Would this work - Advice please

[adamsp123]

This is a follow up thread that I posted earlier regarding purchasing a long focal length scope for Galaxy imaging and for other small targets.

Having got some good advice I realised that scopes like RCs, Meade AFCs etc were just too expensive so I was considering a Skywatcher 300 F5 Newt but that turned out to be 25Kgs and just too heavy, lighter OO models going back to being too expensive.

One suggestion was to use a Barlow and this got me thinking - and hence the question...

What if I combined a F4 imaging Newtonian with a 2X Barlow?

I do have an F4 200mm GSO Newt that I could try this out but long term I would look at getting the SW 250mm F4 Imaging Newt.

Before buying an ED Barlow would this combination actually work?

Would I be able to focus?.........Does the barlow throw the focus point further out or in??

Would I need a coma corrector at F8?

Could I get decent clear images 2x binning my Atik 383L+ camera? (thus bringing back the "speed" of the system)

Finally would the Skywatcher ED barlow be adequate?

Thanks for looking and any advice much appreciated

[Bloom]

Well, if you bin the camera, I guess it will work. But if you first use a 2x barlow and then bin the camera, you just return to the initial image scale you had , without the barlow lens and without binning.

You do not mention it, but it seems you want to go to an image scale smaller than 1 arcsec/pixel. If something like an RC or a catadioptric telescope is expensive, why don't you consider changing your camera for a new Atik model, with smaller pixels?

I try my hand with galaxies a lot. I use a 6 inch f/5 newtonian and an Atik 450, which gives a decent image scale of 0,95. From my experience, galaxies are demanding! They do not really need narrowband filters, but they have low surface brightness. These things are really faint. Because they are extended sources of light they need significant amounts of capturing time; but above all, they need fast focal ratios. So, using a barlow is not the best of ideas.

Using a barlow lens, even if it is called "ED" may create a new issue: colour fringing. Even if it is ED, some colour problems will occur.

[ollypenrice]

You mention galaxies 'and other small targets.' If the other small targets are bright planetaries then I think your Barlow idea might work. I've seen that done on here, I think by Rob Hodgkinson. But as already stated, galaxies are faint.

You can think in terms of numbers if you like, 'resolution' being defined in arcseconds per pixel, but I don't believe it works like that. It would work like that if you always had enough signal to reach the limit of your system but if your system is F8 you're going to need a boggling amount of data to get there. Take out the Barlow, get four times as much signal in the time, and push the sharpening in post processing and this might get you more real detail ('resolution' is a word with too many meanings) than the longer focal length system with inadequate data.

The more AP I do, the more I want a fast F ratio. The last thing I want in my box is a Barlow and I don't want an F8 scope. F7 is bad enough...

Olly