Which scope for VA?

[Stu]

Just a general question really. My assumption is that what works for imaging also works for VA? Is this correct?

So...a fast scope gathers more light more quickly over a wider area so will be good for getting bright views more quickly? A longer focal ratio scope for smaller targets such as galaxies?

Do people use power mates/barlows to increase image scale where required?

I have a nice Sky 90 with focal reducer/flattener to f4.5 which seems like it might be nice. How about the Edge HD (non reduced at f10), or even the Vixen 150ED at f9?

Thoughts on a postcard please.....

....or just reply to the thread [emoji6][emoji12]

[nytecam]

Deep sky objects due to their faintness need fast optics in VA/EAA to match the typically brief exposures (5s - 60s) and usually achieved via focal reducers in the f/5 - f/3 range. AP, with l-o-n-g exposures typically >20min, is not directly comparable - treat VA/EAA as a different field.

Personally I use my SCT occasionally at native f/10 on bright and small planetaries but gx always @ f/3.3 irrespective of their size. With the sole exception of M31 most gx fit the 1/2" sensors typically used. M31 best with a fast long camera lens and not a telescope. My deepsky gallery linked below :-)

Good luck - experiment is the best tutorial:-)

Nytecam

[Macavity]

II would much echo the above. You rarely go wrong with fast optics.

For general DSOs/galaxies, I find my 8" f/4 (800mm) Newt about right.

Right price, size, field etc. But not without "mechanical" challenges?

Small video chips are anyway tolerate of aberrations tho'! Obviously,

the same could be achieved with a *focal reduced* (larger aperture)

SCT. You can of course easily/substantially focal reduce MAK/SCTs.

I have e.g. some useful shots of Uranian satellites with f/12 MAK150,

but it struggles to image DSOs beyond "globulars and planetaries"?

However greater than 8"(?) scopes always have... "Grunt" power?

For wider fields, I use a fairly generic 66mm f/6.5 f/400mm ED APO.

Mostly because it accommodates the full lunar / solar disk on typical

1/2" video chips. For more oomph, get a 0.8x reduced 80mm APO?

For even wider fields, I bought a Canon 70-200mm f/4 Zoom lens.

I may yet buy a standard/zoom camera lens (Camera body even!)

But really that's all I intend to buy re. video astronomy "scopes"...

Of course, if I had a really LARGE Dobsonion etc., I'd be tempted

to try it out somehow for VA, whatever the technical difficulties...

[Stu]

Thanks both, very useful info.

I have quite a few options to try. As a portable setup, the Tak Sky 90 with reducer to f4.5 is probably the first I will try, hopefully the StarAdventurer will handle it although it's quite heavy.

So, does aperture count in the same way that it does for visual, or not? For instance I have an f6.3 10" dob which I could stick on an EQ platform, how is that likely to perform?

Forgive the questions, I take heed of the 'just go and experiment' advice, but just was interested in understanding the basics.

I believe the EdgeHD needs a super duper expensive reducer so I won't be doing that until the lottery win (next week [emoji6])

[Macavity]

There isn't too much definite information? But here's a table

from Massey & Quirk's fine book "Deep Sky Video Astronomy".

http://www.amazon.co.uk/Deep-Sky-Video-Astronomy-Patrick-Practical/dp/0387876111/ref=sr_1_1?ie=UTF8&qid=1445514823&sr=8-1&keywords=deep+sky+video+astronomy

This gives limiting (stellar?) magnitudes for typical VA setups...

Perhaps if I reproduce (part of it) here in return for publicity?

12" f/4.5 reflector +16.911" f/10 SCT        +16.210" f/5 reflector   +16.28" f/10 SCT         +15.58" f/5 reflector    +15.88" f/4 reflector    +16.26" f/5 reflector    +15.63" f/5 refractor    +14.2300mm f/4 lens      +13.6135mm f/2.8 lens    +12.88mm f/0.8 Vid Lens  +8.5

(Sorry about the psychadelic colours! lol)

But I think it gives an indication of the general dependency

on both the aperture and f-number. As we know, a fraction

of a magnitude can mean a *lot* more objects visble....

Yet you have to consider surface brightness for extended

objects -- Once you get over the threshold for the typical

galaxy... +12 to +14, many scopes work perfectly well.

I have no doubt an 10" f/6.3 Dob would be a fine thing.

If there is sufficient in-focus, you could try to *modestly* 

focal reduce it? Or simply buy a more sensitive cam!

There are quite a few variables in this, as you imagine?

But, for a few minutes exposure, to get some flavour of

a virtual scope of 2-3x any aperture can't be too bad? 

[HiloDon]

Hi Big,

I believe you're going to use the SCB2000. That was my first cam and it was a good choice. It has a 1/3" sensor that gives a pretty narrow FOV. Your choice of starting with shorter faster optics is a good one. A focal length of 400mm with the 1/3" sensor will give a nice FOV for many objects and will be easy to track. The Edge and Vixen will require some serious focal reduction unless you want to get objects like planetary nebulae. Tracking with those lengths will also need more attention.

If you really get into EAA, your Edge will be ideal with Hyperstar and the Lodestar or Ultrastar cameras. At F1.9 or 2.0 and those cams, your views will be real near real time.

Hope this helps. Let us know how things work out.

Don

[Stu]

Hi Big,

I believe you're going to use the SCB2000. That was my first cam and it was a good choice. It has a 1/3" sensor that gives a pretty narrow FOV. Your choice of starting with shorter faster optics is a good one. A focal length of 400mm with the 1/3" sensor will give a nice FOV for many objects and will be easy to track. The Edge and Vixen will require some serious focal reduction unless you want to get objects like planetary nebulae. Tracking with those lengths will also need more attention.

If you really get into EAA, your Edge will be ideal with Hyperstar and the Lodestar or Ultrastar cameras. At F1.9 or 2.0 and those cams, your views will be real near real time.

Hope this helps. Let us know how things work out.

Don

Thanks very much Don. I'm fairly sure I will stick with the Sky 90 for quite a while just to get going and understand what I'm doing. The Vixen sits very nicely on the AZEQ6 though, so I might give that a go at some point. I run it in AZ mode, but I doubt field rotation would be an issue at 10s exposure would it?

Can anyone confirm the sensor size of the SCB2000. A friend looked it up for me last night at 4.54mm x 3.42mm which I assume is correct? The fov doesn't look too bad so I should be ok.