3" refractors for astrophotopgrahy

[Jingguo]

hi guys. i'm just wondering if aperture is an important factor for astrophotography or is f-ratio more important? Any1 try using 3 inch refractor or deep sky photography? thanks alot.

[MooEy]

http://www.tmboptical.com/picsRecord.asp?pic_id=24&

~MooEy~

[weixing]

Hi,

hi guys. i'm just wondering if aperture is an important factor for astrophotography or is f-ratio more important?

Both are important if you are using prime focus.

Any1 try using 3 inch refractor or deep sky photography? thanks alot.

If you are imaging those large DSO or wide field astrophotography, you only need a small scope... sometime small scope is prefer, becuase if the object is big and in order to frame the whole object, you need a shorter focal length or you need a very fast scope if the scope aperture is large. In fact, a lots of nice image out there are taken using small good quality scope.

http://www.tmboptical.com/picsRecord.asp?pic_id=24&

Ya... The scope is small... a very expensive small APO scope and expensive CCD camera and mount.

Anyway, all scope can be use for astrophotography and it all depend on your expectation of the result image. I think the general rule is: it is easier to produce good image with good (also mean expensive) equipment.... That's why they said astrophotography is for those with great patient and deep pockets.

Have a nice day.

[ariefm71]

http://www.tmboptical.com/picsRecord.asp?pic_id=24&

~MooEy~

The same image taken from a much cheaper scope (a C8 ): http://www.ghg.net/akelly/m8lrgbl.jpg

Other images taken from a C8: http://www.davidbarmore.com/

Of course, the mount used may cost more than 5 times the scope :-)

[MooEy]

for astrophotograhy, no point comparing sct to apos. the picture from the c8 doesn't put up any fight la, just look at the size of those stars. the apo gives very small dots whereas the stars from the c8 looks like blobs. details from the tmb is way better than the c8.

but for visual, dso purposes, i must agree the c8 would show more than wat a 3" apo would show.

the SV version of the tmb 80/6 doesn't cost alot more than the c8 anyway. roughly 1.8k with the feathertouch focuser, whereas the c8 is prolly 1k, reducer and dewshield sold separately.

for astrophotography purposes, the 80/6 would work well with a smaller ccd like the starlight xpress, whereas the c8 would be needing a larger size ccd.

~MooEy~

[weixing]

Hi,

for astrophotograhy, no point comparing sct to apos. the picture from the c8 doesn't put up any fight la, just look at the size of those stars. the apo gives very small dots whereas the stars from the c8 looks like blobs. details from the tmb is way better than the c8.

I don't agree with what you say about that an APO will give a "smaller" stars image than a SCT. As we know, all stars should be pin-point, but due to the resolution of the scope, it appear as a small dot. And resolution is related to the scope aperture.... The larger the aperture, the higher the resolution. As a result, a scope with larger aperture will give a "smaller" stars.

As why the stars appear as a larger dot in the image is because the long exposure will make brighter star "bigger". Look at the dim stars in the image background, they are just a small dot.

Have a nice day.

[ariefm71]

not only resolution, contrast is also a function of aperture.

As for the effect of central obstruction to contrast, the following article explains it all:
http://perso.club-internet.fr/legault/obstruction.html
in short, an 8" scope with 33% c.o. will have the same contrast as a 5.3" apo, an mn56 with 20" c.o. will have the same contrast as a 4" apo, etc.

On the other hand, resolution is not affected at all by obstruction.

So, not to go off-topic, for astrophotography, the quality of optics and mount is more important than aperture. of course, for visual, NOTHING beats aperture :-)

[MooEy]

zzz can't be bothered arguing. the amount of nice astrophotographs i have seen taken with refractors are way more than sct. of course, small objects like gobulars taken with RC looks gd too.

~MooEy~

[ariefm71]

to the AP gurus (jy, tg, mcym, etc):
can give us step-by-step tips on how to capture RAW images and process them to get this kind of image?

http://www.astromart.com/classifieds/de ... _id=328646 ?

cheers,
arief

[JY]

Hi,

That's an impressive image !

After trying several sotwares to develop and proces raw images, I have settled with ImagePlus.

The BIG plus of this software is that it comes with a tutorials DVD, hours of tips to develop, align, rotate, stetch, ... the images. Each menu is explained as a tutorial, very Neat.

Now if we could get rid of the clouds !

To step in the discussion:
- To take nice wide field images, go for a small APO
- To take interesting images of faint galaxies, go for a SCT

Cheers

Jean-Yves