iStar 150mm F/5 R50 Semi APO Doublet Mounted in Custom Made OTA

by Skylight Telescopes of London


Ian G. Ranson, United Kingdom


iStar 150mm F/5 R50 Semi APO Doublet Mounted in Custom Made OTA by Skylight Telescopes of London


I have had a liking for low power wide field telescopic views as long as I can remember. I was looking for a telescope to supplement my Istar 200/1200 WFT that would be portable enough to easily transport in a compact car and utilize a 3" format 30mm 100° field eyepiece.  The Istar 150/750 semi-apo lens was perfect as it would give me 25x and a huge 4° true field of view. I had Skylight Telescopes of London, England design and build the telescope around the Istar lens and a 3.5" Feathertouch focuser.

I have now had several opportunities to use the telescope under clear and relatively dark skies. The first time out with the ES 30mm 100° 3" eyepiece and diagonal was a real joy and confirmation that the project was a success. Milky Way fields through this eyepiece with its 4° field of view are amazing.

I also tried my TV binovue binocular eyepiece with 24mm Panoptics. The telescope was designed to use the binocular viewer with a zero power glass path corrector. This presented a beautiful view of the dumbbell nebula at around 30x. More structure was visible than in the big 30mm eyepiece. The binocular view providing a darker sky background and higher contrast due probably in part to using both eyes.

So, to sum up so far:

Most dramatic view: 30mm 100° ES eyepiece for exceptional milky way sweeping at 25x and 4° field. Best enjoyed from a truly dark location.


Best corrected low power view: 31mm Nagler with 2" TV diagonal. For purists this setup is going to provide the best wide field views the objective is capable of giving with near perfect star images over a field of view that is more than 3° in diameter.To bring out dso detail: TV Binovue with 24mm Panoptics. The views with this combination and a zero power glass path corrector to preserve the wide  2.2° field make the telescope a cost effective alternative to large aperture binocular telescopes. One that is lighter, more compact and a fraction of the cost.

I am really impressed with the quality of images produced by this objective. It is a world apart from the readily available 6" f/5 achromats. When approaching focus there is a lot of false colour visible. When focus is reached - and believe me it really does snap into focus - the false colour is gone and a pure image with excellent colour fidelity remains.  Anyone who delights in low power, wide field viewing will be very impressed with its performance. Colour correction in focus at the low powers I have been using (between about 4x and 8x per inch of aperture) is exceptionally good. Towards the upper end of this range Venus in a darkening twilight sky exhibited a sharp edge to its crescent shape with no discernible false colour halo. Further evidence of the colour fidelity is shown in star colours. Albario in a wide field eyepiece showed blue and yellow colour saturation of its components that was a pleasure to see. The colour of faint stars in the double cluster in Perseus was very dramatic and reminded me of the view of this object through 40x150 ED binoculars.

Anyone looking for a large aperture, rich field refractor with exceptional performance would do well to consider the Istar 150/750 lens.


Ian G. Ranson

United Kingdom