ETX/DS TECHNICAL TIPS Last updated: 15 March 2002

ETX SCT field widener review

Sent:	Saturday, March 9, 2002 11:55:08
From:	thestarman@atlascomm.net (DC)
I recieved the Shutan part # 5956290, called the SCT adaptor. This was
ordered to see if I could use the Celestron f6.3 corrector and the
Shutan SCT adaptor to widen the ETX field and reduce the focal length.

The test was done on a standard 90mm ETX tube assembly (the spotter) and
was placed on a Celestron G5 german EQ mount. (This same mount is
available from ORION under a different name, called the EQ 2).

I actually am doing all of this to widen the field of view on the ETX 90
to be used as a guidescope for my LX200, and the problem was to find a
guidestar you need a wide field, bright stars, but the ETX being a MAK
is often too dark with a 9mm eyepice and those glowing red dual
crosshairs on the MEADE illuminated & adjustable reticle eyepiece!

But as this combination worked so well, I wanted to share it with ETX
users as it is also a visual dream come true.

Keep in mind, the whole assembly sticks out about 3.5 inches from the
back of the ETX tube, so it probably will not clear the ETX 90 forks!
But being careful, it can still be used except at zenith.

As I do not have a digital camera to attach to the scope for a star test
and photos, I can tell you the view is crystal clear, with little
diffraction problems. Maybe in the future I can send you scans done from
my OM-1 35mm at a later date, but there was no clear skies for this test
on the date of this article.

The Shutan ETX SCT adaptor ring fits well. The included Shutan info says
to NOT tighten it too much. It appears that if you did, these things can
stick and getting them loose is a problem. Also it is possible to break
the ETX metal thread loose from the plastic housing. (Hear that MEADE?).
But I see not problems for a careful user of this part.

The corrector is a Celestron f-6.3 field widener, or a focal reducer
used for SCT photos work. They work well on any SCT and MEADE also makes
a similar version. I have tried both and they seem identical in every
way, so both are good. It weighs about 6 ounces. The Shutan SCT adaptor
weighs about 6 ounces, and then you have to add a visual back, a star
diagonal and of course an eyepiece. But the total weight is about 1.2
lbs. This may be too much for a ETX on the To Go mount, but maybe not.
As I do not own a full To Go ETX, I do not know.

OK, here we go. The test for a wider field was done using a MEADE 25mm
plossel, a 45 degree upright star diaonal (So I could read measurements
on a yardstick at 21 feet). I sat the ETX tube with the complete widener
assembly attached and looked at a yardstick exactly 21 feet away. The
standard ETX at f-13.5 showed 3.5 inchs in the eyepiece across, and the
widened filed showed a full five inches!

As the corrector is called a f-6.3 field corrector, that means it
converts a SCT f-10 to f-6.3. This in percentages is a 63 % reduction in
focal length, and a 37% wider field on any SCT. The ratios are the same
on the ETX.

The formula came out yielding a new ETX ratio of Focal Length 787mm, at
f.8.7. The field of view is 37% wider, and power is reduced 37%
accordingly. And again the actual yardstick measurement at 21 feet was
f-13.5 = 3.5 inches, and f-8.7 = a full five inches! Much wider! A
success! Upon doing the SAME TEST using a Celestron G5 SCT f-10 tube,
it's field of view at 21 feet was FOUR inches (no corrector). So this
makes the ETX a wider field than any SCT at F-10 and the estimate of
focal ratio is really about f-8.7.





The cost are..

Shutan SCT adaptor $36 plus shipping.
Celestron F 6.3 corrector $119 (shop around!)
An SCT visual back ring (price unknown)
An SCT star diagonal (about $49 bucks)
and use youe existing eyepieces...






So for around $200 bucks you can do all of this, a bit pricy.

You can go the other way and order a more lightweight field widener, I
understand some are out there in the form of a reducing star diagonal,
but this way you can use SCT parts, and that means I can attach my OM-1
35mm camera using the OM-1 T bracket ring or do whatever I need to do!

While I was not rigged for astrophotos at the time of this article as
seeing was not good enough, I did get to look through passing clouds and
got a clear view of Saturn and Jupiter. They of course seem smaller, as
the power is reduced, but the field of view was much wider! ! Clean and
crisp and full contrast views with perfect all the way to the edge
focus.Putting on a Televue 11mm, the rings if Saturn showed the Cassini
division clearly and bands on Saturn and 3 moons. Jupiter looked as good
as in the regular ETX and better than my LX200! I add that an ETX Tube
only is front heavy. Adding all this gear balances it almost perfectly,
so for ETX tube owners, this is the way to go! It makes you a real nice
90mm APO f-8.7 refractor like scope! There was no visual distortions I
could find. Perfect focus just as the regular ETX we all love to use!

I am well pleased I did all of this, but as I already had all parts
except the Shutam SCT ring, the conversion only cost me $42 with
shipping. Shutan shipping cost was over $6 bucks..a bit high, but it
came fast and well packed with a Shutan catalog. The guy at the store
could not find the part number. But just tell them it is an ETX -SCT
adaptor ring, they know what to ship.




Hope this helps someone, and please tell people who wish to use the ETX
as a guidescope about this.

Anyone wishing more info write me ( I may be slow getting back as I have
to work and all that stuff).

Dan Callahan
thestarman@atlascomm.net
The Springfield Astronomical Society

Return to the top of this page.

Go back to the Tech Tips page.

Go back to the ETX Home Page.