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Equinox ED80 Pro - focal length

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I am probably being very stoooopid this am.

However, I keep looking at specs for this and they consistently show this as 500mm focal length, 80mm aperture f6.25.

Have used this a few times with the SkyWatcher field flattener, described as "A high-quality, non-focal-reducing Field Flattener optimised for DSLR imaging on refractor telescopes with focal ratios around f/5.5 to f6.0", with my Canon DSLRs.

Decided to use a colour CMOS that I have, so, measured from lens back 500mm to get good stab at focus, and it is waaaay further out that the DSLR looked.

So, put the DSLR on and measured to sensor and it's 40cm/400mm (no not deadly accurate measurements) but enough to make me wonder, and certainly not 4" of error :)

1. is it 400mm or 500mm focal length

2. The Skywatcher field flattener is actually 0.8 reducer too when used on this f6.26 OTA?



Edited by iapa
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The focal length of the scope will not change unless you chop it up or use a reducer, I do not know if you need to add the distance your camera achieves focus at to determine the total focal length of the set up.
I believe the varying distances to achieve focus from the scopes focal point is another matter altogether but I am no AP expert.

I am not sure if this helps in anyway :happy8:

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I know it won't change unless I use a reducer - of pay with the lenses (not an option) :)

Everything I have read says the sis 80mm aperture f6.25 600 500mm fl.

I am measuring from the approx mid depth of the lens to approx sensor position.

My tape measure says 40cm

That is with the field flattener and DSLR, or the colour CMOS guide/planetary camera with an extension tube.

Either way, lens to sensor measures out was 400mm.

If also backups that the Skywatcher field flattener doesn't do much, if any, focal reduction.

Edited by iapa
correct 600mm fl to 500mm fl
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41 minutes ago, Davey-T said:

Not sure what you mean, if the actual metal tube is 500mm long how do reach your 400mm measurement.


From front of tube, where the lens is, to the sensor of the camera measures as around 40cm.

37 minutes ago, Pig said:

Are you using a digital tape measure ?

I am using a metal Stanley tape measure.

Going to try and take a picture one handed



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20 minutes ago, Davey-T said:

Dunno, I've got 100mm f6 and it measures 600mm from the objective to the camera sensor when focussed.

Are you using a focal reducer to make it f/5 ?


No additives :)

OTA, focuser, and touptec colour CMOS.

My Star Travel 80mm f5 measure at 400mm correctly.

Wonder if they have changed the glass int eh Equinox Pro?

It measures the same physical 400mm when I have the field flattener and DSRL attached.

Not complaining mind, if I've got the fluorite and Schott glasses and its f5 instead of f6.15, what's to complain about :)

I must be doing something :cussing: wrong :icon_scratch:, its such a fundamental thing.

Focal length is the distance from the lends to where the image on the sensor is in focus.... or is it? :)



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14 minutes ago, iapa said:


Focal length is the distance from the lends to where the image on the sensor is in focus.... or is it? :)



I think so, how far out is the focuser if you use a diagonal ?


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