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My true focal length?

Gadget

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Gadget

Gadget

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Hi all

Just wondering what the true focal length of my scope is the stock ota is f10 2350mm but by the time I add the diagonal and Crayford style focuser has it changed very much or is it still about the same ?

I only ask as I want to know that when I work out what mag I am using for example 10mm ep = 235mag is this going to be much different with my bits on or still about the same?

　

Thanks

Kevin

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steppenwolf

steppenwolf

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The focal length is unchanged by the addition of the diagonal and Crayford, they are just part of the light path - the focal length is dictated by the optics alone. A Barlow lens would alter the 'effective' focal length so if you use one of those, you would need to factor that in too.

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steppenwolf

steppenwolf

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Although a pendant would point out that the moving mirror complicates matters slightly.....:icon_salut:

I considered commenting on this but it was just outside the original question! You are quite right though, although I have never tested an SCT to discover just how much the focal length does actually change.

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GazOC

GazOC

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Thanks Steve I was not sure as I read on another forum where they were discussing Crayford style focusers saying that on a sct one of the downsides was that it added to the focal length

　

Kevin

Adding a Crayford means you have to move the primary mirror forward in the tube and closer to the secondary to achieve focus, this increases the effective focal length of the system.

I'm not sure of an actual formula for this but I've seen figures of 10-20% bandied about.

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Gadget

Gadget

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Thx for the answers

So it means the focal length of my scope could be anything from 2585mm to 2820mm meaning that my 10mm eyepiece is not giving 235x as I thought but anything between 258x and 282x

Not that it really makes a difference but I have a list of all my eyepieces and what mag they give written down so it is not as accurate as I thought

Thanks

Kevin

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Peter Drew

Peter Drew

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When you advance a primary mirror in a catadioptric (SCT/Mak) system, the back focus increases by the amplification factor of the secondary squared. For example, most SCT's have a primary focal ratio of F2, the amplification of the secondary is 5X giving the standard overall F10. If you advance the primary by 1" the new back focus would be plus 25", this principle is why it is so easy to reach focus with almost any accessory. The other side of the coin, however, is that when the scope is cooling, every .001" of inter mirror change results in a .025" change in focus, this is why you always seem to be refocusing with this design compared to a refractor.

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Gadget

Gadget

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Yeah - you learn something new every day in this hobby - just when you thought you had it sussed something else comes along :icon_salut:

Nice scope Kevin - get some binoviewers on that sct if you haven't allready - stunning views :(

Thx brantuk

Binoviewers is next on the top of my never ending list more to do with that im hoping the wife and kids may enjoy better viewing as they seem to have trouble at the eyepiece I don't know why but they switch from eye to eye with their heads going up and down and squinting everywhere and they never seem to see anything

It is painful to watch

Kevin

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Gadget

Gadget

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In my F15 Mak, adding binoviewers increases the focal length of the scope by a shocking half-a-meter (!) making 20mm eyepieces behave like 16.5mm ones.

How did you work out the focal length when adding binoviewers gb

So if I was also to get some binoviewers would this increase also have to be added the increase made by the Crayford? If so is there anyway I can get an accurate idea what my focal length could be

Thanks

Kevin

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great_bear

great_bear

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I did it by a rough drift measurement.

I timed how long it took a star to completely cross the field-of-view with the eyepiece in the focuser, and then retimed the same star with the same eyepiece in the binoviewer. The ratio of the two gives you the factor that the focal length increases by.

Since I was doing that, I took the opportunity to extend the test with each of the two OCA (barlow) lenses that I use in the binoviewer, so as to measure their true magnification.

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glowjet

glowjet

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Chris Lord investigated the change in focal length due to mirror shift in a SCT.

I've included his formulae in my reducer spreadsheet.

Not wishing to hijack the thread, but I see the name Chris Lord mentioned, obviously a well educated man, you should have a look at his article in the public domain on "Franhofer Diffraction and its affect on aperture masks", it strips down the Bhatinov Mask to a point where he proves that three thin strips of material placed across the telescopes aperture opening, provides as good or better results to aid precise focus with web cams/ digital cameras.

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