New LRGB 1.25inch filters and back focus

ap1954

Hello
I have just bought asi1600mm, efw and the new LRGB 1.25inch filter set and read somewhere that these will change the 55mm backfocus a little. The rule seems to be divide the filter thickness by 3 and add to 55. Given these filters are 5.5mm thick does this mean the back focus is about 57mm?
Thanks in advance
Alan

w7ay

ap1954 Given these filters are 5.5mm thick does this mean the back focus is about 57mm?

Alan, yes, if the filters are really 5.5mm thick, then the mechanical backfocus needs an extra 5.5/3 mm from what is specified by the focal reducer/flattener. If the reducer specs say 55mm, you will indeed need a spacing of 56.8mm between the reducer and your sensor.

Chen

w7ay

ap1954 Given these filters are 5.5mm thick

By the way, are you using very large square filters? 5.5mm is really thick for a glass filter. Even the good 2" filters are only 3mm thick.

I don't know of such thick filters, and I use Radian Quads, Chromas and IDAS filters.

Check your specs again to make sure it is the glass thickness, and not the thickness of the filter holder (which of course, will not contribute to any additional optical path).

Chen

ap1954

Thanks for the prompt reply Chen. I bought the complete ZWO package which includes the asi1600mm plus the filter wheel and set with the NEW 1.25inch threaded LRGB filters. From what I could see on zwo website the specs indicated thickness of 5.5mm but it was not entirely clear to me if that was just the glass. Any additional info is greatly appreciated
Alan

w7ay

ap1954 From what I could see on zwo website the specs indicated thickness of 5.5mm but it was not entirely clear to me if that was just the glass.

I just did a quick Google search (I do not use ZWO filters), and that 5.5mm is the thickness of the mount (with the threads extending another 2.5mm).

ZWO's narrowband filter glass is 1.85mm thick. See here:

https://astronomy-imaging-camera.com/product/zwo-new-narrowband-1-25-filter

I suspect their LRGB set is also somewhere around the same thickness.

Good manufacturers like Chroma stick with the same glass thickness for all of their filters so that you won't need to refocus after a filter change between LRGB and narrowband.

In any case, a difference in glass thickness is no more than about 1.5mm. That means a optical path difference of 0.5mm from one filter manufacturer to another.

If you are using a ZWO camera, their sensor flange distance is specified only to an accuracy of +/- 0.5mm anyway. (ZWO cameras are hobby class quality.)

Just start with some nominal value (like 1.85mm, 2mm or 2.5mm), and then fine tune the backfocus by looking at shape of the aberration of stars around the edge and corners of the frame. It is very easy to tell if the backfocus is too short or too long. See halfway down this web page:

https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus

Chen