Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Distance to view to find focal length

  • Please log in to reply
20 replies to this topic

#1 tag1260

tag1260

    Vanguard

  • *****
  • topic starter
  • Posts: 2491
  • Joined: 07 Oct 2012
  • Loc: Ohio, USA

Posted 11 December 2018 - 09:57 AM

As posted before, I'm building a 153mm SS f8 scope.  I have constructed a scope on a stick to work out focal length and measurements and such. I've read several different things and seems to be  no concise answer so....

 

What sort of distance do I need to view in the day time  in order to find this measurement?  I've read 1 mile and I've read 10 miles.  I know 10 would be better but that just ain't gonna happen where I live.

 

Thanks.



#2 sg6

sg6

    Aurora

  • *****
  • Posts: 4587
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 11 December 2018 - 10:19 AM

What's an "SS" scope?

 

153@f/8 is 1224mm (1.224mtr) (expected longer for some reason).

 

So using an object at 1Km (1000mtr) is:

1/1.224 - 1/1000 = 1/Image posn = 1.2255mtr.

So if using an object at 1Km away the difference between image and focal plane is 1.5mm.

So how accurate do you want and how accurate can you measure.

 

If at all possible use the moon, those craters are a good guide to in/out of focus.



#3 highertheflyer

highertheflyer

    Apollo

  • *****
  • Posts: 1016
  • Joined: 08 Jul 2005
  • Loc: Aledo, Texas

Posted 11 December 2018 - 10:25 AM

I believe he is referring to a homemade Surplus Shed (SS) telescope project.

Jim



#4 tag1260

tag1260

    Vanguard

  • *****
  • topic starter
  • Posts: 2491
  • Joined: 07 Oct 2012
  • Loc: Ohio, USA

Posted 11 December 2018 - 10:29 AM

Thanks for the reply.

 

SS= Surplus Shed+ Wallensak 153mm x 1200mm

 

I didn't realize it would be that small of a difference (1.5mm)  . 

 

Never built a refractor before and BOY do I have a LOT to learn!!!!!

 

Thanks again



#5 GShaffer

GShaffer

    Knight of Ni

  • *****
  • Moderators
  • Posts: 6076
  • Joined: 28 Feb 2009
  • Loc: Bogart, Ga USA

Posted 11 December 2018 - 10:51 AM

As posted before, I'm building a 153mm SS f8 scope.  I have constructed a scope on a stick to work out focal length and measurements and such. I've read several different things and seems to be  no concise answer so....

 

What sort of distance do I need to view in the day time  in order to find this measurement?  I've read 1 mile and I've read 10 miles.  I know 10 would be better but that just ain't gonna happen where I live.

 

Thanks.

Didnt happen without pics smile.gif

 

The further it is the more accurate it will be......That said while I did play with it in the daytime a little for kicks I actually took my real measurement at night focusing on the stars. Not ordering a long enough tube was a real issue for me as I was dealing with a 8" f/16 and with the truck freight it pushed the cost of the 9" tube over $300.....no mistakes needed on that LOL


  • Mike I. Jones likes this

#6 tag1260

tag1260

    Vanguard

  • *****
  • topic starter
  • Posts: 2491
  • Joined: 07 Oct 2012
  • Loc: Ohio, USA

Posted 11 December 2018 - 11:00 AM

Well, pics isn't something I'm good at doing but if you wait a bit, I'm out to work until NEXT Wednesday morning . I have it torn down to the stick and focuser assembly. I will remount the objective then.  Don't trust letting it sit around put together.



#7 Mike I. Jones

Mike I. Jones

    Skylab

  • *****
  • Posts: 4331
  • Joined: 02 Jul 2006
  • Loc: Fort Worth TX

Posted 11 December 2018 - 11:20 AM

Just don't focus using the Moon, it's not at infinity.

grin.gif grin.gif grin.gif grin.gif belushi.gif


  • GShaffer and Pinbout like this

#8 ccaissie

ccaissie

    Surveyor 1

  • *****
  • Posts: 1664
  • Joined: 13 Sep 2010
  • Loc: Whitefield, Maine

Posted 11 December 2018 - 01:09 PM

Just don't focus using the Moon, it's not at infinity.

grin.gif grin.gif grin.gif grin.gif belushi.gif

And...just how much DOES it displace the focus, smartypants...


  • Mike I. Jones and cmooney91 like this

#9 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 21974
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 11 December 2018 - 01:30 PM

Thanks for the reply.

 

SS= Surplus Shed+ Wallensak 153mm x 1200mm

 

I didn't realize it would be that small of a difference (1.5mm)  . 

 

Never built a refractor before and BOY do I have a LOT to learn!!!!!

 

Thanks again

test it against an oil flat...from the second floor.



#10 roscoe

roscoe

    curmudgeon

  • *****
  • Posts: 9617
  • Joined: 04 Feb 2009
  • Loc: NW Mass, inches from VT

Posted 11 December 2018 - 01:46 PM

The farthest thing you can see will get you pretty close, as noted above..... with near stuff focusing further out than faraway stuff.  If in doubt, give yourself an extra 1/4" of in-travel beyond your closest-focusing EP, and you should be safe.



#11 gr5org

gr5org

    Explorer 1

  • -----
  • Posts: 64
  • Joined: 06 Sep 2018

Posted 11 December 2018 - 02:36 PM

So if you know the simple equation 1/d1 + 1/d2 = 1/f

 

Where d1 is the distance from mirror to eyepiece focal plane (the focal plane of a 25mm eyepiece is about 25mm behind the lens).

Where d2 is the distance to the object you are focusing on

Where f is the focal length of your mirror which you already know is roughly 1224mm.

 

then you can focus on anything you want if you know it's approximate distance and like the second poster did above you can calculate how much error there is.  At 1km it is 1.5mm.  To focus on closer things you need to move the eyepiece *farther* from the main mirror so if focusing on something 1km away you will want the eyepiece to be able to move at least 1.5mm closer to the telescope to get things at infinity.



#12 gr5org

gr5org

    Explorer 1

  • -----
  • Posts: 64
  • Joined: 06 Sep 2018

Posted 11 December 2018 - 02:38 PM

On the other hand if you aren't really confident of the math it might be best to focus on the moon at night.  It's a gorgeous crescent tonight in early evening (but may set before you have a chance to set up).



#13 Mike I. Jones

Mike I. Jones

    Skylab

  • *****
  • Posts: 4331
  • Joined: 02 Jul 2006
  • Loc: Fort Worth TX

Posted 11 December 2018 - 03:48 PM

And...just how much DOES it displace the focus, smartypants...

Well, lessee (licks pencil), the average distance to the Moon is 238,900 miles = 15,136,704,000 inches.  Let's do the 200" Hale at f/30 Coudé (6000" EFL) first. The lens law tells me that relative to the stars, the Moon would be 0.00238 inches out of focus.  Simply unusable without refocusing.  The OP's scope would be somewhat less affected.

😁😁😁😁



#14 dave brock

dave brock

    Surveyor 1

  • -----
  • Posts: 1584
  • Joined: 06 Jun 2008
  • Loc: Hamilton, New Zealand

Posted 11 December 2018 - 04:40 PM

That's how I tell which galaxies in a cluster are furthest away. By checking the focus.😏
  • Mike I. Jones, roscoe, Oberon and 2 others like this

#15 tag1260

tag1260

    Vanguard

  • *****
  • topic starter
  • Posts: 2491
  • Joined: 07 Oct 2012
  • Loc: Ohio, USA

Posted 11 December 2018 - 05:49 PM

Am I right to assume that perfect collimation isn't required to find the focal length?



#16 dan_h

dan_h

    Mercury-Atlas

  • *****
  • Posts: 2659
  • Joined: 10 Dec 2007

Posted 11 December 2018 - 07:13 PM

Am I right to assume that perfect collimation isn't required to find the focal length?

Yes, you are right.  When I build a scope on a stick to find the focal length of a lens or mirror, I don't bother with an eyepiece. I just get the best image I can on a white screen.   

 

Let's face it:  you don't need the measurement to be any better than a few percent.  In your final build, you will leave a little wiggle room to focus an eyepiece since the actual focus position will change from one eyepiece to another. I have a 32mm that focuses about 3/4" different than my 24 Pan does and I have a bunch in tween these two. 

 

In reality, a target at infinity gives the best measurement but anything 20 focal lengths distant or more, will work to provide reasonable  build measurements. 

 

dan



#17 GShaffer

GShaffer

    Knight of Ni

  • *****
  • Moderators
  • Posts: 6076
  • Joined: 28 Feb 2009
  • Loc: Bogart, Ga USA

Posted 12 December 2018 - 12:08 AM

Am I right to assume that perfect collimation isn't required to find the focal length?

 

Correct.....I did however build my scope on a stick so I could get it pretty close using a laser.....wasnt necessary though.



#18 Bob4BVM

Bob4BVM

    Surveyor 1

  • *****
  • Posts: 1644
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 13 December 2018 - 02:45 AM

Thanks for the reply.

 

SS= Surplus Shed+ Wallensak 153mm x 1200mm

 

I didn't realize it would be that small of a difference (1.5mm)  . 

 

Never built a refractor before and BOY do I have a LOT to learn!!!!!

 

Thanks again

You're worried about nothing. Refractors are the easiest to build because of the simple, easily 'breadboarded" light path.

I second using the moon with you stick build. Don't forget to account for the star diagonal, you're going to want one of those for anything above 30*.  A little extra focuser travel cures many such problems.

CS

Bob



#19 tag1260

tag1260

    Vanguard

  • *****
  • topic starter
  • Posts: 2491
  • Joined: 07 Oct 2012
  • Loc: Ohio, USA

Posted 15 December 2018 - 08:42 PM

Going to test with my star diagonal as I've never viewed without one. Is there a sort of standard eyepiece to test with. And is there a certain rule of thumb on where to have the drawtube set for best use?  I'm using a 2" focuser with 4.5" of travel. Do I set it in the middle or where?



#20 GShaffer

GShaffer

    Knight of Ni

  • *****
  • Moderators
  • Posts: 6076
  • Joined: 28 Feb 2009
  • Loc: Bogart, Ga USA

Posted 16 December 2018 - 12:00 AM

Going to test with my star diagonal as I've never viewed without one. Is there a sort of standard eyepiece to test with. And is there a certain rule of thumb on where to have the drawtube set for best use?  I'm using a 2" focuser with 4.5" of travel. Do I set it in the middle or where?

 

Best method is find your two EP's that come to focus the furtherest in and the furtherest out.....and setup based on that....


  • dave brock likes this

#21 Jim Dixon

Jim Dixon

    Sputnik

  • *****
  • Posts: 38
  • Joined: 23 May 2013
  • Loc: Little Rock, AR 72211

Posted 13 January 2019 - 04:02 PM

With my second scope, a 16", I cobbled up a platform from sawhorses, 2x4s and other boards, and clamps. Set up where I could see something about 2 miles away. Then adjusted the distance until my favorite eyepieces could come to focus.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics