Focal Length Question

[Aussie]

Hey guys, I have been looking about different scopes (thank you cloud's ) and have a question. What would be the difference in the image seen in a telescope with the same aperture size but different focal length.

For example an EVOSTAR-150 which has a longer focal length versus a STARTRAVEL-150 which has a shorter focal length.

I know they weigh different etc and would need different mounts but it's just the image itself I'm wondering about. What would be the differences between the two?

[brantuk]

Hi Aussie - you may found this page useful:

The New CCD Astronomy Home Page

Download "CCDcalc" - it's free - you can enter any telescope details and get an image size comparison both numerically and visually - nice bit of software

[Rossco72]

The longer focal length will have less false colour than the shorter one I would expect. With Achromats the longer the focal length, the less false colour and Violet/Blue halos around stars etc.

[BazMark]

Not sure if this is the answer to what you're asking but, if you use the same EP in each scope, the longer focal length scope would give you greater magnification but a dimmer image whereas the shorter focal length scope would give you less magnification but a brighter image.

[brianb]

if you use the same EP in each scope, the longer focal length scope would give you greater magnification but a dimmer image whereas the shorter focal length scope would give you less magnification but a brighter image.

True ... but in theory if you use the same magnification the view is independent of focal ratio of the scope (if the aperture is the same too).

In practice, a short focal length tends to have more optical deficiencies (unless the design compensates for them by adding correctors, which increase the price), are much more sensitive to collimation errors and are much less tolerant of the cheaper sort of eyepieces more likely to be used by beginners. IMHO a focal ratio of f/8 - f/10 is ideal for a general purpose beginners scope.

[mark7331]

I agree with Brian. For a beginner F8-F10 is about right.

Shorter focal length = brighter view, wider field of view but more optical problems.

Longer focal length = dimmer view, narrower field of view but less optical problems.

If you're thinking of using a CCD with the scope then you need to match the pixel size of the CCD with the focal length so that you get a field of view that works for the objects you'll be imaging.

[ollypenrice]

I also agree with the others. The optical challenges facing the designer increase as the focal ratio gets lower (faster). The problem is considerably worse for refractors than for reflectors. 'Fast' refractors can only give top class results by being very expensive!

In any event, as Brian says, f8 to f10 is user friendly all round, though not what you want for deep sky imaging.

Olly

[Aussie]

Thanks for the help guys. What do you want for deep space imaging then?

[Moonshane]

I actually disagree with my much more experienced fellow SGL'ers.

I feel that a focal ratio of f10 would ptentially make life more difficult for a newbie to find things (other than maybe the moon and the larger planets) as often the field of view for a decent aperture is quite narrow and may make it difficult for someone inexperienced to find things.

I think an f6-f8 scope has adequately fast optics to make most things look OK with even the most modest of EPs (other than some of the extremely poor ones that come with new scopes) and the aperture can be reasonable too without being too cumbersome.

this is the main reason I think the 8" f6 dob is one of the best starter scopes there is unless the person concerned intends getting straight into the expensive route of astrophotography (which I think generally is not recommended right away??).

by way of example, a 120mm 1200 focal length (f10) scope would with a field of 1 degree with a standard 25mm plossl and a maximum usable magnification of say 180x.

the 200mm dob with the same focal length (f6) would have the same field of view approximately but be a lot brighter and you'd see more of the fainter objects. also the maximum usable magnification (subject to seeing) would be around 300x.

to get the same aperture at f10 the focal length would have to be 2000mm on the 200mm scope which would make in pretty unmanageable or you'd need a 'folded' design.

I am sure the others will now tell me why I am wrong but I thought I'd add this to the fray

you'll note from my signature though that I suffer from dobsonianosis.

[Moonshane]

Thanks for the help guys. What do you want for deep space imaging then?

a quite large budget. I think you can do planets and moon quite cheaply but to do justice to DSOs such as nebulae and galaxies etc you really start to add the cost up. others can provide more meaningful estimates on this. I think a typical kit is mount £500-700, wide field ED refractor £250-???, a laptop, a CCD imager/DSLR, possibly a guidescope and probably lots of other things like a lot of time and software knowledge.

[ollypenrice]

For DS imaging you really want a fast f ratio with (for me) an upper limit of f7.5 though less would be more! This speeds up capture allowing for more data while the sky is clear and shorter subs for easier guiding. The focal length that you want simply depends on your targets since you work at prime focus. Small objects need long f length and large ones the opposite. Long focal lengths for imaging need better tracking accuracy.

Olly

[Aussie]

Thanks guys that answered that question nicely. I already have my beginner scope though and was just browsing through others and was wondering that distance. From what you all said I think I would prefer shorter focal lengths versus longer ones just because I like to it all at once.

Don't worry though, the next scope I buy wont be in a rush and ill definitely be trying it out before i buy it.

[Rossco72]

If your thinking of imaging, then a fast achro isn't really going to do much good, i think you will be very dissapointed and frustrated by the images.

If you're going down the imaging route then a good quality doublet is the absolute minimum with a triplet or better preferable. There are many folks using ED doublets and are happy with them.

[Aussie]

doublet being? sorry I'm new remember. Is that one of those scopes that looks like a combination of refracter and reflector? short, fat and with the eye piece sticking out where a refractors would be?

[brianb]

doublet being? sorry I'm new remember.

Refractors ... doublets have two elements in the objective (the big lens that's at the opposite end to the one you look through), triplets have three elements.

"ED glass" is a way of reducing the chromatic aberration. Some top end scopes use calcium fluorite (cut from a large synthetically grown crystal) instead of glass for one of the elements of a triplet, this gives the best possible performance, but some e.g the William Optics FLT 110 "fluorite triplet" use a special glass which has similar properties to real fluorite instead ... still very good but not as well controlled in the ultra violet. The difference is of course irrelevant for visual work.