Barlow-ing Advice!!

[Lioness_Stargazer]

Good morning!

Last question for a while hopefully!

I attempted to image the planets with my 6se before I exchanged it for my C8.. I used my D5100 attached to a 2x Barlow and to be honest didnt get great results.. the image was small and no detail at all... I know webcams or ccd's are better for this kind of imaging anyway but as my dslr can take hd moves and can be broken down into invidual frames in registax as well, im giving this ago before I spend more money!

What I wondered is now that im using a c8 obviously the aparture is bigger but the focal lengh is still the same (f/10) will I get similiar results if i continue to use a 2x barlow or shall I invest in a 3x or even a higher one (4x/5x).

Any recommendations??

Thanks all!

Claire

[rowan46]

the problem with dslr on planets is the pixel size, because the pixels are so small the image scale is reduced its one of the reasons webcams are used it's not just frame rate they have larger pixels.

[Lioness_Stargazer]

Oh ok, I didnt know that! Thanks!

So same question applies -if i pick up a webcam/ccd would a 2x barlow be good enough to get a decent image with my set up or shall i go for a bigger barlow?

Thanks

[rowan46]

I only used 2x when I did it the more you magnify the dimmer your image. However I only have a 5" scope. an 8" scope has a lot brighter image

[Macavity]

There is a fair bit of discussion on this (sometimes). <G> My belief, as a rule of thumb, for planetary imaging, one should "oversample the typical resolution of your setup by THREE times". Queue web search?

The calculation is easier if you find a website to do it? I know that e.g. with my MAK150 plus Watec VIDEO camera (8.5 micron pixels) the basic scope config give approx. ONE arc second per pixel. So (with optimal seeing) I aim to use (say) a 3X Barlow... or an effective focal length of 5400mm - f/36.

I sense though, with the average Catadioptic scope, you would be fairly safe starting with a 2x - 3x Barlow? An f-number of around f/24-f/36. Other folks go MUCH higher... to f/60 even? If the default proves insufficient, try increasing the "power" of the Barlow with extension tubes. I recently bought a 2x, 2" Barlow, with *detachable* lens. It has filter threading, so I can use T2 extensions to adjust the "power" settings.

Sorry if the above is overly cryptic, but I [too] was trying to get to the "essentials".

Unfortunately the choice of Barlow is fairly scope / setup AND seeing specific?

[Lioness_Stargazer]

Wow... Er, thanks!!! ;)

Web search here I come...!

[Macavity]

Heheh. I'd imagine, with a Celestron C8(?) ... f/10; f=2000mm etc.

- A 2x or a 3x Barlow would be a reasonable START, whatever!

(Brightness of image [for focussing], UK seeing, might dominate?)

[StuW]

I found the only realistic way of getting planets with a dslr (or the case I tried, with a HD video camera) is to use EP Projection / Afocal. Luckily my Baader zoom has a thread specifically for this as I think most Baader EP's do. Not that I'm anywhere near competent with imaging of any kind

[NickK]

Hmm.

If the image of a planet is 1 cm diameter, then the smaller pixels only serves increase the resolution (the image covers more pixels) and the number of photons arriving at a pixel is reduced (because the pixel area is reduced).

A larger scope will allow you to capture more photons which allows you to reduce the size of pixels at the fill rate. Note that smaller pixels have a smaller capacity.

Putting a barlow/powermate in would simply increase the size of the image over the sensor. So the 1cm diameter planet is now 2 cm. The same amount of light exists between them so the image for 2cm barlow'd image is dimmer (longer imaging time) but more pixels are covered.

There is something called dawes limit, which is the limit at which a wavelength provides resolution. That is roughly R=11.6/D; so 11.6/20.0 = 0.58 arc seconds in an ideal environment. Smaller than this and your image becomes increasingly blurred, however it is still possible to compensate and even gain additional resolution by post-processing.

I can image a 0.80 arcsec when my scope's dawes limit is 1.10 arcsec with my 2x powermate. However the downside is a 4x increase in imaging time.

Lastly - focusing is a PITA once you start increasing the focal length. So think about motorised micro focusing.