Orion 120mm F8.3 Focuser Upgrade

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  • Member since
    July, 2003
  • From: Eastern SD.
Orion 120mm F8.3 Focuser Upgrade
Posted by johnjohnson on Sunday, January 27, 2008 6:07 AM

Due to the bad weather we have had lately in S.D., I decided to upgrade the focuser on my Orion 120mm F8.3 refractor. As many of you know the focusers on these refractors are lacking in many ways. Focus shift at high power, and stars may not be perfect points of light because of focuser droop, especially with heavy eye pieces. I found a 2 speed Crayford style focuser available from Agena Astro products.
GSO Crayford
It is an exact replacement to the old focuser. Remove the three screws holding the old focuser in, install the new focuser, tighten the screws! This thing is a dream compared to the old focuser. Now I have  2 speed focusing, no image shift and it will handle heavy camera loads. It also allows you to rotate the focuser so when you get into an awkward viewing position with an equatorial mount.
Modification Images
I decided to check its rotational accuracy by collimation. It was off a bit as you went through a 360 degree rotation, however there are three set screws on the focuser assembly that allow you to center the focusing tube. After several iterations of adjustment and collimation, I determined that the focusing tube held collimation through out a full rotation.

Last night was clear so I set the unit up and wanted to confirm the telescopes collimation. I used Arcturus with a 8mm Baader hyperion EP. I did the star test in eight locations of rotation through the full 360 degrees of focuser rotation. Perfect star tests through out the rotation. Concentric rings showed before focus and behind focus. In focus showed a beautiful airy disk. I was extremely pleased.

Actual observing. I wanted to start out with a wide field view of an object so I started with M45. I used a 21mm Baader Hyperion EP and was rewarded with a magnificent view. This gave me a view at 48X with a 1.43 degree FOV. I was swept in. Stars were tack sharp across the FOV. Nebulosity was noted around the brighter elements in the formation. The Moon was fairly bright but I wanted to try a Globular cluster. M3 was up high so I aimed the scope there and put in a Baader Hyperion 8mm. This gave me 125X and a 0.54 degree FOV. With the use of the 2 speed focuser I could focus a few tiny resolvable stars in the fuzzy globular area even with the bright moon. I was pleased with the view. Now a planet, Saturn, still using the 8mm Hyperion I placed the telescope on Saturn. The view was beautiful. Saturn revealed slight banding on the Orb, ring shadow on the Orb, and Orb shadow on the rings. The Cassini division came and went with seeing conditions. Two moons were noted (I'll have to check to see which ones they were). My last test of the focuser / collimation was on the Moon. I used a 13mm Hyperion for this. It yielded 77X with 0.88 degree FOV. This made the Moon almost fill the EP. The terminator was glorious. Deep shadows, revealing very high contrast. Focus was tack sharp in the entire field. The true test of the collimation is revealed in the telescopes chromatic aberration. With the perfect focus that can be achieved with the new focuser, and the collimation, the only noticeable CA was a light violet fringe around the limb of the Moon. It was tight to the edge and no other colors were noted. This is not bothersome at all. I can handle this slight aberration easily.

In summation. Although this focuser costs 2/3'rds the price of the telescope, it is really worth it to be able to eliminate focus shift and maintain perfect collimation. The collimation helps to reduce chromatic aberration and maintain point sharp stars in the field. I am extremely pleased with this telescope combination. I highly recommend this modification to anyone wanting to upgrade their telescope to a higher degree of accuracy. The accuracy shows in the eye piece. The cost was rewarded by the views!


20" F5 Obsession, OMI mirror .987 Strehl. 10" F4.7 reflector. 6" F5 ST reflector. 120mm F7.5 EON. 80mm F11.3 guide scope. SkyWatcher EQ-6 Hyper Tuned.   Flicker Astro Site   More Astro Images

  • Member since
    November, 2007
Posted by astronig on Sunday, January 27, 2008 10:56 AM

Great post John ! , I've been eyeing this unit also , It's available at Scope Stuff too , Adapter plates available for Synta Refractors and also the AR5 , AR6 & Antares !....It's a fraction of what Moonlight wants for thiers!.................Great info , Thank for the post!CoolDave

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