Which barlow?

[Stephen Greenfield]

Was trying to image the ring nebula last night on my equinox 80, it was extremely small and so though would try the 5x barlow i have , to no avail couldnt get it to focus

so , which Barlows would be best (above a x2) for the

Equinox 80 apo

Celestron C8

for astrophotography

[OzDave]

so , which Barlows would be best (above a x2) for the

Equinox 80 apo

Celestron C8

for astrophotography

For the eq80, which i also have, probably you are pushing it with anything above a 2x barlow. You will just be exceeding the maximum useful magnification of the scope. Consequently the ring nebula isnt the best target for this scope as it is quite small. The eq80 is much better on bigger DSOs like m31 or m42.

To do better with smaller DSOs like the ring nebula, you need a scope with a longer focal length, but also ideally a bigger aperture so it will have a fast enough focal ratio.

David

[dmahon]

I thought the point behind a Barlow was to increase focal length (so you don't need to buy a scope with a longer focal length)?

Would an 80mm aperture scope with a focal ratio of 20 be better than an 80mm scope with a focal ratio of 10 using a 2x Barlow or one with a focal ratio of 5 using a 4x Barlow (assuming a top quality Barlow)? What were high powered Barlows (x5 for example) designed for? If they were intended for use with Fast scopes, and, for example, you can use a x5 Barlow on an F4 scope, you should be able to use a x3 on an F6.25 scope (Equinox 80), no?

[OzDave]

Yes, Barlows do increase focal length, but you can't just arbitrarily increase it in this way without hitting some limits.

Firstly, when you put in a Barlow it increases the focal length and at the same time spreads out the light cone more, meaning that the image is less bright. Thus, if you are shooting DSOs, you'll need to expose for much longer to capture the same number of photons as you would without the Barlow.

I think on the EQ80, the maximum Barlow you could hope to get working is about 3x. But in order for this to work, you'll need exceptionally good seeing so that you can focus. If the seeing isn't up to it, the Barlow won't work. And in any case, you'll need to be exposing about 9 times as long with the 3x Barlow to get a decent image captured.

I'm not an expert on Barlows, but they tend to get used more for planetary imaging where the targets are quite bright. Therefore, when you spread out the light cone with the Barlow the reduction in brightness isn't such a big deal because you are typically exposing for less than a second anyway on these targets. Also, when you do planetary imaging, you generally capture thousands of frames in a movie and use something like Registax to produce your final image from that. Most of your frames might suffer from poor seeing with a big Barlow in place, but every so often, you get lucky and 1 frame jumps into perfect focus for long enough to get a great image. Thus, the hope is that with thousands of frames, you'll get enough of those moments to stack up a great image.

So, although it would be great to just be able to add a Barlow to the EQ80 and get excellent results when imaging targets like the Ring Nebula, in practice it just doesn't really work out for the reasons I've outlined above. Give it a shot though (maybe with a 2x Barlow) and you'll see what I mean, it's a good learning experience - and for me meant that I wanted a second scope that was good for smaller DSOs.

To answer your question about scopes, all the combinations would be the same since they have the same focal ratio in the end. In order to get a decent size image of the Ring Nebula you'll need a longer focal length *AND* and larger aperture. Otherwise, you'll end up with a small image of the nebula in a widefield and you'll have to expose for quite a long time to capture much detail.

David

[Stephen Greenfield]

so , the celestron c8 is the better option for the ring nebula?

[OzDave]

Well, given the choice of scopes, the C8 is probably better. However, at f/10 it's on the slow side for DSO imaging. However, with it's long focal length, you'll likely get a decent size image of the Ring Nebula.

What you might want to consider in the future is a focal reducer. Celestron make an f/6.3 reducer which is designed to convert your scope into an f/6.3 focal length which is much better for DSOs. It will make them look a bit smaller, but they will be brighter and you can expose for less time. The downside is you'll probably get some vignetting around the edges of the image, but that's probably ok.

David