ok..... heres a question for you ? ( star diagonal )

[Guest billywhizzzzz]

can anyone help , because this has blown my mind , even though to those in the know there will be a very simple explanation .

 

Right then I have just taken charge of my new Williams 2" star diagonal http://www.firstlightoptics.com/diagonals/william-optics-2-dielectric-diagonal-with-sct-adaptor.html

anyway I thought seeing as im going to have a play tonight , thought I would fit it on an just have a look through ( as you do ) to make sure all was ok , however ......

I thought I would leave the stock diagonal on first with my Faithfull 25 mm Ep  , point the scope ( celestron C8 SCT ) at a wall about 500 yrds away , have a look and the swap to the new diagonal and see if there was any difference ,

 

and yes , there was a massive defence , I took notice of how many bricks a could see in the EP then swapped over and I the fist thing I noticed was I have to refocus , not a little , but loads . when it came into focus I would say the image was nearly double .

 

my question is WHY ? would the new diagonal change the focal length of my scope ? I will say the image was Chrystal clear though . before anyone asks  I was not using a 2" Ep , just the standard 25mm 1.25 .

 

regards steve

[+RonC]

Is the stock diag. a 1.25"?

[dawson]

I guess anything which alters the distance from the first point of "distant light - telescope interaction" (in your case the glass at the front of the scope) and the point at which that light path is brought into focus on your retina, will alter the perceived focal length. So if the new diagonal was 'bigger' (extended the light path by say an extra 5mm) then the focal length will be increased by that amount, and as such the field of view will be slightly altered (making, say, your f/10 scope an f/10.1 one).

 

But philjay (or the other clever ones) may correct me here.

 

I've still not picked up my optics book, I'm working my way very slowly through other books.

 

I'll be interested to see what the others say.

 

James

[Guest billywhizzzzz]

Ron ..

 

yes mate , it is

[+RonC]

What James said is about right I think. I use both (1.25 and 2") depending on what I'm observing, I never thought about the f ratio etc just my observing! 

[Guest billywhizzzzz]

• thanks chaps !

blimey do you mean I was sort of right ...... see its amazing what you pick up just by reading / listening to people that know there stuff .

 

must say its looks a cracking bit of kit

 

thanks again lads

 

steve

[Guest billywhizzzzz]

a few pics

 

steve

[Tweedledee]

Have a look at post #20 here...

http://www.eastmidlandsstargazers.org.uk/topic/5037-increasing-focal-length/

 

It may help or confuse.

 

With a cassegrain type scope, you usually focus by changing the primary to secondary mirror separation. When you need to refocus to push out the focal plane to accomodate the longer optical path of a bigger diagonal, this does increase the focal length in accordance with a mathematical formulae. A small change in the mirror separation results in a larger change in the effective focal length.

 

A refractor or newtonian will always have the same focal length, you simply move the eyepiece to coincide with the focal plane. A cassegrain that uses mirror focusing is a different animal.

[dawson]

That's right, thanks Peter, clearly that whiskey does your memory for old threads the world of good

 

James

[Tweedledee]

White wine tonight James, cheers .

[red dwalf]

i have the 2" william optics diagonal also had a 1.25" version which i did a comparison with the stock 1.25" diagonal, williams was a brighter and clearer view, worth the money, now a couple of 2" eyepieces and your away.

Edited June 26, 2013 by red dwalf

[Guest billywhizzzzz]

i have the 2" william optics diagonal also had a 1.25" version which i did a comparison with the stock 1.25" diagonal, williams was a brighter and clearer view, worth the money, now a couple of 2" eyepieces and your away.

oh yeah there  next on the list , it does make a big difference though , I was looking at M13 last night and with the new diagonal I would say I could see a good 10% more of the cluster .

 

did you notice a diff in mag when you swapped the diagonal over , seeing  as you have a similar set up as me  ?

 

steve

[red dwalf]

i suppose there was an increase in mag but to me the stars look brighter or cleaner with alittle more clarity to them, the reflective mirrors are alot better than the standard diagonals, annoying when you have already spent alot of money on a scope you need to get a better diagonal to get the best out of it.

i love the 8" sct`s alot, it would have to be something special for me to sell mine.

Edited June 26, 2013 by red dwalf

[Guest billywhizzzzz]

yeah same here , I find it a joy to  use