New goto/tracking mount or DSLR + photography class first?

[VigdisVZ]

The mm-number in lenses describe how far the light has to travel to come to a focus, its called focal length. It has nothing to do with the size of the aperture.

But if you divide focal length by aperture you get something called an F-number or Focal Ratio, wich is a rough description on how much light the setup gathers.

So, my scope has 750mm focal length divided by 150mm aperture, I get a F/5 scope. Which is considered bright.

When you start working with lenses you see that your bundled zoom lens will probably at its most open setting give you an F/3.5 at 18mm. When you zoom in to 55mm your aperture wont have changed, its still at the widest, but since the lens is taking in light from a smaller area the F value follow suit and can thus be used as a rough estimate on how "effective" a setup is. This means less with visual observing in scopes, but means a lot with photography.

The lens size matters, but since the lens description gives you a focal length and a ratio, you could probably calculate how big the aperture on the lens is, but it's exact aperture size might be irrelevant to many people since they want to know how wide it is (focal length) and how much light it lets in (F-number).

Some other examples of this:

My newt has the 750mm / 150mm = F/5 ratio,

A Celestron SCT might have 2032mm / 203.2mm = F/10.

Practically this means my newt has a smaller opening, but since it gathers light from a bigger area, its considered brighter than the celestron, that has a bigger opening, but picks up light from a more concentrated (zoomed) area.

Atleast this is how I understand it

[Chris]

I started out with just a DSLR and a tripod and had lots of fun trying star trails and wide field shots, I learnt alot about my camera before getting my EQ mount and I'm glad I did it in this order. I have a mint condition 1100D for sale boxed as new on here (190 inc postage, or I'll do 220 inc postage with the 18-55mm kit lens) if you want to take a look, it might save you a few bob compared to a new or refurb one, PM me if your interested (My apologies to the mods if touting is slightly bending the rules but it might benefit Kens wallet:D )

[kenmyers]

Thanks Carl, for some strange reason I thought focal length of the camera lense was how wide it was (probable because that's how it works with my telescope eyepiece lenses)

So for camera I want lower focal ratio for brighter images even though for my telescope I like my larger focal ratio for planet spotting and dimmer image.

Great explanation. Confusing, but I think I get it.

[VigdisVZ]

Thanks Carl, for some strange reason I thought focal length of the camera lense was how wide it was (probable because that's how it works with my telescope eyepiece lenses)

So for camera I want lower focal ratio for brighter images even though for my telescope I like my larger focal ratio for planet spotting and dimmer image.

Great explanation. Confusing, but I think I get it.

If we bring in eyepieces I am at a loss, I haven't got that calculation into my head as of yet

But for cameras and scopes the focal stuff is pretty straight forward and gives a rough idea on brightness.

For instance a fixed 50mm at F/2.8 is many times considered a worse lens since a 50mm at F/1.4 will allow for much shorter exposure times to deliver the same amount of light to the sensor. Then we have the whole issue of optical quality, brands and fixed vs zoom and that's a whole other ballpark.

I'm glad I'm atleast not confusing you anymore.

[skfboiler]

As far as learning photography, all I knew how to do is turn the camera on when I first bought it. So I bought the book by David Busch called "David Busch's Mastering Digital SLR Photography". After reading the book cover to cover, I'm able to take pictures in all of the manual modes with confidence. I now know what the terms folks on these sites when talking about astrophotography such as exposure, shutter speed, aperture etc. He also has books specific to the camera make and model. Reading the book prior to takling classes may be help.

[scoobee]

As far as learning photography, all I knew how to do is turn the camera on when I first bought it. So I bought the book by David Busch called "David Busch's Mastering Digital SLR Photography". After reading the book cover to cover, I'm able to take pictures in all of the manual modes with confidence. I now know what the terms folks on these sites when talking about astrophotography such as exposure, shutter speed, aperture etc. He also has books specific to the camera make and model. Reading the book prior to takling classes may be help.

When I was doing my City and Guilds in photography, David Busch books was a norm in our bags and refered to by all, well worth a buy.

Matt

[kenmyers]

Thank you @ Matt and @ skfboiler,

I will have a look and a good read.