Sergi Verdugo is a very clever chap http://astrophoto-sv.com/index.php?p=1_78 🙂

I have been thinking today about setting up a second Canon 200mm lens on the mini-WASP array for the wide field work.  To this end I was looking for 72mm HUTECH IDAS light pollution filters (I have one of these on the Canon 200mm lens I am currently using) and was drawing a blank.  Searching for HUTECH IDAS filters I came across Sergi’s (very nice!) work and found his ingenious solution.

First let’s go to basics and what I did.  I put a 72mm LP filter on the front of the Canon 200mm lens and stopped the lens down (using its internal diaphragm) to f#4.5.  Why did I stop it down?  Two reasons: one so that the filter will work (it is not designed for f#2.8 which is the lens wide open) and secondly so that I get good quality stars across the whole APS-C size sensor.  I also get the 8 highly annoying diffraction spikes around bright stars from the diaphragm thrown in for good measure.

Here comes Sergi’s solution.  Fit a 72mm – 48mm step down adapter to the front of the Canon 200mm lens and put a 48mm IDAS filter in that.  You automatically stop down your lens to f#4 where the filter will work and you will get good star shapes.  In addition you can now work with the diaphragm wide open – so no diffraction spikes – how brilliant is that then?

I bought the 52mm IDAS filter instead of the 48mm, so I get a tiny bit more speed out of the lens, and I bought a 72mm – 52mm step-down adapter from e-Bay.  I already have a 52mm – 48mm step-down adapter, so I can put in my 48mm narrowband filters when I want to do some narrowband work – and all with no diffraction spikes.  How cool is that then?

Thank you very much Sergi, a great solution – and one that I should have come up with years ago – and didn’t.  I guess I’m getting old.

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