Telescope Reviews: 5" F12 Istar Refractor project

Well I had a pleasant evening last night. The conditions were similar to the previous night, clear sky and seeing about 3 out of 5, not ideal but not too bad. I made an aperture stop out of black card for the 6 inch D&G which made it a 5 inch f18.3 (2330mm focal length). The Istar 5 inch f12 has a focal length of 1500mm. This naturally gave the D&G an advantage and it did perform better overall. Less false colour and steadier images. The Istar was very good though and I am not dissappointed in any way. It is a fine lens and I recommend it as good value for the price.

It was not possible for me to match magnifications but even so it was an interesting and fun experiment. With a 13mm Nagler in the D&G (179x) and a 9mm Nagler in the Istar(167x) I started on the moon. The images were sharper with less false colour in the D&G (almost none visible). They both showed tremendous detail. The Istar was also disadvantaged due to a less steady mount which made critical focusing more difficult so with that in mind I feel further testing on a better night is required.

I tried swapping diagonals between the scopes (GSO dielectric and AP Maxbright) but this didn't seem to make any difference. I also tried some star testing with a green filter (Baader Solar Continuum). For this I used the 9mm Nagler in the D&G (259x) and a 5mm Nagler in the Istar (300x). The star was Beta Centaurus. There was atmospheric disturbance in both images but otherwise they were similar inside and outside of focus with the slightly clearer (steadier) images coming from the D&G probably due to the focal length advantage. I would like to do this again on a better night when the chance comes.

Star testing without the green filter both scopes showed false colour (violet haze) on Beta Centaurus at the above mentioned magnifications. There was less in the D&G as expected due to the longer f ratio. In trying to quantify the level of false colour I removed the aperture stop from the D&G and the false colour increased as expected. The false colour shown in the Istar 5 inch f12 at 300x was similar to the D&G 6 inch f15 at 259x. While not being an exact test and certainly subjective I found this interesting as Istar classify this lens as R30 which refers to a reduction in false colour of 30% compared to a standard achromat of similar size and f ratio. I don't know if this means 30% across the whole visual spectrum or just at certain wavelengths. I really am not too concerned as a little false colour does not bother me too much as long as the lens is good in other aspects.

A standard 6 inch f15 has a CA ratio of 2.5.
A standard 5 inch f12 has a CA ratio of 2.4.

Based on my subjective test done in less than ideal conditions using different magnifications I would say that the 5 inch f12 Istar has similar false colour level to a D&G 6 inch f15.

Take that for what it's worth (not much). It is certainly not a fair test but an interesting side note for now. I did not buy the Istar lens expecting an apo for the price of an achro. I wanted a quality longer focal length 5 inch lens delivered in a reasonable time and that is what I received.

After the testing I just had fun observing various objects. I looked at Saturn and then some brighter DSO's. At lower magnifications the views in the scopes were more or less the same. High contrast, flat fields and sharp stars as expected of good refractors.

A 5 inch f12 is certainly a great deal more compact and managable than a 6 inch f15 and I can see that a 5 inch f15 will be sudstantially larger also not just in tube length but also tripod height. My work schedule means I won't be able to construct the 5 inch f15 D&G for another few months but I will test the two scopes side by side then and continue to use the Istar in the mean time.

Andy