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Posts Made By: Allen Tracht

August 31, 2005 03:40 PM Forum: Eyepieces

Eyepiece Suggestion

Posted By Allen Tracht
I'm looking for a 2" eyepiece for a home-brew finder scope. Ideally it should be about 32mm FL, 70 degree AFOV (for a 46 mm field-stop diameter), but 35 mm FL, 65 degree AFOV might be more practical (even if the exit pupil is getting large).

Weight is the most important issue. I can't have the eyepiece weighing as much as the OTA.

I'll sacrifice optical quality to save weight and cost.

I think I may be asking too much. Still, I welcome your suggestions.

Allen

August 31, 2005 06:54 PM Forum: Eyepieces

Field stop size and AFOV

Posted By Allen Tracht
I thought the field-stop diameter in a distortion-free eyepiece should be focal-length x 2 x tan(AFOV/2).

However, for a 31 Nagler T5, with AFOV = 82 degrees, this gives a field stop diameter of 31 x 2 x tan(41 degrees) = 31 x 2 x 0.869 = 53.9 mm, but the TeleVue spec says 42 mm.

For a TV 32 Plossl, FS = 32 x 2 x tan(25) = 29.8mm which is still 10% bigger than the spec'd 27mm.

Why don't these numbers match? One guess is that the focal "plane" is spherical, then 'tan' would be replaced by 'sin'. Now, for the Nagler, we get 40.6 mm and for the Plossl we get 27 mm, much closer to the actual.

The actual focal surface cannot be spherical, because the edges of the field would be far out of focus (7.6 mm for the Nagler, and 3.0 mm for the Plossl), but the numbers don't work for flat focal planes.

What am I missing? Distortion? Vignetting? Marketing vs. Engineering?

Allen

September 21, 2005 06:50 PM Forum: Equipment Talk

Basket Carrier

Posted By Allen Tracht
I can only get two of the following three in my Toyota Avalon: family, luggage, and scope. So far, I've left the scope(s) at home on family outings, but what's the fun in that?

Changing vehicles is out of the question, but I've see "basket carriers" like these http://www.accessconnect.com/hitch_mount_basket_carrier_class.htm and think they might give me the extra capacity I need, with much less cost and trouble than even a small trailer.

Do you have any experience with these?

Thanks,

Allen

September 23, 2005 04:51 PM Forum: Takahashi

Clamshell PIggyback

Posted By Allen Tracht
The clamshell on my FS-128 has a threaded stud protruding from its top that is obviously designed for piggybacking some other types of equipment. Have you used it? If so, for what and how?

Allen

September 27, 2005 08:21 PM Forum: Eyepieces

What's a Warranty Worth

Posted By Allen Tracht
I was surprised to learn that TeleVue's 5-year warranty is non-transferrable. Therefore, a pre-owned TeleVue eyepiece, regardless of condition (even new, never used) or age (even one day since purchase), has no warranty.

I know that TeleVue's reputation suggests that you shouldn't ever need the warranty, but it is good to have, at least for peace of mind.

So, what discount would you want before buying an otherwise perfect TeleVue eyepiece without the warranty? 5%, 10%, or even 20% or more?

On the other side: have you ever needed TeleVue's warranty service? For what?

Allen

October 1, 2005 05:41 AM Forum: Equipment Talk

Semi-custom Eyepiece Case

Posted By Allen Tracht
My wife was kind enough to give me a relatively large (15" x 12-1/2" x 7-1/2") camera case for my astronomy gear. At first, I used the supplied pluck foam, but I didn't like it. It didn't fit the profiles of my eyepiece very well, and it wasted lots of space.

When I started to get a more substantial collection of eyepieces, I knew I had to use the space more efficiently, so I removed the foam and installed a 3/4" plywood plate that was drilled with a variety of 1-1/4" and 2" holes. The plate stands on 1" long feet to allow the eyepieces to be inserted more fully. This allowed me to store my eyepieces vertically and leave lots of room for more.

January 17, 2006 08:43 PM Forum: Equipment Talk

Collimating with older eyes

Posted By Allen Tracht
When collimating a Newtonian, one of the recommended methods for adjusting the secondary tilt is to make the primary's center spot line up with the cross hairs in the sight tube.

I find that this is practically impossible to do well. First of all, I cannot simultaneously see both the center spot and the cross hairs sharply: if I focus on one, the other's a blur. This makes their alignment unclear (pun intended).

Secondly, there is the potential for further error if your eye is not centered within the peephole in the sight tube.

Together, these error might result in misalignment by up to one degree (but probably less). Is this critical for an F/4.5 scope?

Are there any tricks for working around these issues? I know that there are other tools I might use, but I'm trying to get the most from this one for now.

Allen

February 6, 2006 08:03 PM Forum: Reflectors

Paracorr and Miscollimation

Posted By Allen Tracht
Does a paracorr help or hurt if the telescope is moderately mis-collimated?

As I understand it, one of the important goals of Newtonian collimation is to aim the optical axis of the primary along the center of the focuser, so the part of the image with minimal coma is centered under the eyepiece.

According to the paracorr instructions, with an F/4.5 primary, only the central 3mm of the image has less distortion from coma than from diffraction. With the paracorr installed, this "sweet spot" enlarges from 3mm to 18mm.

But what if the telescope is miscollimated so the sweet spot is a few millimeters from the optical axis of the paracorr? Could that make the image be even worse than without the paracorr?

I'm sure my scope is never perfectly collimated, even if it is close. How close does it have to be for the paracorr to help?

Perhaps an optical designer with knowledge of the paracorr's design can give us some guidance.

Allen

March 1, 2006 10:31 PM Forum: Refractors

Re: ap 102 refractor

Posted By Allen Tracht
I'm not sure this addresses the exact scope, but it does have lots of interesting AP lens info:

http://www.tmboptical.com/itemsGrid.asp?cat_id=28

Allen

March 6, 2006 10:27 PM Forum: Equipment Talk

Thousand Oaks Optical -- Phone Problems?

Posted By Allen Tracht
I've been trying to contact Thousand Oaks Optical for a few days to get some product details. I sent an email on Thursday and did not receive a reply. So I tried calling them, both on Saturday and today (Monday). Each time I've tried calling, their phone has been busy.

Has anyone had recent contact with them?

Allen

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