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IR Cut Filter & Star Bloat

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I'm just starting out imaging with my first dedicated camera (The Zwo Asi 533). Am I right in thinking i'll need the above filter to control star bloat, i'll be imaging with the Samyang 135ED f2 to start with. Also is this one ok for the job and would I need the 2" or 1.25" https://www.firstlightoptics.com/uv-ir-filters/baader-uvir-cut-filter.html and lastly, will I have to take the filter into consideration when working out backspace?

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You will probably need a uv/ir cut filter, yes, although other users of that lens will be better placed to confirm.

I'm not sure how well corrected the optics are, but if its less than perfect, you'll probably want to look for a filter which cuts above just 400nm and just before 700nm. The Astronomik L3 filter worked well for me when I was using an ED doublet: https://www.firstlightoptics.com/uv-ir-filters/astronomik-l-3-uvir-blocking-luminance-filter.html

Here is a handy calculator to check what filter size you'll need: https://astronomy.tools/calculators/ccd_filter_size. I put your equipment info in, and you should be alright with 1.25" as long as you can get the filter no further than about 25 - 30mm from the sensor. 

And lastly, yes, you should add 1/3 of the filter thickness on to your backfocus requirement.

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@nephilim I use an L2 with my Samyang 135mm + ASI294MC setup and I’ve used an L3 with my Samyang and ASI1600MM NB setup. Both have been very beneficial imho. I now use the L3 with my RedCat + ASI1600.


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