ZWO EOS Filter Drawer

brianboyle

Ok - so I must be doing something really stupid.

I bought the ZWO EOS filter drawer to use with by Canon EF lenses and ASI 2400MC. Everything fits fine - and I have the 44mm of back focus needed for Canon lenses.

Only one problem - I can only achieve that focus with no filters in the drawer - as soon as I add a filter (L-enhance, UV/IR cut) it increases the back focal distance by about 1-2mm and I can’t achieve focus.

Now I expect filters to increase back focal distance by a little, but I am surprised it is so much that a filter holder sold for use with Canon cameras cannot actually cope with filters. Nor is that any extra spacer supplied with the kit.

What am I doing wrong?

Tech@ZWO

brianboyle
hello
Did you add the spacer？

w7ay

Given that glass has a refractive index of about 1.5, the optical path will be stretched by 33% of the thickness of the glass. For a 3mm thick filter, the mechanical distance between the EOS lens flange and the camera sensor must be 43mm, instead of 44mm.

Chen

brianboyle

w7ay That's what I figured too. So why was no spacer supplied? [And I think you mean the back focal distance will increase by 1mm from 44mm to 45mm.

brianboyle

I received the filter holder plus an adapter ring to fit onto my ASI2400 body. But no spacer.

There was no spacer shown in the diagram manual - or in the picture where the filter holder is attached to the full frame ZWO camera. I haven't seen the picture you showed above before. Where is it form.

Do I need to buy a spacer as well?

w7ay

brianboyle I think you mean the back focal distance will increase by 1mm from 44mm to 45mm.

You are right. The extra glass moves the focus of the lens about an extra 1 mm away from the lens flange. To compensate, move the sensor 1mm further from the flange.

Chen

brianboyle

If if I were to put on a spacer the adapter ring is now stuck fast to the filter drawer body. Even a rubber wrench can't get it off!

w7ay

brianboyle If if I were to put on a spacer the adapter ring is now stuck fast to the filter drawer body.

By the way, if you are looking for metal spacers, AgenaAstro has some.

I don't know what the hole size is for the ASI2400, but Agena has the Blue Fireball brand (Blue Fireball's other stuff are pretty good, too) metal washer spacers for M54. The worst thing I have come across is the Baader one-time use Delrin spacer (I suspect ZWO's might be Delrin too), but I am not sure Baader even has the 54mm size.

I have an ASI6200, and an ASI2600 which was converted to an M54 tilt plate, so I run M54 pretty much everywhere nowadays.

Chen

gammerpc

w7ay
"ASI2600 which was converted to an M54 tilt plate" Did you just install the M54 Tilt Plate to the ASI2600MC Pro?
Thanks

w7ay

gammerpc Did you just install the M54 Tilt Plate to the ASI2600MC Pro?

Yes, that is what I did.

I also have an ASI6200. I changed the tilt plate on the ASI2600 to make everything uniform.

Chen

brianboyle

Many thanks for all the suggestions. I have now fixed things up using a combination of brute force and some home-machined solutions.