TMB 105 

In November 2003 I found myself at the showroom of Bray Imaging in Leighton Buzzard, Bedfordshire. They had recently taken delivery of a TMB 105 (4.1") apochromatic refractor with the new lightweight tube. TMB - Thomas M Back has a reputation for good optics, and the test figures on this unit were very good. The 650 mm focal length and f6.2 focal ratio were just what I was looking for to use with my MX camera for medium field imaging, and the lightweight assembly made it an ideal go anywhere 'scope since it would ride reasonably well on my EQ3 mount.

So I purchased the tube there and then and brought it home with me.

However I soon found that there was a problem with the design. The tube was very compact due to a sliding dew shield and draw tube, incorporating a truly excellent Starlight Instruments Feather Touch focuser. But the sleeve in which the drawtube slid was simply lined with thin felt which can wear and compress. Consequently after only a small amount of use the drawtube became sloppy and the eyepiece was able to move out of alignment to such an extent that the finder needed adjusting as the mount changed position, also the collimation was compromised.

Fortunately Bray were most obliging and engineered an excellent solution, although the original tube was from APM in Germany. They manufactured two rings which clamp to the front (internal) and rear projections of the drawtube sleeve, and locate the drawtube firmly with lockable nylon grub screws. Not only does this allow adjustment for wear, but enables the drawtube to be accurately aligned with the main tube axis. Many thanks to Chris and Geoff! I believe that later models were fitted with Teflon strips which solved the problem.

With the drawtube and dew shield retracted the unit is very compact. Don't worry - the grass was dry!

The telescope now performs as expected, with a textbook Airy disc and crisp images free of false colour. The first image taken since the modification is excellent - see the M65-66 image.

MUCH later! As the years passed and I gradually acquired CCD cameras with larger sensors, it became evident that the field of the instrument was not flat enough for successful imaging - stars towards the edges and corners were badly distorted. In July 2015 I resolved to 'do or die' and on browsing the 'net, I found a solution - a non-reducing field flattener from Teleskop Service. This one. Duly purchased direct from TS (at the time the Euro was low, so cheaper than buying in the UK!), it proved to do the job admirably, thereby saving the otherwise excellent telescope from probable sale, and at the same time giving me a medium field imaging option.

It is necessary to space the flattener correctly as noted on the TS web page, but although TS are able to supply custom made extensions, I was able to make suitable parts on my small lathe, a useful saving! These before and after images were taken using my QSI683 CCD camera with a chip size 18 x 13.5 mm. The bright star is Deneb. Click on the thumbnails (flattened is the lower one) for larger images. Full size corner crops are here.

As a bonus, the flattener also improves the images using my Meade 127 refractor, although that is quite good without the flattener.