120mm reflector plus powermate for small objects??

[Pomerol]

Have become reasonably proficient at photographing the larger nebula/galaxies with my F4 200mm Newtonian/Canon 450d combination.

I would like to go for some of the smaller targets and am thinking about buying a Skywatcher ED 120Pro and a x2 Powermate.This would give a 42x28 arcmin FOV compared with the 95x64 with the Newtonian.

Would welcome any advice about this combination or would welcome any alternative suggestions to achieve the same end result.

Thanks

[Macavity]

Another possibility might be a Maksutov? If you're actually going to "Barlow" the refractor to F12-15, the MAK would take you there, by default. And maybe even give a few more inches of aperture, for the price...

[ollypenrice]

No, no, don't do it like that! For the deep sky you absolutely must have a fast f ratio, especially with an uncooled camera with which you cannot simply go on increasing exposure time. (On a DSLR the heat and noise build up too much.)

At the moment you are riding high on F4, very fast. F8 would give you a quarter the signal on DSOs in the same time. Signal on DSOs goes as the square of the f ratio so you can see that f15 or so would be quite impossible. That is why there are no Barlows in DS imaging except very occasinally if the basic scope is very fast to start with and the imager has a bright target.

So to get more image scale you need more focal length but, alas, more aperture as well to sustain a fast f ratio. My own personal limit for DS imaging is f7.5 and I would much prefer to be faster than that. You have been spoiled by f4!

Olly

ollypenrice's Photos

[Whippy]

IIRC, SteveL did a cracking M13 with a MN190 and x2 powermate, that's f10.6 so it is do-able but I would imagine it'd be limited to the brightest targets. I think your problem may lie in getting a barlow big enough (2" minimum I guess) not to show any significant vignetting with your DSLR.

Tony..

[ollypenrice]

IIRC, SteveL did a cracking M13 with a MN190 and x2 powermate, that's f10.6 so it is do-able but I would imagine it'd be limited to the brightest targets. I think your problem may lie in getting a barlow big enough (2" minimum I guess) not to show any significant vignetting with your DSLR.

Tony..

Yes, it was Steve's M13 I was thinking of when I included my caveat. Wonderful image. Rob also barlowed his scope for a bright planetary. It is very much an exception, though, and Barlowing an F7.5 ED120 would take you to f15 and that really would find very few deep sky targets especially uncooled.

Be aware, also, that the need for accurate guiding goes up dramatically with focal length, and also with exposure time, so if you increase FL and exposure you face a double whammy on the guiding.

Olly

[Pomerol]

Thanks for the good input. Spent a couple of hours yesterday searching the available reflectors for anything faster than F7.5 at a reasonable price, most were F7-F10.

My conclusion from all this is that the 120/powermate would be ok for planetary/moon etc but to do any quality work on DSO, a cooled camera with longer exposure capability is essential.

[ollypenrice]

But I'm not sure why you want a refractor if you want to pump the focal length for small targets. The strength of the ED120 is that it gives excellent results at its native focal length/focal ratio.

Have you thought of the Skywatcher Mak Newtonian which runs just over f5 at a metre of focal length. In extremis that can be barlowed to just over f10 which, though slow, is not unthinkable.

It needs at least an NEQ6 mount and certainly autoguiding. Guiding at 2 metres of FL is not within the comfort zone of an NEQ6 but may be possible with care.

Olly

[Pomerol]

Olly, hadn't really thought about using a Barlow on an MN 190 and being able to achieve F10 plus. I am using an EQ6 and guiding with PHD on a fixed pier so "technically" shouldn't be a problem.

Have just been reading about the MN and reviews are good with "no coma" well advertised. As I still suffer from this (enough for me to notice) on my TS F4 200mm even using a Baader MPCC I am keen to avoid it in any future purchase.

I am cetainly favouring the MN solution..............will let you know where the money goes.

Many thanks for your constructive input.

[Pomerol]

Thanks for all input.............have ordered the 190mm M-N plus powermate.