SW 9x50 finderscope focal length ????

[Skipper Billy]

Anyone know what the focal length is of a SW 9x50 finderscope ???

I am using it with a QHY5Lii as a guide scope but need to know the focal length to input into PHD2 - cant find the focal length info anywhere !!!!

Thanks in advance.

[acey]

Don't know, but here's how to measure it. Unscrew and remove the eyepiece section of the finder. Aim the finder (which now has only its objective lens in place) at a distant light source (e.g. Moon, or Sun if you aim using the finder's shadow and take due precautions). Project a focused image of the light source onto a piece of card. Measure the distance from the finder objective lens to the card. That's the focal length.

Alternatively use a nearby light source (e.g. light bulb). In that case, let d1 be the distance from light source to finder objective, and d2 be the distance from finder objective to focused image. Then the focal length is f = 1/(1/d1 + 1/d2). For distant light sources we can take d1 as infinity, in which case f = d2.

[MattJenko]

Post #6 in this thread looks useful : http://www.astronomyforum.net/telescope-accessories-forum/170309-9x50-finder-scope-how-determine-focal-length.html if you haven't seen it on your interweb travels.

[glowjet]

Skywatcher 9x50 focal length is given as 180mm, info via Astro Bin, Google. The Orion 9x50 RACI however, if anyone wants to make a note, is according to the general consensus of opinion on the CN forum, is given as162mm. HTH

[Skipper Billy]

Marvellous - thanks very much !!!

[knobby]

Old thread I know but the skywatcher finder guider platesolves to 181 mm 👍

Which backs up the above.

[jiberjaber]

On 13/10/2019 at 00:51, knobby said:

Old thread I know but the skywatcher finder guider platesolves to 181 mm 👍

Which backs up the above.

I get a different number from my calcs

Rearanging the formula from https://www.lightvortexastronomy.com/tutorial-pre-processing-calibrating-and-stacking-images-in-pixinsight.html

[Arcseconds Per Pixel] = (206.2648 x [Camera Pixel Size in μm]) / [Telescope Focal Length in mm]

FL = 206.265 * (pitch/fov)

Using Astrometry.net and a capture from my finderscope with QHY5-II mono fitted.

http://nova.astrometry.net/user_images/3619149#original

I get FOV 4.39 arcsec/pixel and the specs say the pitch is 3.75um so the resultant FL is 176.2mm 

Sample of 1 image of course!

 

Maybe the difference is in the mounting of the camera or focus? On my SW finder I have unscrewed the eyepiece and replaced it directly with the camera.

 

Edited May 1, 2020 by jiberjaber

[knobby]

It's weird isn't it ... Several different answers on web, mine is a bit heath Robinson with some bits hammered in and not screwed but when I plate solved it came back as 181mm. Maybe some are different ?

[jiberjaber]

16 minutes ago, knobby said:

It's weird isn't it ... Several different answers on web, mine is a bit heath Robinson with some bits hammered in and not screwed but when I plate solved it came back as 181mm. Maybe some are different ?

How did you get the FL from just the plate solve, was it a web site?   It would be useful to give it a try to compare

Edited May 1, 2020 by jiberjaber

[jiberjaber]

I've just had a quick look at mine and it is screwed quite far in to the body on it's adaptor.

 

 

[knobby]

Can't quite remember but I know I used all sky plate solver ... I'll try to dig it out over the weekend 

[Spile]

Sorry to dig up this old thread but I am drawing radius circles over my maps (I need to get out more) and want to do the same for my Sky-Watcher 9x50 Finderscope

According to the specs at https://www.firstlightoptics.com/finders/skywatcher-9x50-finderscope.html it has a True Field of View of 5.6"

That means if the FL of the EP is 20mm and the apparent FOV is 45° the FL of the Finder must be 162mm?

This can be checked agt https://astronomy.tools/calculators/field_of_view

[Mr Spock]

I have a Celestron, which I believe uses the same lens. I attached the eyepiece from a Skywatcher to it as the Celestron eyepiece is a measly 40° AFOV.

What I get is 20mm focal length with a 50° AFOV, which gives 5.6° FOV and x9. I measured the focal length at 180mm and this checks out with the observed magnification.