retman does anyone know if the Baader CMOS-Optmized (which has 2mm glass so the spacers for backfocus would be the same as with my 2mm glass Antlia filters) will fit in the ZWO drawer.

Yes, but barely.

As you know, ZWO and Baader use slightly different thread pitches. But in the case of the "CMOS-optimized" Baader, it will thread pretty much almost all the way in:

Compounding the problem, notice that the Baader filter ring is also quite tall. You will need to unscrew the ZWO locking grub screw (shiny grub screw in the next photo) all the way out for this Baader to slide into the filter holder. You just need to carefully align the filter holder and the filter drawer so that it works.

What I have done with the Baaders that don't fit (I have a Baader Neodymium like that) is to get a junk Chinese UV/IR filter, like the ZWO or the SVBony, throw away the glass, and replace the crap glass with the glass from a Baader. So, that can be your "last resort" solution.

Compared to an Astronomik L-1, the Baader cuts quite a bit off the IR end (it behaves more like an Astronomik L-3 on the IR end), but will let SII and Hα pass without problem. However, I wouldn't use the Baader "CMOS-optimized" UV/IR on a guide scope, where more IR is a good thing (nowadays, I use an Astronomik 642 Bandpass on a ASI678MM guide camera to cut the blues (more sensitive to atmospheric turbulence) and get more IR, but not a full lowpass IR since the FMA180p guide scope is not quite "APO" enough to maintain good stars into the deeper IR region -- there is about 200nm worth of passband from the 642 BP, instead of 300nm for a UV/IR filter, so plenty of sensitivity left after filtering).

FWIW, my copy of the "CMOS optimized" Baader measured out to be about 2.01 mm thick, using a filter thickness gauge.

Chen

retman the Askar part needs to go in the image train post filter (i.e., between the filter and the camera sensor)??

The Askar backfocus adjuster is the best thing that has appeared in the astrophotograhy scene in the last 10 years. You don't find YouTube shills talk about it (not bribed enough), so it is relatively unknown.

I have them permanently installed in a couple of my OTAs that has sufficient backfocus to allow it. And yes, you can collapse it down to 16mm (I have it adjusted to 16.7mm on one of my set ups).

It is indispensible if you have a faster OTA (e.g., f/5 or faster) that requires very precise (better than 0.1mm) backfocus. I have them permanently fixed to my FSQ-85, Pleiades 68 and Epsilon e-130, and have used them temporarily to discover the backfocus of fast camera lenses that don't have enough backfocus to simultaneously include both the backfocus adjuster and a tilt adjuster.

Would be better if it could also adjust tilt (hint: you need to first zero out tilt before fine tuning backfocus) and if it also comes in a thinner version. However, the M54 version of the Baader tilt adjuster (only 10-11mm backfocus needed) is coming out real soon now, and that should solve the tilts from OTAs that do not come with a built-in tilt adjusters (and can adjust backfocus of camera lenses, when a range of only 0.5mm is needed). I also have the M68 version of the Baader on my FSQ-85.

The Askar is for backfocus adjustment, so, placed anywhere between the flattener/reducer and the sensor. No need to restrict it to between the filter and the camera. In fact, you probably do not want it in between a 48mm filter and a full frame camera since a filter-Askar-sensor train will likely vignette the ray from a fast scope. The aperture of the M54 version has 46mm diameter, which is slightly larger than the clear aperture of a typical M48 ("2-inch") filter; Antlia "2-inch" filters have a clear aperture of 45mm, for example.

It also works as a very precise non-rotating helical microfocuser.

It is kind of ironic, since Askar does not even sell fast scopes that requires accurate backfocus :-). But it is a convenient drop-in replacement for their 18mm thick camera end drawtube adapters.

Chen

retman if i add, say a 0.1mm spacer (post filter) to my triplet with field flattener do i need to refocus?

Yes, if you have a flattener/reducer and the filter is in between that and the camera sensor, you will need to both adjust backfocus and then refocus the OTA, when the glass filter thickness changes.

I know a lot of people who use an EFW (or even filter drawers) misses the need to change backfocus when they change their filters, thinking they can just remotely change the filter in the EFW.

Exceptions are Petzval OTAs(which have non-critical backfocus demands) that don't have a reducer, where you will only need to refocus, or EFW with a proper set of filters that are all parfocal, where you don't even need to refocus.

Any change in optical distance to the sensor will need refocusing.

Chen