A few images taken with SDM Dob

[erick (Eric)]

Re that Ghost of Jupiter.

That beautiful image - done and dusted in under a minute - amazing!

[telecasterguru (Frank)]

Just amazing. wonder how this setup would work with planetary? Maybe a Toucam or the like?

[alexch (Alex)]

Originally Posted by telecasterguru

Just amazing. wonder how this setup would work with planetary? Maybe a Toucam or the like?

I have DMK21 with a filter wheel and was planning to give it a go later

[Dave47tuc (David)]

Originally Posted by alexch

I have DMK21 with a filter wheel and was planning to give it a go later

I think the results will be I can not wait to see how it goes

[renormalised (Carl)]

A small, light CCD camera like an Atik 314L+ would be better than the DMK for DSO's, but the DMK would produce pretty spic pics.

Or you could try a Mallincam or GStar camera (good for live viewing).

[iceman (Mike)]

I'm amazed at how good these are! 22" brings in a lot of light, such short exposures and so few of them!

You're obviously very talented and any sort of photography is no problem!

This is now IOTW.

Cheers

[alexch (Alex)]

Originally Posted by iceman

I'm amazed at how good these are! 22" brings in a lot of light, such short exposures and so few of them!

You're obviously very talented and any sort of photography is no problem!

This is now IOTW.

Cheers

Mike, you are too kind.

Thanks for IOTW!

Cheers,
Alex

[Big Dave (Dave)]

Well done Alex.

[prokyon (Werner Probst)]

Wow, great! I like the way you make your pics.

[paulF (Paul)]

Congratrs Alex on those mind blowing images!

[sasup (Stacey)]

ok im blown away.

[Lester]

You are on a winner Alex, thanks for the wonderful views, and all the best for more top images.

[peter_4059 (Peter)]

Impressive for such short subs - well done.

[Satchmo]

I am not a photographer ( and always seeking to clarify what I do know ) , but seems some posters are attributing the depth of the images in such a short exposure being attributable to the 22" aperture.

As far as I know , any lens or scope operating at the same F# ( in this case F 4.14 with the coma corrector) using the same camera , exposure times and stacking and processing techniques would get the same results, albeit with a different image scale. I think it is the image scale of these images ( which is attributable to the 22" aperture) that is impressive about these short exposures.

[strongmanmike (Michael)]

Originally Posted by Satchmo

I am not a photographer, but there seems to be a bit of confusion here amongst some posters attributing the depth of the images in such a short exposure being attributable to the 22" aperture.

As far as i know , any lens or scope operating at the same F# ( in this case F 4.14 with the coma corrector) using the same camera , exposure times and stacking and processing techniques would get the same results, albeit with a different image scale. I think it is the image scale of these images that is impressive.

Oh oooh open a can of worms

In a nut shell, limiting magnitude of point sources is aperture dependant. Faster focal ratio = higher signal to noise in a given exposure time on extended diffuse objects.

so you want aperture and fast focal ratio

Mike

[jjjnettie (Jeanette)]

Light Buckets Rule!

[Satchmo]

Originally Posted by strongmanmike

Oh oooh open a can of worms

In a nut shell, limiting magnitude of point sources is aperture dependant. Faster focal ratio = higher signal to noise in a given exposure time on extended diffuse objects.

so you want aperture and fast focal ratio

Mike

oooh perhaps better in another thread then.

I guess I was referring to the speed of imaging extended stuff. So , with point sources like stars its the increased focal length that allows us to image fainter stars ( I assume that deepest magnitudes can be reached by increasing exposure time for a given aperture ) So given we want as fast a system as possible for extended objects, fainter limiting magnitude at a given exposure is reached via longer focal length gained by keeping F # constant and increasing aperture?

[alexch (Alex)]

Originally Posted by Satchmo

oooh perhaps better in another thread then.

I guess I was referring to the speed of imaging extended stuff. So , with point sources like stars its the increased focal length that allows us to image fainter stars ( I assume that deepest magnitudes can be reached by increasing exposure time for a given aperture )
So given we want as fast a system as possible for extended objects, fainter limiting magnitude at a given exposure is reached via longer focal length gained by keeping F # constant and increasing aperture?

Not quite, as far as I understand with point sources it's the aperture that defines the deepest magnitude and it is independent of focal length. For example my 14mm f/2.8 lens with 90mm front element gets fainter stars than 35mm f/1.4 lens with 55mm front element with the same camera and exposure length (although this is subjective - I have not done any measurements)
Somewhat outdated but still relevant info:
http://www.eskimo.com/~rachford/widefield/calc.html

[Edit: added another link]
http://starizona.com/acb/ccd/projectsfaint.aspx ("Capture the Deepest Amateur CCD Image" paragraph)
Alex

[Satchmo]

Thanks Alex. Great links. It makes sense as the star is a blob of light at the focal plane that is swollen by seeing , tracking errors and optical errors. At the end of the day its a photon stream that increases in brightness with aperture. Just out of interest what is your scale of pixels /arc second with this camera /scope combination and what sort of FWHM were you getting on the night you took these exposures ?

[clem]

Wow! Aperture envy! Must get to making my own large aperture Dob, pronto! That mirror hanging on the wall in my bathroom must be bigger than 30 inches, I wonder if that will do? :-)