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#1
18-07-2018, 10:12 PM
 Zuts Registered User Join Date: Mar 2007 Location: sydney Posts: 1,431
Optics Help Needed

Hi,

I was trying to collimate my 9.25 F10 SCT with an artificial star at home. With the longest distance I could put between the star and the scope ( in my backyard) I could not reach focus with the eyepiece in a diagonal.

By pulling the eyepiece out around 20 cm from the diagonal I could reach focus.

I still had about a metre of backyard left so moved the star back this distance however the focus point had only moved in around a few centimetres.

With a 20mm eyepiece in a 2350 mm scope how much extra distance would I need to reach focus?

Since the magnification is around 100x is it

additional backfocus required by magnification, ie around 20 cm by 100 = 20 m?

The scope is inside the house and the star is on the back fence. There really is no more room. Would it work if I moved the scope say a metre closer to the star to give me room and got a poly pipe to go the extra 20cm of backfocus?

Cheers
Paul
#2
19-07-2018, 12:45 AM
 OzEclipse (Joe Cali) Registered User Join Date: Jun 2012 Location: Canberra, Australia Posts: 948
Paul,

formula for a simple lens is

[1/OBJECT DISTANCE] + [1/IMAGE DISTANCE] = [1/ FOCAL LENGTH]

Providing you are not changing distance between mirrors, the SCT will conform to this rule.

Cheers

Joe
#3
19-07-2018, 07:52 AM
 Zuts Registered User Join Date: Mar 2007 Location: sydney Posts: 1,431
Quote:
 Originally Posted by OzEclipse Paul, formula for a simple lens is [1/OBJECT DISTANCE] + [1/IMAGE DISTANCE] = [1/ FOCAL LENGTH] Providing you are not changing distance between mirrors, the SCT will conform to this rule. Cheers Joe
Thanks but I dont quite understand, using this:-

1/OD + 1/ID = 1/FL

assuming you mean image distance from the front of the primary which if the image is formed 20cm behind the scope then this would be say 25cm or 250mm. So with a FL of 2350mm I get

1/OD = 1/FL - 1/ID
1/OD = 1/2350 - 1/250 = 0.0004 - .004 = -0.0036
OD = -279mm

So maybe I am not understanding but....the star is about 30 metres from the scope not -279mm.

Cheers
Paul
#4
19-07-2018, 08:15 AM
 multiweb (Marc) ze frogginator Join Date: Oct 2007 Location: Hinchinbrook Posts: 18,328
In practice if you have your artificial star at least 20m away you won't rack your mirror forward off the baffle tube so you hit the retaining clip. Just use extension tubes at the back to reach focus. It's good enough for collimation. The optimum distance between the mirrors is there so you don't get spherical aberration which is important for viewing or imaging but doesn't matter for collimating. Visually you'll only see a difference in brightness in and out of focus but the rings will still be concentric.
#5
19-07-2018, 08:46 AM
 Zuts Registered User Join Date: Mar 2007 Location: sydney Posts: 1,431
Quote:
 Originally Posted by multiweb In practice if you have your artificial star at least 20m away you won't rack your mirror forward off the baffle tube so you hit the retaining clip. Just use extension tubes at the back to reach focus. It's good enough for collimation. The optimum distance between the mirrors is there so you don't get spherical aberration which is important for viewing or imaging but doesn't matter for collimating. Visually you'll only see a difference in brightness in and out of focus but the rings will still be concentric.
Hi Marc,

There is a laneway next to my house which will probably give me enough distance so I'll try there.

A bigger backyard without trees would be a good upgrade for my 25 buck artificial star but funding it may be an issue atm

Cheers
Paul
#6
19-07-2018, 10:44 AM
 multiweb (Marc) ze frogginator Join Date: Oct 2007 Location: Hinchinbrook Posts: 18,328
Quote:
 Originally Posted by Zuts Hi Marc, There is a laneway next to my house which will probably give me enough distance so I'll try there. A bigger backyard without trees would be a good upgrade for my 25 buck artificial star but funding it may be an issue atm Cheers Paul
I setup in my garage and place the artifical star across the road on the grass reserve under the power lines when I do mine. You can make your own artificial star with a bit of optic fibre and a LED torch, that's all you need. You're not testing optics, you're collimating ballpark. That's good enough. The final tweak do on a star test.

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