Monday, July 01, 2013

20130701 Cheung Chau, using a Canon 100-400mm f4.5-5.6L as a telescope

Visual observation basically,except I helped some of my friends to take images of the Milky-way area with the help of a tripod.

This is a camping event with the youngsters from the church.  This time I brought my Canon 100-400mm and used it as both a camera lens and telescope!  If we treat it as a telescope, it's just very compact and extremely portable, and it allows zoom. 

In order to use it with a diagonal, I've added a Tele Vue 2x barlows in front of the diagonal so as to give enough focus travel.  Given its 4x zoom range, it's versatile enough with just a single 20mm Widescan Type III.  Actually, I also had a 4mm TMB planetary with me, but I was too lazy to pull it out.  Before I went for this observation, I tried these combinations at home at terrestrial objects to make sure infinity focus is alright with certain image clarity.

Conventional estimation gives 3x in such a configurations of barlows and diagonal, so I have 15x with a real field like 5 degrees and it is huge!  Searching for target is rather easy, but I didn't bring any finder or zero power sight devices, so I need some scanning which is not exactly enjoyable with my 410 head.  Maybe due to the complex lens configurations, the view is rather dim at its widest.  Also, notice that the Canon 100-400mm has f4.5 effective focal ratio at 100mm, therefore, the effective aperture is just 22mm!  So technically speaking, it is a stronger reason why it was so dim.

Stars were basically pinpoint across most of the huge 80 degrees field, open clusters were well resolved but again, it's dim.  Pushing up the magnification does not give dimmer image  since it has more than 70mm effective aperture at 400mm.  Swinging it to Saturn reveals it's oval shape even at 15x, by pushing up to 400mm, we have 60x which has its ring resolved very nicely!

Conclusion: given its small effective aperture at 100mm, it is not really a wide field instrument but its wide field capability could make it serve as a finder of its own.