K3's Equipment - Telescopes
"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4

Cooling Fans Modification of my 8" Newtonian Telescope
or How to Enhance "Local Seeing"
I own my Orion Optics Europa 200 F6 scope almost 2 years. During that time I enjoyed it very much. But I encountered problems with seeing. Long time I was in the conviction, that my problems with turbulence were caused by my balcony location (in the middle of block of flats, heat currents from the lower flats...).

But recently I read very interested message in the Orion Optics discussion group from Jon Grove, who tested cooling fans to speed up telescope cooling. The results were very impressive. I was also motivated by Dave Scott's telescope modification and also Chris Heapy Newtonian modification.

The matter of seeing problem is that the overall seeing consists of 2 components - atmospheric seeing and local seeing. While atmospheric seeing cannot be influenced, the local seeing condition can be affected. Newtonian scopes suffer from internal air currents it their tubes. They are caused by temperature differences inside tube. As my scope is normally stored indoors, it takes a long time for getting thermal equilibrium. In my case the scope must be placed outside at least 2 hours before observation. In many cases the ambient outdoor temperature is changing all the night, so sometimes it is not possible to get thermal equilibrium at all.

The modification of the scope resides in adding cooling fans to the rear side of mirror cells. They suck in the air at ambient temperature and blow it to the back of main mirror and then outside of optical tube assembly (OTA). Practical tests showed me, that cooling down process with my fans was shortened from more than 2 hours only to 20 minutes!!!

The whole modification was done without any special tools in our kitchen. I write abot this, because I would like to encourage those owners who hesitate about the modification.

Parts list:

Part description Type Pieces
12V/1.9W cooling fans (60mmx60mmx15mm) SUNON KDE1206PHS2 3
fan grills   3
power supply connector   1
bolts M3x25 6
bolts M3x30 6
nuts M3 18
washers M3 12
rubber washers (bought in waterworks shop) M3 12
rubber sealing O-ring 6/4" 3
old 3.5" black floppy disk Verbatim ;-) 1

Construction photos:

Pictures can tell you much more about the modification than my English ;-). Click the pictures to see them in full size.

Before modification:
Original back cover with 3 collimation bolts and nuts

The primary mirror cell mounted to back telescope cover. The primary mirror is removed.

The primary mirror cell dismounted. It remains untouched during all modification.

Back cover without primary mirror cell. The holes for motors are drawn.

The most time consuming process was cutting the holes for fans. I used jigsaw.

While the first hole cutting took me about 40 minutes, the third hole was done in 20 minutes :-)

Finally the holes for fans are cut. Also conical openings for sinking M3 bolts were drilled.
Now we can start to mount fans grills.

Fan grills are mounted from inner side of back cover. The fans are isolated from the back cover by rubber washes - to suppress vibrations. Because of gap between fan and cover (created by mounting grills and rubber washers) the O-ring rubber sealings were added around fans perimeters (O-ring were cut into 4 pieces).

Detail of fan mounted. You can see rubber washers and O-ring mounted.
The inner bolts were shortened to 23mm to keep them from touching primary mirror cell.

View from outer side of back cover. You can see fan grills mounted.

All fans are mounted, so we can create light shields.

Light shields cut from old defective 3.5" floppy disk. They prevent light from entering into OTA.

Light shield mounted.

Mirror cell on its place again...

Detailed view of modification. Modification is almost finished - it remains to put the main mirror to its cell and mount the back cover to OTA.
Don't forget to collimate the scope!!.

The modification is done!!!

My first tests confirmed, that modification is really useful. The scope was cooled down in 20 minutes and I could see Cassini division of Saturn inspite of its low altitude (30 above horizon). Unfocused star showed very nice diffraction rings, so I was able to collimate the scope without problems.

As the construction of back cover is open now, I also tested the behaviour of the scope when direct light is applied to back cover. I used battery torch for my tests. When torch was applied directly to fan holes, I noticed some reflections, inspite of light shields were installed. The reflections originated in glass of main mirror.
I plan to use black textile (used for speaker boxes) for covering the fan holes. I did preliminary tests with it and I am totally satisfied.

Out of doubts, I am very satisfied with the modification. Now I have to wait for better weather for deeper tests.

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Last Update: 01.04.2003