New Meade Deep Sky Imager

swee sin · Post by **swee sin** » Wed Sep 15, 2004 6:31 pm

yes, this ccd imager is good when you need it for guiding your autostar compatible mount! you won't need to mod your webcam which can easily fry them during the process!

long exposure like 5min is not practical over here! light pollution coming earlier than that! plus with that resolution, it won't beat the qualtity of Afocal with digital camera! but if it can do 1min exposure, then sure it will be a good guider!

swee sin · Post by **swee sin** » Wed Sep 15, 2004 7:01 pm

but if it can do 1min exposure, then sure it will be a good guider!

sorry, should be "can do 10-20sec exposure without extra mod, then it would be a good guider"! 1min guiding interval period is too long!

Post by **weixing** » Wed Sep 15, 2004 7:40 pm

Hi,

correct me if i'm wrong, some high end ccd cameras are capable of doing imaging & guiding at the same time. i don't remember the exact details....

May be there are some ccd camera capable of doing that, but because most object is quite dim, so it is quite difficult to find a suitable guide star in the same field of view as the object.

Have a nice day.

MooEy · Post by **MooEy** » Wed Sep 15, 2004 8:02 pm

hmm..dunno y everyone thinks that astrophotography is abt 1hr of exposure. when we talk about exposure, 3 factors comes in. iso, aperture and shutter speed. they are often measured in stops.

iso is the speed of the medium, whether it's film or ccd. the higher the iso, the faster the speed. as i said earlier, we measure in stops. iso 100 is 1 stop slower than iso 200. while iso 400 is 1 stop faster than iso 200.

aperture is measure with the focal ratio, not just diameter. a typical fast camera lens is maybe f/1.4. a fast zoom lens is maybe f/2.8. but for telescopes, usually f/8 is consider slow, f/4 being fast.

for aperatures, each stop is 1.4x, ie f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16. if u notice, most poeple who actually expose for 1hr are using rather slow scopes of f/8. the medium that they use are also extremely slow, iso 100 and 200 stuff.

shutter speed i dun think i need to explain much, each stop is either doubling the time, or halfing the time. if i need 1hr exposure on iso 200 film at f/8, then by using iso 400 film i only need 30min.

another factor is the brightness of the object. most people who use extremely exposure shoot dim stuff. if u shoot brighter stuff u will need alot lesser exposure.

~MooEy~

swee sin · Post by **swee sin** » Wed Sep 15, 2004 9:06 pm

correct me if i'm wrong, from what i know, ccd imager especially webcam like ccd imager cannot change iso or apature value like other slr or digital camera! all they can do is to increase the time for the sensor to expose to the light in order to get more signal!
the apature value is depend on the scope you used which is the F number. sometime bigger scope enable to get more signal than small scope! so it mean more faster than small scope! so if same amount of exposure time on different setup will produce different result.

Post by **weixing** » Wed Sep 15, 2004 10:02 pm

Hi,

the apature value is depend on the scope you used which is the F number. sometime bigger scope enable to get more signal than small scope! so it mean more faster than small scope! so if same amount of exposure time on different setup will produce different result.

The F-number (f-ratio) is the ratio between focal length and aperture (focal length / aperture). A scope with a smaller f-ratio will require less exposure time than a scope with larger f-ratio (both scope with same aperture), this is because a longer focal length will generate a higher magnification, so the image is dimer than a shorter focal length scope (smaller magnification).

So generally speaking, a scope with a smaller f-ratio will require less time to attain a certain level of exposure than a larger f-ratio scope.

By the way, above is only true for prime focus astrophotography only.

Have a nice day.

carlogambino · Post by **carlogambino** » Wed Sep 15, 2004 10:05 pm

hihi, hope this is not off-topic, but wonder how you can increase magnification when doing prime focus astrophotography. Is it totally dependent on the focal length of the scope? Thanks.

swee sin · Post by **swee sin** » Wed Sep 15, 2004 10:12 pm

actually prime focus is refer to the lowest power of magnification which the scope can produce while imaging. so if want to increase the magnification, then barlow/teleconverter is needed for this! but that time it's refer as negetive projection(according to astrophotography for the amateur book).

MooEy · Post by **MooEy** » Wed Sep 15, 2004 10:26 pm

use a medium with a smaller size lor. a piece of 35mm film measures 43mm diagonally. if u use a dslr, it will have abt 1.6x magnification compared to the 35mm film. if u use ccd with small sensor, it will be even more magnified.

~MooEy~