How to work out if finderscope is 8x50 or 9x50

[bendiddley]

I've got what I think is an SW 9x50 straight through finderscope but I'm wondering if it may be an 8x50 (it came with a second hand blue 200P scope that I bought a while back), is there any way of working out how?

[John]

I think the Skywatcher 50mm ones are all 9x50. An old Skywatcher brochure I have showing the blue trim scopes says 9x50.

To be honest the tolerances that these things are made to may well mean that these figures are approximates rather than exact. There are a number of barlow lenses for example that say one amplification but in reality are a bit different to what is marked on them.

 

[bendiddley]

Ok cool, thanks John.

[Ouroboros]

You could disassemble the scope and measure the focal lengths of the objective lens and eye piece. Magnification = focal length of objective / focal length of the eye piece. Getting these accurate enough might be tricky to tell the difference between 8 and 9 times. 

Why is it important?   

I agree with John above. It's probably a 9x50. It sounds like my blue finder scope which is 6 years old now. 

[Charic]

Can you detect any difference in the exit pupil size (were talking small,  about .75 mm difference ) assuming the larger 6.25mm exit  would equate to the 8x finder and the 5.5mm relating to the 9x finder?

[spaceboy]

From what I have read over the years I don't think it matters much if they are 8x50 or 9x50. What is a concern though is that you will most likely find both finders are not actually 50mm. Finder scopes are often stopped down to "improve the view". This was a naughty trick used in some cheap ass refractors to give the impression that the objective was larger than it actually was. Yes the objective was eg: 60mm but the edge performance was so poor that the first baffle would stop down the aperture to the center 50-55mm of the lens using the better corrected centre and increasing the focal length of the scope making it slower and so more forgiving on eyepieces. Finder scopes are intended to find things not give super views of the night sky so optical performance is not a high priority so there is a very high probability stopping down is employed in most mass produced finders. The only finder I have found not to be the case is the celestron 8x50 sold with C100ED refractors. The coatings on the objective were also superior.

[Charic]

According to the information Flo! provides, with regards to the present 9x50,  which is probably the same info provided from Skywatcher, their 9x50's are not stopped down and have  a true field of 5.6°.
I've seen the finder-scope for the 200P  referenced  a few times in the past, being sold, as an  8x50 , yet I'm sure my info, my scope,   is fitted with the 9x50.
Its in the detail, nice to know, but as for the difference in views, probably unlikely to see a difference,  using my eyes!

[bendiddley]

2 hours ago, Ouroboros said:

You could disassemble the scope and measure the focal lengths of the objective lens and eye piece. Magnification = focal length of objective / focal length of the eye piece. Getting these accurate enough might be tricky to tell the difference between 8 and 9 times. 

Why is it important?   

I agree with John above. It's probably a 9x50. It sounds like my blue finder scope which is 6 years old now. 

Cheers guys, thanks for the advice. The reason for asking is that I'm using a 9x50 (I think) Finder guidescope for my guided astrophotography rig, and PHD keeps not detecting any star movement and is failing the calibration, I've tried fiddling with I don't know how many settings and I was wondering if maybe I had an 8x50 and there just wasn't enough magnification for PHD to detect the star moving, probably totally not the case, but it had me wondering and wanting to rule it out of the myriad of possibilities.

[Davey-T]

Have you tried fiddling with the calibration step size  ? if not, alter it by a large amount and see if it helps.

Also make sure you're not trying to guide on a hot pixel.

Dave

[bendiddley]

3 minutes ago, Davey-T said:

Have you tried fiddling with the calibration step size  ? if not, alter it by a large amount and see if it helps.

Also make sure you're not trying to guide on a hot pixel.

Dave

Cheers, yep tried those.

[Davey-T]

Can you see the star move if you slew manually using the control in PHD ?

Dave

Folks used to use half an old pair of binoculars so a finder scope should be no problem.

[bendiddley]

13 minutes ago, Davey-T said:

Can you see the star move if you slew manually using the control in PHD ?

Dave

Folks used to use half an old pair of binoculars so a finder scope should be no problem.

Ok, no I haven't tried that, I'll give it a go. What would it mean if it does move or if it doesn't move?

[Davey-T]

Just now, bendiddley said:

Ok, no I haven't tried that, I'll give it a go. What would it mean if it does move or if it doesn't move?

If it moves manually it means the cable is OK, if it doesn't move it could be the cable but not necessarily, easiest way to check is try another cable, always good to have a spare

Are you using  ST4 or pulse guiding ?

Dave

[bendiddley]

I've tried another cable and had same results. I'm using ST4. The set-up is a QHY5 mono with an ST4 cable to the ST4 socket on my NEQ6.

[bendiddley]

My next port of call was to try using PHD1 to see if that makes any difference or not. There's quite a lot of posts on SGL with settings for a finder guider using PHD1 and was going to try using the settings suggested, not a lot around for PHD2 and a finder guider.

[Davey-T]

Does it move manually ?

Dave

[bendiddley]

31 minutes ago, Davey-T said:

Does it move manually ?

Dave

I haven't tried it unfortunately so don't know, I'll give it a go next time I'm out and let you know.

[Ouroboros]

All that matters regarding  PHD is the focal length of the primary lens in the finder. This is 190mm for the 9x50 Finder.  I use PHD2, which has a calculator into which you input various parameters [Guide camera pixel size, focal length of guide telescope etc] and it works out the appropriate settings.  It also has a useful simulator so you can see how PHD2 should behave when it's working properly. I must say I find the instruction manual with PHD2 much more informative.