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Don Pensack
02-01-2007, 09:40 AM
Arie Otte is a Dutch reader of these forums. He recently sent me a personal e-mail to which I could not respond because his e-mail service considers my e-mails to be SPAM. The following is his e-mail to me and my reply. I post it here in case other readers have had the same questions:

Dear Don,

I live in The Netherlands and I'm currently finishing making a truss
dobson with a 12" f/4.0 mirror. Hence I use a Paracorr to correct for
coma. This works well, but I have a question that I hope you may be able

My question concerns the position of the focal plane of an eyepiece. When
designing the secondary mirror cage the distance between the secondary
mirror and the focal plane is critical for the degree of illumination of
the primary mirror. This in turn determines how efficient you make use of
your mirror. I designed the cage by defining the position of the field
stop as the position of the focal plane. For the 17 and 26 mm Nagler I
have, the field stop practically falls together with where the eyepiece
touches the focuser. Hence the edge of the focuser provides a workable
position to measure. Now comes in the Paracorr.

With the Paracorr you have to refocus, so the question is now: where lies
the focal plane? I figure you have to consider the Paracorr-Nagler as one
optical system now. But, for the calculation of the degree of illumination
of the primary it is totally different whether the focal plane lies where
the Paracorr touches the focuser (like with the original eyepiece), or
wether the focal plane still lies at the position where the Nagler touches
the Paracorr. There is at least 1.5" difference in between and it changes
the way I should (re-) construct my secondary cage. My intuition says that
the focal plane will have shifted to somewhere half-way the Paracorr, but
who'd say.

Measuring the distance that is needed to refocus upon inserting the
Paracorr may give an indication, but I'm not sure. If that is correct, one
should just consider the Paracorr-eyepiece combination as a new eyepiece
and postulate the position of focal plane as the site where the new
eyepiece enters the focuser.

Anyway, I hope I made my point clear. I can nowhere find anything on this
subject, hence I hope you can give me either the right answer or give me
indications where to look for the answer.

Arie Otte

Arie,

When you focus an eyepiece, you are bringing the focal plane of the telescope coincident with the field stop of the eyepiece. Though different diopters of eye correction change this somewhat, this is essentially what happens when we focus a telescope.

When the Paracorr is added, the eyepiece is lifted out away from the telescope's focal plane. This would require in-focusing in amount equal to the height of the Paracorr, except the Paracorr does move the focal plane inward slightly. As a result, the Paracorr does not require as much infocusing as its height would imply.

Add to that the different adjustment of the top for each eyepiece, which means the amount of infocusing will vary according to the tunable top's position. The farther out the eyepiece's top adjustment, the more infocusing is required on the Paracorr.

Depending on the eyepiece, infocusing can be as little as 1 centimeter, or as much as 3 centimeters. I've never heard of any eyepiece requiring more than that.

Hence, when designing a newtonian to work with all your eyepieces AND a Paracorr, it is customary to set the final focal plane position of the scope at least 3 cm above the top of the focuser when the focuser is racked all the way in.

Yes, that may require a slightly larger secondary, but you have a choice in that:
1) design the scope to provide 12mm (+/-) of 100% illuminated focal plane in the center of the field.
2) Design the scope to provide a 70% illumination at the edge of the field of your largest field stop eyepiece (this is my recommendation).
3) Design the scope to provide only a few mm of fully illuminated field in the center (the planetary-specific scope)
4) Design the scope to use a low-profile focuser (such as an AstroSystems, which can be as low as 3.5cm from the tube) so the focal plane can be closer to the tube and a smaller secondary can be used.
5) design the scope so the upper tube assembly vignetted the image slightly. That means a smaller UTA and a smaller secondary-to-focus distance. I don't like this, but it is often chosen. A simple experiment with trigonometry shows the inside diameter of the UTA should be at LEAST the size of the primary mirror PLUS the diameter of the field stop in your lowest
power eyepiece. For me, that's 12.5" (317.5mm) plus 38.7mm (field stop on a 35 Panoptic).
Your lowest power eyepiece will probably be a little shorter (say, 28mm) so the field stop could be a little smaller and your UTA a little smaller.

In my case, I got lucky. My focal plane was only 2.5cm above the top of the focuser when racked in all the way, yet all my eyepieces have focused with the Paracorr, and none has required the focuser move all the way in. Technically, with the collection of eyepieces I currently own, I could have designed my scope with slightly longer trusses. But, since I may choose to use a Barlow lens at some time, I may desire to have a little more in-focusing
capability, so I am content to leave it the way it is, even though my secondary is 21% of the width of the primary mirror (at f/5).

I hope that helps you.

Happy New Year,
Don Pensack

davidpretorius
02-01-2007, 10:48 AM
thanks, heaps of detail there.

really appreciate you posting