doublet for widefield narrowband

[alistairsam]

hi,

I'm looking at getting a relatively inexpensive fast doublet to do widefield narrowband and was wondering if the skywatcher 102mm F5 scopes are any good.
I have a mono sbig with a filter wheel and I will be re-focusing between filters so would that be sufficient to remove CA?
What sort of flattener/reducer will I need?

I really want it below F4 but I can't find an inexpensive one that's native F5 or below.
I'm looking at either http://www.skywatcher.com/product.php?cat=3&id=39 or
http://www.skywatcher.com/product.php?cat=3&id=38

Will this reducer/flattener work with either of these scopes?
http://www.bintel.com.au/Astrophotography/Reducers--Correctors--Flatteners/Orion-0-8x-Focal-Reducer-br--for-Refractor-Telescopes/1184/productview.aspx

This would drop both F5's to F4 which is what I like to image at.

I'm using an STF8300m with an OAG and lodestar and the sharpsky focus controller.

Cheers
Alistair

[Profiler]

In terms of bang for your buck they are excellent - but - it is a fast Achro so expect optical performance imperfections (ie CA etc)

[JohnH]

Hi, there is no reason why you cannot use a scope like this for narrowband work - as you have spotted CA will not matter (SA will though) and, as you point out, you will need to re-focus between filters and that will be a pain...these scopes typically do not come equipped for imaging - the focuser may be poor and have flex and/or image shift when locking. You will be tempted to upgrade and maybe automate the focuser - as you will be hanging quite a bit of weight on there - so perhaps it is better to consider a scope built for imaging. And yes you will need a flattener too (or to crop your images) and these are best matched to the scope and are sensitive to spacing - thus you may find issues with accommodation of the FW and or OAG while still getting the FR spacing spot on, check the dimensions/specs on all the components before you buy.

[alistairsam]

Thanks John,

With SA, is this what flatteners fix? or is field flatness different to SA?

I thought of the focuser as that's usually the weak link in these scopes, but I was just looking at something as a portable secondary scope as my primary is a 10inch F4 with a motorized moonlite.

I will motorize the focuser for this and use threaded drawtubes all the way.
I will check the spacing for the reducer/flattener.
Its currently 55mm as I use a Baader mpcc with my OAG/filterwheel. I'm not sure if the flatteners need more than this.

Cheers
Alistair

[JohnH]

In my (very) limited understanding of optics SA is the error that causes the focal plane to be curved, all refractors will suffer from this, so yes this is what flatteners are intended to correct. An FR is another animal though and will make curvature worse (more of the outer parts of the field where this distortion is at its worst will now fall on your ccd). Some devices do both (WO spring to mind). It seems to be a bit of a black art with no system being perfect for all uses, so you may need to experiment and/or check what other users of the scope are doing for FR/Flatteners.. I used a 127ED with a mono ccd for narrowband it worked out ok but I always found Ha sharper than Oiii and with RGB B was always softer. Only way out of that is a mirror or large $$$....(shields up!).