From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Ralph Seguin" 
>
>Oh yeah.
>I also forgot to inquire about sizing for secondary
>and tertiary mirrors.
>It would seem to me that I would need to place the
>secondary closer to the primary, and thus might need a
>larger secondary mirror.

Hi Ralph
 Yes, the secondary needs to be larger. You can use the
normal telescope programs on the net to determine the size, depending on
the additional length of light travel, to the eyepiece.
 One should note that you'll mostly be using the bino at
50X to 100X. This means that it is desirable to have a larger 100%
illuminated field spot than would otherwise be chosen.

>
>I'm hoping to use star diagonals for the tertiary
>mirrors to simplify the building process and save time
>(in short supply).

 Each has their advantages and disadvantages. The prism
type shortens the optical path a little ( 75% of the path length in the
prism ) but suffers a little from spherical aberrations. The mirror is the
opposite.

>
>Thanks.
>-Ralph
>
>--- Ralph Seguin  wrote:
>>
>> Ok.
>> In an insane rush to get things done before our trip
>> to Hawaii next week, I want to put together a
>> binocular telescope that I can bring with us.
>>
>> I have a pair of matched 6" f/5-ish mirrors (would
>> prefer 8", 10" or larger, so if anybody has any...).
>>
>> My questions are:
>> -What are the necessary adjustments in order to get
>> proper image fusion?

 There are a number of things that can be out of adjustment.
1. Field overlap
2. Field rotation
3. Image overlap
4. inter ocular adjustment
5. focus
6. normal collimation

 Some of the things above interact some as well.
Here is what causes each to be off:

1. Field overlap
 This depends on the relative angles of the eyepiece's
optical path ( not necessarily parallel with the eyepiece's mount ). What I
call field overlap is how well the field overlap when looking at something
like blank sky. This is not the same as image overlap.
 It should also be noted that the impression of a larger
field of view can be had by slight changes in field overlap. This is a good
reason to have this adjustable so that you can experiment and determine
which you like best. 2. Field rotation
 This is an interesting one. Actually, most people have
little problems fusing the image with quite a bit of field rotation. The
problem is that it quickly cause eye fatigue.
 This is adjusted on the typical centered eyepieces by
rotating the eyepiece/tertiary assemblies on an axis from the tertiary to
the secondary centers. 3. Image overlap
 There are a number of ways to deal with this. From experiences
that I have had with this, it is best to adjust on OTA on a horizontal and
the other OTA on a vertical. Most systems the try to mix both on the same
tube end up causing problems. Using adjustment at 45 degrees will cost you
hours of setup time, even though it keeps both OTA's identical.
 The adjustment knobs should be in easy reach of the person
viewing.
 These adjustments, if practical, should pivot around the centers
of the secondaries. This is usually not practical on larger binos.
4. inter ocular adjustment
 Sliding things around can usually handle this. It should
be noted that most star diagonal housing will interfere for those with
close set eyes. The requirements for exact inter ocular are relieved a
little with larger exit pupils ( bigger primaries and lower power ). Still,
this is always a difficult adjustment. Some have solved it by rotation of
the entire OTA's. This can effect #3 if not designed correctly. If you
intend to have other that are not trained in how to readjust #3 use the
bino, you might consider linear slides as a simpler option. This will still
require more exact angling of the secondary than is normally required for a
simple Newton.
 It should also be noted that this effects the focus as well,
giving points back to the OTA rotation method. 5. focus
 It is desirable to have some method of focusing a single
eye, as well as a tracking focus, like on regular binoculars. If there are
only a limited number of people using it, separate focus adjustments are
OK. Helical focusers work well for separate focus, as long as they are
small enough. Chris Angelos and I have been using just the setscrews for as
focus adjustments for some time and that works for most.
 You should also give up all hope of using 2 inch eyepieces.
There just isn't enough room between peoples eyes. 6. normal collimation
 A lot of things that were not all that critical, like
secondary offset and OTA center on the other axis become a bigger issue
with Bino's. Rather than fighting every possible thing that can be
adjusted, I've been know to slightly miscollimate in order to get on with
viewing. It is unlikely that you'll be able to do really high power and
keep the alignment of #3 and be able to move the telescope. Just keep this
in mind. It is better to sacrifice a little of something than to not view
at all. Note what didn't work right and play with it later during the
daylight.

Some other thoughts:
 Make a dual target with a lot of exact copy detail to
use during the day. It will be confusing to use object at normal distances.
Still, focusing will be difficult if you can't get the target to about 1/8
miles or so.
 Have the cells mounted such that you can easily change the
distance down the tubes. Most cell adjustments are not enough.
 Keep things as square as you can. Things like squaring
the focuser and the secondary ( that are not really critical in a normal
Newton ) can cause all kinds of problems in a Bino. The only thing that you
might want to tilt slightly out of square are the eyepieces ( see #1 ).
 Hope this helps some.
Dwight

>>
>> -How do you handle IPD adjustment?  I've thought of
>> using a linear bearings/slides for moving one of the
>> entire tubes, or by having focussers on one or each
>> tube to handle side to side (IPD) translation.
>>
>> Problem with the first solution is making the
>> structure super stiff.
>>
>> The problem that I see with the 2nd solution is that
>> it will require additional in/out translation to do
>> focusing (not a big deal).
>>
>> -How much of a problem is field rotation?
>>
>> -How do you deal with it?  I've been told that field
>> rotation is the biggest headache cause for fusing
>> images.
>>
>> -Prisms or mirrors for tertiary mirrors?
>>
>> -How do I carry this on a plane without having them
>> assume I am a terrorist?
>>
>>
>> Thanks.
>> -Ralph
>>
>>
>>
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