Barlow - focal ratio for planetary imaging with SCT

[riklaunim]

I switched from fast newtonian to 8" slow SCT so 5x Barlow won't work. I'm looking now at two Barlows for planetary imaging:

- GSO APO 2.5x Barlow

- UO Klee 2.8x Barlow, second hand

Looking at few equations at f/20 I reach max details with 5,6x5,6 pixels so f/25 or f/28 is slightly bigger than that while it should still keep good view quality. Any advices on those two or something similar below 3x?

I've seen also some images from overscaled setup at f/35-40 but thats probably will work nicely only with newer CCDs like the ICX618

And by the way - how well does SCT keeps parfocal filters trully parfocal?

[brianb]

I have enough trouble finding seeing good enough to justify f/20 (with the DMK21 i.e. 5.6 um pixels). If I could find a decent 1.5x Barlow that worked at 1.5x not 2x with the extra extension of the filter wheel I'd be very pleased.

I find the Celestron Ultima 2x is pretty good, it's what I do most of my solar work with. The 4x Imagemate I get on quite well with too but it's too much for the SCT. I've had a couple of 3x barlows I haven't got on with (Meade series 5000 3x tele-extender, Antares 3x barlow) and will probably buy a 2.5x Televue Powermate to get a bit more power than I get with the Celestron.

Don't know whether I'm helping or confusing you so I'll shut my trap

Except just to say that I've found that my "parfocal" Astronomik type 2c RGB filters are so close to parfocal in my SCT that it's not worth the effort of attempting to refocus after a filter change. I usually just focus through the red filter & let the others look after themselves.

[riklaunim]

There is Antares 1.6x but 2". I'm bit angry at GSO. They make a lot of barlows. All of them are ED/APO... except 2x in 1,25"

[brianb]

There is Antares 1.6x but 2"

Got one (it's branded "Proxima 1.5x" but appears to be identical). Slightly over 2x with the extra extension of my filter wheel. Much prefer the Televue 2x Powermate when used this way.

[TheThing]

Have you tried the Astro Engineering AC555? 1.5x and screws into EPs, web cams etc.

[brianb]

Astro Engineering AC555? 1.5x and screws into EPs, web cams etc.

Got one of those rattling round somewhere ... If I'd been impressed it would be in the "ready to hand" accessories box. Gave rather more than 1.5x screwed straight into the camera nosepiece but did nothing for the optical quality of the image, in fact there was more resolution with the thing removed.

If Al Nagler is listening, I want a 1.5x Powermate - preferably in 2" fitting but I'll accept 1.25", T thread or C mount threads.