(Un)Acceptable deviation of stated(marketed) f/l vs real f/l for imaging scopes

[mytola]

All,

Take a look at this image run through an online astronometry app:

http://nova.astrometry.net/user_images/92471#annotated

This was taken with an Atik 314L+, which has a pixel size of 6.45 um. Based on this, we can calculate the focal length of the optics used to take the picture:

arcsec / pixel = 206.3 * ( pixel size [um] / focal length [mm] ) 

Doing this I get that the focal length of the optics is 543 mm. Now, in my scope specs, it is stated that it should have a f/l of 478 mm...quite different, and in my opinion, not really acceptable. For imaging, it will be f/5.9 vs f/6.7.

Can I have done something wrong here? Would be nice to have some opinions on potential mistakes. But I know others have come to similar results with the same scope and different sensors. I have sent this to the manufacturer, asking if they agree, and if there's been a mistake in the marketing, but no reply...

Anyone have similar experience?

[jambouk]

Alter the size of the sample from the image the app uses and see how much that alters the calculated focal length. I suspect changing the sample area by a small amount, massively alters the calculated focal length.

James

[Adrian]

Could it be that although the pixels are the stated size, they have gaps between them and so the effective size of the pixels is larger than specified?

Edit: that would increase the calculated F/L anyway   

[ronin]

2 thoughts come to mind:
1: Where does the subtended angle come from? The image you link to of NGC5194 had a bit of it outside the big green circle. So I would suspect that 5 different reference for NGC5194 will give you 5 different sizes.

As an example Wikipedia seems to say 11.2 arc min, SEDS says 11.7 arc mins, so SEDS is 4,4% bigger then Wiki in one go.

2: What is pixel size? Distance between the centres of the pixels or the length of a side of the active sensory area, there will be a seperation band between one pixel and the next on all sides so the above values will differ.

So we could question both the size of a pixel and the size of the object.

Which scope are you refering to?
Sounds like imaging and that makes me think it is the GTF-81.

WO say the focal length of this is 535mm and if you are getting 543mm then with the inaccuracies of object size and pixel size I would say the GTF is exactly what it is claiming to be.

http://www.williamoptics.com/telescopes/gtf81_spec.php

The GT-81 is specified at 478mm but that is not the GTF-81 which is what your signature says you have.

[mytola]

Thanks for your answers people! Might be some gap between the pixels, I don't know, but several people have gotten similar results that I have with different cameras.

Ronin,

WOW, WO has changed the spec on the page, it was listed as f/5.9 before! Dealers still list it as f/5.9:

http://www.teleskop-express.de/shop/product_info.php/info/p1323_William-GTF81-5-Element-Flatfield-APO---81mm-f-5-9---FPL-53.html

[ronin]

I suspect that the lens focal length is 478mm the same as the GT-81, but the inclusion of the flattener extends this to 535mm.

The flattener must have some effect and WO will not have made a new lens.

Would say you have a 478mm focal length objective that is extended to 535mm with the inbuilt flattener. Optically the flattener will be negative and that will extend the focal length. So I think you have you have a 478mm focal length objective that when used in conjuction with the field flattener is operating as a 535mm focal length optical scope.

[mytola]

Ronin,

That's what I'm thinking as well, but until just now, after people have noticed this using photometry, they've marketed the scope as f/5.9. They certainly did that when I bought the scope.