astrophotography terms

Jingguo · Post by **Jingguo** » Sun Oct 03, 2004 11:59 pm

Hi guys. 1) I would like to know what is BACK FOCUS? i read a book and it states that to be able to place the image plane at least 5 cm behind the end of the eyepiece tube. 2) It also state that Newtonians may not be enough back focus for astrophotography, why is that so? I'm quite puzzle by the term back foucs. Hoping that someone can help me explain in detail. 3) Isit true that newtonians have to be modify to overcome back focus. 4) what does the term vignetting means? how does it affect the photograph? thanks alot.

Post by **perseid** » Mon Oct 04, 2004 2:18 am

Hi,
I dabbled in astrophotography for some time now and I hope I can provide you with some answers that make sense.

Back focus is the ability for the telescope to reach focus. To focus an object with your telescope for your eye is different than with a camera attached to it. Back focus is rarely an issue with refractor. But for newtonian this is a major issue. This is due to newtonian design. To keep the secondary small, the focus is kept at a minimal distance from the tube. Usually, this is in the number of 3 to 5 inches. For viewing, this is fine. As the secondary is smaller, you'll get a better contrast on planets. Now the problem comes in. When you want to attach your camera, you will first remove your eyepiece (for SLR type). You'll then find that to focus object on the camera, you'll need to rack in the focuser. Before long, you'll notice that you'd rack in all the way and still can't focus.
One way is to move your primary mirror up a couple of inch or so. You may need to change your secondary mirror to a larger one.
The other option is to change your focuser to low profile focuser, usually called the helical focuser.

I guessed I have cover question 1 to 3. For question 4, vignetting refer to the darkened effect around your image. To check for vignetting, just adjust the contrast of your image to maximum and you may notice that it appears darker at the edge than in the middle. Usually this is caused if you have a short focal length telescope or the secondary mirror is too small. Fortunately, vignetting can be reduced later in image processing.

Hope these answer satisfy you for now.

regards,
CK

Post by **weixing** » Mon Oct 04, 2004 12:56 pm

Hi,
I think this problem can occurs on all telescope if the telescope manufacturer does not take astrophotography into consideration when designing the telescope... unfortunately, it happen most on Newtonian

. I think this is because Newtonian don't use a diagonal, so as what perseid mention, in order to make the secondary mirror(reduce central obstruction) as small as possible, they design the focus point too "near".

By the way, my Sky-Watcher 6" Newtonian don't have this problem if I connect directly to a SLR, but may have this problem if I put an off-axis guider in between the SLR and the focuser. Anyway, can always use a guide scope for guiding

.

Have a nice day.

Kamikazer · Post by **Kamikazer** » Mon Oct 04, 2004 8:03 pm

in vignetting, how does it cause the images to darken at the field edge?

wont a off-axis guider cause a barren patch in photos?

Post by **perseid** » Tue Oct 05, 2004 1:43 am

Vignetting is caused by part of the light path that is blocked by the focuser (usually). To reduce vignetting, use a longer focal length telescope or simply crop off the edge.
The prism by the off axis guider will indeed block part of the light from reaching your film or electronic sensor.

Kamikazer · Post by **Kamikazer** » Thu Oct 07, 2004 10:27 pm

something about f-ratio:

assuming 2 different lens of different focal length, opened at the same f-ratio;
will the total amount of light collected be the same? (assuming all other factors constant)

Post by **weixing** » Thu Oct 07, 2004 10:35 pm

Hi,
The total amount of light collected is alway depend on aperture (the number of elements in a lens will also reduce a very small amount of light), not f-ratio.

Have a nice day.

MooEy · Post by **MooEy** » Fri Oct 08, 2004 7:43 am

hmm..on the subject of off-axis guider, so far i have only seen the celestron radial guider. i doubt it will vignette since it is oversized(it fits sct thread, which is bigger than the required 43mm). the prism cut off a little bit of the light which will not end up on the film.

~MooEy~

Post by **weixing** » Fri Oct 08, 2004 9:20 am

Hi,

assuming 2 different lens of different focal length, opened at the same f-ratio;
will the total amount of light collected be the same? (assuming all other factors constant)

Need to add something on my previous post. In this case, the longer focal length lens will collect more light than the shorter focal length lens, because the longer focal length lens will have larger aperture than the shorter focal length lens. BUT because the longer focal length also produce a larger(higher magnification) image than the shorter focal length lens, the final image produce by the longer focal length lens may not be brighter than the shorter focal length lens.

May be others who are more experience in camera may want to comment on this.

Have a nice day.

Kamikazer · Post by **Kamikazer** » Sun Oct 10, 2004 2:48 pm

sorry...i think i i didnt make myself clear.
let me rephrase:

take 2 scopes, a 4" f/5 and a 6" f/5 used for prime focus photography.
assume that narrower fov emits more light (to remove the effect of magnification on light), and the same shutter speed and film iso.

would both film have the same amount of saturation?