How can I determine the focal length of a Cass?

[jimao22]

Hi guys,

I received from a friend a new telescope (new for me, but the telescope is not new) - a classic Cassegrain. The telescope is made by someone very handy, I realize that when I dismantle it to fine tune it and to see how can I collimate the scope (is a new type of scope for me). 

My problem is I don't know how to calculate the focal length of this type of scope. Is not so easy like doing this for a newtonian, because of the curved shape of the secondary mirror. Have anyone a clue how to calculate this?

 

 

[ollypenrice]

I have no idea whether it can be calculated or how that might be done but it can be measured. One way would be to take a picture through the scope and then plate solve the result.

If you scroll down to brianb's post in the thread below you'll see another way which is simple and ingenious.

Olly

[jimao22]

Thank you Olly, the plate solving is a very good and very accurate method, I completely forgot this.

About the other method, I didn't know till now. It is very ingenious, indeed. I will solve this as soon as I'll have a decent sky.

Thank you again!

[Astrobits]

An alternative method that does not need clear night skies is to use a low power eyepiece and aim the telescope, focussed at infinity,  at a bright surface/blue or cloudy sky. Then carefully measure the diameter of the exit pupil. The ratio of the exit pupil to the diameter of the mirror is the same as the ratio of the focal lengths of the eyepiece to the telescope.

Nigel

[jimao22]

Thank you, Nigel - I just sow your reply. I will try your method since the weather is not cooperating with my intention to do a proper plate solving...

[CraigT82]

I'm not sure how accurate this would be but you could take a picture through the scope of an object of known angular size (Jupiter for example). Then analyse the image to see precisely how many pixels it occupies.  Assuming you know the pixel size of the camera used, calculate focal length by:

fl = (object size [pixels]*pixel size*206.3) / object size [arc secs]

P.S the mount in that picture looks like a work of art! What is it?

[jimao22]

Hi Craig, that's a worshop made alt-az mount. Is for visual only. My astrophoto setup is on an EQ6.

[skybadger]

Plate solve is one, object size is another  - could measure the solar/lunar diameter at prime focus during the day. I like the eyepiece method, or you could make an assumption and model it as somewhere between f/10 and f/15 for a normal Cass. 

I normally use the solar method  - for example my f/10 6" gives a 22.5mm image at prime focus.

Do you know the specification of the mirrors ? If so you can calculate the focal length using the cassegrain equations. You'll need the mirror spacings, their focal lengths and the back focus distance. 

regs

Mike

 

[CraigT82]

4 hours ago, jimao22 said:

Hi Craig, that's a worshop made alt-az mount. Is for visual only. My astrophoto setup is on an EQ6.

Looks very interesting! Would love to see more pictures of both scope and mount 

[jimao22]

On 5/6/2016 at 12:00, CraigT82 said:

Looks very interesting! Would love to see more pictures of both scope and mount 

Sorry it took me so long for a reply, I was very busy at work, traveling a lot. I saw your post but I should have my astro-computer with me to answer you about that. This mount is very sturdy, with parts of aluminium thick enough, and is quite heavy. The alt-az adjustments are made with some depth measurement calipers connected to cranks on one side and to the movement shafts on the other. The main shafts can be blocked with some allen screws if necessary - the clutch (that's why you can see there the blue allen key). The mount is not made by me, is was made by a fellow from Romanian astronomy forum who now is "out of the astro business". It was a real bargain, I think the price for raw materials was bigger than I paid for it.

Now it stand into my bedroom, waiting for some nice weather (is raining cats and dogs here from more than a week).