Jump to content

what does this mean

steve30b

Recommended Posts

Stu

Stu

Posted
Posted

It means the focal length of the mirror is 11.8 times longer than the diameter of the mirror.

Yep, it is important in terms of defining the magnification you will get with different eyepieces. The scope has a focal length of 1500mm, with an aperture of 127mm making it f11.8.

A 20 mm eyepiece will give you a magnification of 1500/20=75.

A shorter focal length would give you lower mag with the same eyepiece.

The benefit with your scope is you can get higher mags with longer focal length eyepieces which tend to have better eye relief so are more comfortable to use. The downside is that you can't get very wide fields of view but to be honest, most objects will be fine if you use say a 32mm as a low power.

Cheers,

Stu

Link to comment
Share on other sites
ollypenrice

ollypenrice

Posted
Posted

If you are a visual observer this number is of only passing interest. You'll be more interested in the field of view and magnification that different eyepieces will offer in your telescope.

If you have the intense misfortune to become addicted to astrophotography (as I have!) then the Focal Ratio becomes a matter of obsessive concern since (ignoring some other complexities) it determines the length of exposure you will need. F5 is four times faster, photographically, than F10. A ten hour image becomes a forty hour image. *

Olly

*I do understand the 'F ratio myth' so am assuming a co

nstant focal length. My aim is to help the OP, not get involved in this famous generator of hot air!!

Link to comment
Share on other sites
JamesF

JamesF

Posted
Posted

As everyone else has said.

If the aperture of the telescope is a (in mm or whatever other units you prefer) and the focal length f in the same units, then a = f / 11.8 in this case.  The smaller that number the "faster" the telescope and the larger the "slower".  Faster telescopes are often very demanding of optics generally used for wide fields of view and viewing or imaging extended objects such as galaxies and nebulae.  Lower quality fast scopes can show significant colour abberation where different parts of the spectrum do not focus in the same place, leading to colour fringes in the views.  Slower telescopes can be much more forgiving of optical quality and are often used for high magnification viewing or imaging of targets within our solar system.

Those are guidelines more than actual rules, mind.  I've used my 127 for viewing planets, the Moon, the Sun (with a suitable filter) and deep sky objects as well as imaging all of those too.  As long as you accept that it has its limitations it's a very capable little scope.

James

Link to comment
Share on other sites
Paul73

Paul73

Posted
Posted

Interesting answers from knowledgeable people - Cue the tedious question from the imaging simpleton.

All of my photography to date has been done in daylight where the general idea is to get as much light into the camara as possible so that you have more options re. magnifcation, depth of field, shutter speed, sensitivity etc. I've never quite understood why a fast scope which typically capture a lot of light is worse than a "slow scope"? Surely with the tiny amount of light on offer, you would want to catch as much of it as you can?

So would a really good quality fast scope beat a similar priced slow scope in terms of imaging potential?

Is there a two sentence answer or a thread / website that could put me on the right track?

Please forgive the basic question but imaging is an uncharted ocean for me.

Paul

PS. I know that my scope is probably the least suited for imaging. But I am going to bolt an SLR on the side and see what can be done.

Link to comment
Share on other sites
JamesF

JamesF

Posted
Posted

There's a balance to be had really, much as in daytime photography, but the limitations are rather different.

With DSO imaging you'll generally be attempting to capture light from very faint objects, so lots of aperture helps.  But you also often need a wide field of view which means a short focal length.  This is where you end up with a fast scope.  There's not much you can do about the required field of view and therefore the focal length, but if the aperture is small you can make up for that with longer exposures which is how people often end up imaging with the likes of the ED80.  Ten minute exposures are not at all uncommon and there are those who may use single frames of 30 minutes.

With planetary imaging you're at the other end of the spectrum.  You want loads of focal length giving a narrow field of view because the targets are very small -- Jupiter at its nearest for example is still almost two hundred times smaller across than M31.  But because images need to be taken very fast to prevent atmospheric distortion being visible at those kinds of image scales you also want lots of aperture to get enough light for short exposures to work.

For DSO imaging there are people who are happily using sub 100mm aperture f/4-ish scopes, whereas for planetary work people often want the largest aperture they can get (250mm up) and f-ratios of anything between f/20 and f/40 depending on the pixel size of the camera.

Imaging anything at all with a non-motorised dob is going to be a real challenge, mind.  A driven equatorial platform would probably be the place to start.

James

Link to comment
Share on other sites
steve30b

steve30b

Posted
  • Author
Posted

It means the focal length of the mirror is 11.8 times longer than the diameter of the mirror.

thanks for the info. i have only got the stock EP. 25 and 9. if i setup the scope to lets say the moon. how do the two EP differ when looking at the moon ?. is one a smaller image.. which is the best one to start with ?. daft question i know but i have only just got my scope and its been too cloudy to use it.

Link to comment
Share on other sites
Stu

Stu

Posted
Posted

Start with the 25mm as that will give the lowest mag, and more importantly the widest field of view. This will make it easier to find the moon in the first place! Then switch to the 9mm for higher power views.

If you have a finderscope or red dot finder, then align this on a distant object (like an aerial or tree) during the day so that you can use it at night.

Stu

Link to comment
Share on other sites
Paul73

Paul73

Posted
Posted

There's a balance to be had really, much as in daytime photography, but the limitations are rather different.

With DSO imaging you'll generally be attempting to capture light from very faint objects, so lots of aperture helps. But you also often need a wide field of view which means a short focal length. This is where you end up with a fast scope. There's not much you can do about the required field of view and therefore the focal length, but if the aperture is small you can make up for that with longer exposures which is how people often end up imaging with the likes of the ED80. Ten minute exposures are not at all uncommon and there are those who may use single frames of 30 minutes.

With planetary imaging you're at the other end of the spectrum. You want loads of focal length giving a narrow field of view because the targets are very small -- Jupiter at its nearest for example is still almost two hundred times smaller across than M31. But because images need to be taken very fast to prevent atmospheric distortion being visible at those kinds of image scales you also want lots of aperture to get enough light for short exposures to work.

For DSO imaging there are people who are happily using sub 100mm aperture f/4-ish scopes, whereas for planetary work people often want the largest aperture they can get (250mm up) and f-ratios of anything between f/20 and f/40 depending on the pixel size of the camera.

Imaging anything at all with a non-motorised dob is going to be a real challenge, mind. A driven equatorial platform would probably be the place to start.

James

Link to comment
Share on other sites
JamesF

JamesF

Posted
Posted

Start with the 25mm as that will give the lowest mag, and more importantly the widest field of view. This will make it easier to find the moon in the first place! Then switch to the 9mm for higher power views.

If you have a finderscope or red dot finder, then align this on a distant object (like an aerial or tree) during the day so that you can use it at night.

Stu

And if you divide the focal length of the eyepiece into the focal length of the telescope that will give you the magnification.  A rule of thumb is that magnification beyond twice the aperture of the telescope in mm is a reasonable maximum, so with the 127 Mak and a focal length of 1500mm the maximum magnification is around 254x, whilst your 25mm eyepiece will give you 1500/25 = 60x magnification and the 9mm about 167x magnification.  If you'd like a wider field of view you can probably make use of a 32mm eyepiece (I use one a fair bit in my Mak for locating targets before moving up to greater magnifications) and obviously you could go up to a 6mm eyepiece for 250x.  In practice whatever scope you have, 250x is probably a reasonable maximum magnification for the UK unless the sky quality is exceptional.

James

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.