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how fast is my scope ?


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hello, seen several references to the speed of a scope in discussions over what lenses to purchase, wherein   a scope is described as slow or fast which seems to determine the magnification that can be achieved using different lenses, had a search on the forum but I am still no wiser as to what this actually means, is there a simple explanation, for ref my scope is a skywatcher 127 mak ( I believe fl1250/f13.8 from spec sheet), thanks, mike b.

ps, I hope I haven't bought a pavement scooter instead of a ducati !!!!!

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, I hope I haven't bought a pavement scooter

You have indeed but don't let put you off as it is a fantastic little scope with a massive following on SGL.

Its an amazing scope for its size, great on planetary but not so good on DSO.

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Yes I'm afraid you have! :smiley:, fortunately the "speed" of a telescope is in photographic terms. As such, your telescope would be considered "slow" but it will give a scooter a hiding on magnification and optical performance. A telescope with a focal ratio of F5 or less is considered "fast". 

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Divide the focal length of  your  scope ie 1500 by the diameter of the primary  mirror 127 = f11.81.   For another  example my scope is 1200/203=f5.91.  Hope this helps.

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iv got a few scopes the worst ironicly is the biggest, my 11.5" dobsonian,   its F4 and needs really decent quaility eyepeices to use (or a really expensive televue parracore)   im actually in the process of getting a much slower scope thats a bit more friendly to my lower quaility eyepeices.  (around the same as you F10ish)     so dont worry too much you have plenty of time to go.    different speeds of scopes are better for different purposes,     the quicker the scope  the more its like a light bucket absorbing all the photons and the better it is for astrophotography on DSO's (and visual if you have televue or explorer scientific) the slower it is the better it is on plants, the moon and the more forgiving it is with cheaper eyepeices on DSO's (the bright ones)   :)  i m sure you will have lots of fun with your MAK :)   i beleive there is a 127 club on SGL as well 

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oops, typo on origional post, it should have read 1500-f/11.8, that will teach me to read my notes & post with out my glasses,thanks for the replies it is all starting to make some sense, as a beginner there are many questions to ask, sadly another cloudy night here tho' ,mike b.

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Few people consider the f/ ratio when they buy a telescope, I certainly didn't as it would not have meant much anyway.  All i considered was how much light it would capture so bought the biggest aperture I could afford at the time (my SW 200P).  Later, as I began to find where my true interests lay (planets, double stars etc) I decided to get a scope more targeted to these objects (my C100ED), so now I feel I have the best of both worlds. 

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Speed is of interest in astrophotography. In visual use the speed of the system is the effective focal length of scope/EP combined divided by the aperture. The speed of the scope in isolation is not important.

Olly

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hello, seen several references to the speed of a scope in discussions over what lenses to purchase, wherein   a scope is described as slow or fast which seems to determine the magnification that can be achieved using different lenses, had a search on the forum but I am still no wiser as to what this actually means, is there a simple explanation, for ref my scope is a skywatcher 127 mak ( I believe fl1250/f13.8 from spec sheet), thanks, mike b.

ps, I hope I haven't bought a pavement scooter instead of a ducati !!!!!

You have bought a very competant scope. It is not a fast scope but that is a requirement of it's design which is a Maksutov Cassegrain, these type of scopes in particular the Rumak Maksutov designs ( regertably your's is not one ) offer flat field of view , great resolution at times approaching a much more expensive APO. These scopes excel in planetary both visual and imaging but are considered slow for deep sky imaging. For deep sky observing with a decent EP it will do fine but with a smal FOV. Enjoy the scope and don't worry about its speed. A fast scope BTW has an F ratio in the region of F2.8~ F5.

A.G

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I have three different telescopes with different "speeds" one is an f/4.3, another is an f/14 and another is an f/12 between the three , I can honestly say that my 70mm 7/4.3 has given me the more pleasing results with every ep I use. last night I got some nice results using my 40mm and 25mm eps in that scope and Im definately hooked on the bigger aperture size. but I'll be honest , i have had nice pictures out of a fast scope on chance just as i have had with the slower scopes. it all is just a fun hobby for me taking pictures. I'm actually planning on getting a full sized tripod for my 70mm since it is currently a table top. its great for travel, and I love its compact size. the other two are of longer focal lengths obviously. I own nothing but refractors right now. Do I like other models? Absolutely. but I can say sometmes the speed means nothing. at least from my experience. It is clearly different from other people pov as well. 

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