2600MC Pro and UVIR filters

retman

i use a 2600MC Pro. i realize that it already has a UVIR filter built in but have read about concerns as to the built in filter's quality. i also use a ZWO 2" filter drawer with several Antlia filters (ALP-T and SII EDGE). In the interest of keeping things simple i would like to keep the filter drawer in place when i image broadband targets and simply insert another 2mm (optical thickness) UVIR filter in place of the narrowband filters so i don't have to adjust spacers for backfocus. (i'm using a Stellurvue SVX90T with Stellarvue field flattener with this rig).

Also, if this is a "good idea" 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. OR, any recommendations for other 2mm thick UVIR filters (i have seen some not very positive things about the ZWO filters and the glass is 1.8mm rather than 2mm thick)

i know that w7ay (Chen) has had some comments re the UVIR filter used in the ZWO cameras

Kevin_A

retman I was initially doing what you intend to do to keep the backfocus the same but ultimately decided not to go that route as you will loose a bit of light. If the filter passes 94% light then you will be losing another 6%. I opted to spend the money on an Askar M54 adjustable backfocus adjuster as it is very good quality and it takes only about 10 seconds to adjust it to compensate for the lack of a filter in my filter drawer.

w7ay

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

Kevin_A

i looked at the specs for the Askar part, it says 18MM +/- 2mm so the min thickness is 16mm and can replace the ZWO 16.5mm spacer?
the Askar part needs to go in the image train post filter (i.e., between the filter and the camera sensor)??

thanks for your help

retman

w7ay thank you chen, very helpful, as usual!

w7ay

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

w7ay hello chen, if i add, say a 0.1mm spacer (post filter) to my triplet with field flattener do i need to refocus? i ask because last night i set up and focused (ZWO EAF) with a 0.65mm spacer (filters are 2mm glass) and then added a 0.1mm additional spacer but did not refocus (bad science i guess) and then removed the 0.1mm spacer and replaced with a 0.2mm spacer. all spacers added were between the focuser and the camera but still no refocus. (so backfocus noodling only)

w7ay

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

retman

w7ay thanks chen, i kept having arguments with myself about this!