[gregbradley]

I have used all 5 brands.; Custom Scientific (1.25 inch LRGB), Astrodon (50mm round LRGB and 6nm Ha, 50mm square 5nm Ha, 5nm O111 and 5nm S11), Baader (7nm Ha, O111 and S11 50.4mm unmounted) and Astronomik (LRGB 50mm square), Schueler (1.25 inch 10nm Ha)

Firstly parfocal. Parfocal means same focus point for all filters in the set.
This is determined by the the filters being the same or close to the same thickness not anything to do with the coatings etc.

So it is a question of tolerances. The issue of whether Astrodons are parfocal or not was brought up in a discussion of these filters last year. Basically as I recall the filters are made to a tolerance of I think it was 4 microns. Parfocal requires a tolerance perhaps a bit tighter than that and the Astrodons were being criticised as using a marketing term "parfocal" (much like how the term "APO" gets abused or RC optics got abused). I think it is more accurate to say fairly parfocal and some sets may vary but I believe he sets a thickness tolerance for his manufacturer. Other companies probably do the same and I see Baader claims also it is parfocal. I am not sure what Astronomik claims in this regard but using my 50mm square LRGB set they seem to be as parfocal as the Astrodons. Which is to say you should ideally refocus between filters, most especially the blue filter.

In my case I usually did not focus between filters like many do as I haven't been using a robofocus (even though I have one!). I would usually refocus with temperature changes. But when I did refocus between filters I would notice green and red were the same focus but blue would be slightly different - this was with an RCOS 12.5 inch at F6.75 and using a Tak reducer.

The main issue with Astrodons are:

1. With an STL camera they require a 50mm round filter and the filter wheel opening is 50.8 or so. 50mm is too small and Astrodons can come loose in the wheel as the fastener washer is a bit small and I have had an expensive 6nm Ha filter get damaged because it came out of its housing and rocked on a metal part in the camera and it got scratched badly on the drive down to my dark site.

I also have had the same (it must be cursed) filter fall out unexpectedly when the lid of the camera was off for the same reason. The little washers that come with the STL don't hold it down that well.

Astrodon supplies wider washers to handle this problem. But I found with my STL the shutter clipped them as they were slightly thicker than the original SBig washers and it made an annoying clipping sound every time I took an image and the shutter clipped the top of the screw for the filter.

2. Reflections;

The bane of the Astrodon is reflections. They are a mirror reflection type system of filter. So they reflect a lot of the light (and therefore the colour they appear to be is not the actual colour they let through so be careful when labelling them, make sure you shine a torch through them and not assume oh that is the red filter because it looks red, that is the green filter and the one that looks blue is the red filter!).

I find with different scopes (high quality gear - FSQ106, AP140) I would get annoying bright reflection halos on bright stars most prominently in the green filter. Unfortunately the halos are different sizes and shapes with the red and blue adding to the difficulty of processing them out.

These reflections become critical with the new large chipped cameras that seem more prone to trouble from them. So much so that any scope using a flattener or reducer (and ALL do when we are talking 16803 or 09000 chips) that reflections become quite tough and severe. Perhaps not so bad with an RCOS which you can get away with without using a flattener although better if you do.

The new series are supposed to reduce reflections but I have seen an image from someone using a Proline 16803 and the reflections were still unnacceptable.

Having said the above the filters are good and easy to process the colours more than other filters and that is their main strength. I found it easy to get balanced colour and there was no special working out what the mixing ratios were or tricky processing, it made everything quite simple.

I have Astrodon Narrowband filters and they are great. I particuarly like the Astrodon Ha filter. I also have a Baader 7nm Ha filter and I preferred the Astrodon. my Baader Ha filter I think is slightly defective as it has little black specks in the images that I have to process out whereas same camera and scope the Astrodon's do not.

Baader however have a new LRGBC set and these rival the Astrodons in all respects - reflections, parfocal, 1:1 exposure times, 3mm thick, correct diameter (50.8mm). Colour from them seems to be better as Johanne Schedler has demonstrated with his recent images using them. Superior colour in my opinion. Also their clear filter would be great for galaxies etc.

The Custom Scientific I had were just the bog standard ones Sbig used. I found them very difficult to use to get the colours right when processing the images and frankly no good at all. But that's me, your use may be different and I was beginning at the time but they certainly were a struggle to get the colours balanced and looking like proper colour.

The Astronomik type 11 filters seem very good. They have excellent colour, they do not have virtually any reflections they seem quite parfocal but their main problem is their thickness. They are only 1mm thick. Filter wheels usually have a recess of 3mm.

In my case, I bought 50mm square LRGB. These were not supplied with a spacer to make them work. I don't know what the manufacturer was thinking - oh take the money and let them sort out how to mount our filters? Not my problem as I have their money?

I also have heard from other users that the supplied washer for the 50mm round filters is a bit too thin and does not hold the filter securely. I wonder if this can cause optical aberrations if the filter is leaning slightly and not orthogonal to the filterwheel and camera? Probably.

So keep that in mind. Nice filters but hard to mount and the manufactuer is not taking responsibility for the mounting of his product - that's your problem. Apogee are supplying me with a washer spacer kit so that is good of them doing Astronomik's job for him.
FLI sell filters for their large cameras and they work well and are proven. I think they may be Custom Scientific but I don't know for sure.

So it would seem to me the Baader's are probably the ones to get but they are new to the market and early days but on first light they appear to be the winners.

It is also interesting that different filters do give the same scope/camera combo a distinctly different look. My recent images using the Astronomiks gave me what I thought was more pleasing colour than what I was getting with an STL/Astrodons but it did take more fiddling with ratios etc and it did require normalising the LRGB masters before creating the colour image, something I never had to do with the Astrodons.

Greg.