get thicker glass!! Supremax 33 only real option (what most makers use now) Limited UK suppliers (HV Scan expensive!) Reginato (Italy) Suppliers of up to 1M blanks (only 54mm thick now). Generated and fine annealed. Aiming for 30 inch F3 Ethos 21mm and Televue T2 corrector.

Prices in Euros need to start saving for Christmas again!!!!

Recent mirror purchased now being polished and figured 30 inch F4.6 (45mm thick)

Interferometer shows this to be 114 nm RMS on the surface This was supposed to be a finished optic! Needs to be 24nm RMS or better really

Latest mirror blank (Reginato) 30 inch generated to F3 (54mm thick) £3000 delivered to UK

Upgrading Bath Interferometer High resolution spectrometer for calibration of laser Winch in garage for handling large optics and heavy equipment New test stand that doubles up as telescope to star test 2 points 90 degree apart (vertical) 40 nm PV surface deformation for 30 inch F4.6

0 deg72 deg 144 deg 216 deg288 deg Average 200mm diameter F3 sphere Comparison between Bath Interferometer and 4D Technology Interferometer Images below taken with Bath every 72 degrees and averaged Astigmatism introduced by the Bath (short path length/beam separation)

200mm diameter F3 sphere Comparison between Bath Interferometer and 4D Technology Interferometer Bath = nm RMS surface 4D = nm RMS surface Patterns of error on surface look the same Bath can introduce errors for fast short optics if not measured correctly