Star Test

[vindictive666]

hello everyone

i finally got around to do a star test last night
found a reasonable star not to bright not to faint (dont ask what mag :) )
I used a 9mm 1.25 inch GSO (is this ep ok to use ?)

i had a look at the star iether side of the focus the only thing i could see was no rings what it looked like was a slide moving amoeba under a microscope
plus i had the scope outside for at least an hour or more before trying the test
so cooling not a problem ?

also stuck in the 2x 2" barlow saw the same thing :)

must admit using the same ep setup had a look at the moon wow factor real close and moving quick :) (out of fov)

had a look at jupiter as well the tails on the moons seem to have gone
iam still getting cross hairs thought so whether this is still a cooling problem or i dont know

those four spikes ive never seen before until very recently, before it got collimated the time before i removed the mirrors :) it looks like four light streaks though could be it hasnt cooled down enough
ill keep an eye out for them next time i can go out :)
any hints ?

:bashcomp: :ashamed: :scared3:

[asimov]

Hmmm interesting....The 9mm SHOULD be ok to use for this purpose...How far each side of focus are you going John? The "seeing" conditions have to be pretty good for a star-test. Also the star you're testing with, has to be fairly high up in the sky. Is this a 12" mirror we're talking about here? If so, 1 hour is not enough time for the mirror to cool successfully...My 12.5" takes at least 3 hours..Sounds to me like you're taking it too far each side of focus....perhaps?.....John, the 4 spikes you are seeing is from your 4 vaned "spider" holding your diagonal mirror....this is normal.

[slice of heaven]

The 'amoeba moving' your seeing is probably the boundary layer. Thats the layer of air sitting on top of your main mirror. How turbulent it is shows how cooled down the main mirror is.A really warm mirror will show a lot of movement.
There are ways to remove the layer, maybe someone here has worked on removing the layer on the gs scopes and can jump in with their thoughts.
The 'crosshairs' are the diffraction spikes from the spider.There is no way to remove them apart from replacing the straight spider with a curved one, even then your only spreading them out , not totally removing them.
Use as high a mag ep as seeing will allow to startest. You should have still seen the rings with the 9mm.???? Maybe not perfectly but enough to show you the line up of your optics, which is what your after. Keep the star perfectly centred while assessing the startest. On a good night you can determine the finer details of your optics. A lot of which you cant do anything about.Theirs a link on another thread with some details of startesting with pics.
Your tails are gone so thats good. Reducing the ugliness of the image makes it all worthwhile.

[asimov]

The boundary layer can only be "removed" be either: waiting until the mirror is +- 1 degree of the ambient air temp. Or "blowing" the layer away from the mirror using Alan Adlers technique. Someone correct me if I'm wrong please..

[slice of heaven]

Thats the normal method John, its the setup of fan placement or a baffle I was wondering about. I dont know if these tubes could handle having a fan placed above the top of mirror.
An hour is a reasonable amount of time to cool John's scope with the fan on John.
Try one , youll like it. Great for those 'gee,the clouds have gone,get the scope out' moments. Cool down times are quicker on our fan forced gs than the fanless Parks pyrex, both 12",give or take half an inch.

[asimov]

G'day Slice..Oh yes, with a fan on it takes less time, agreed. I've been experimenting with fans for a couple of years. On John's 12" I dont think he's using a fan...or is he? If he's not, an hour is not enough time....not in my experience anyhow. Of course, that would depend on how far in temp the mirror has to drop to reach ambient. I cheat these days & bung the OTA in the lounge room & put the air conditioner on, taking temp readings of the mirror compared to the temp outside. when the mirror is 2 degrees below ambient, in the saddle the OTA goes.

[Starkler]

What has worked for some is a ring 'baffle' attached to the top of the mirror clips with outer diameter to seal against the tube wall and an inner diameter about equal to the mirror diameter.
A fan is attached to a board sealing against the rear of the mirror cell. This setup has the 'baffle' directing the airflow to create turbulence at the edge of the mirror and to break up and scrub away the boundary layer.

This is on my to-do list.

[asimov]

Iv'e always wanted to follow Alan A's advice and cut a hole in the side of the tube & mount the fan blowing across the mirror & out the other side, but I'm not game!

[Starkler]

That may not be the best option as you cannot ensure that this air is evacuated out the side hole unless you have two fans, one blowing and one exhausting.

[slice of heaven]

The baffle is my preferred option too. Side fans are probably a bit too much of an overkill for small mirrors. The baffle is a much easier choice to play with. Maybe make it stiff enough and fix it to the tube to keep the tube in shape with the mirror cell removed.
If the scopes are really well cooled the layer doesnt bother me, but when their not......

I'm the same as you Geoff, it's on my to do list .
I've enclosed the bottom of the scope and fitted the variable speed to the fan, just need the right baffle, in the right spot.

[fringe_dweller]

I agree with you John I am a bit concerned about pumping dust all over the primary on regular basis! I know a little bit of dust (or even a lot sometimes) isnt a big deal - as it only covers a small portion of the total surface area of course - but i do subscribe to the theory of not washing your mirror very often - a Pain in the neck process - plus it quickens the demise/oxidisation of the mirror coating? - i have heard the modern mirror coatings are much tougher and better but.
A gentle fan at the front end of the tube - pulling the air out - has been suggested to me - but i dont know? :confuse3:
Kearn

[ballaratdragons]

I experience the same effect John (Vind) is talking about. A boiling bubbling appearance to the Star test. The best star test result I have ever had was when I accidently left the fan running (again!) I turned the fan on for the 5 min cooldown and forgot it was on. About 15 minutes later I went back outside and did a star test and it was the best I had ever acheived! Then I heard the fan quietly purring. I turned it off, scoped for about half an hour and Star tested again. Bubble, bubble, toil and trouble. That pesky boiling was back. Turned the fan on again and in about 30 seconds it was gone!

Give that a try John. After a looooong cool down whack the fan on and do a star test after it has been on for about a minute.(leave the fan running during the test)

Let us know how it goes.

[asimov]

Hi Kearn. Iv'e tried putting a fan behind the primary recently & sucking the air down over the primary. The result was not good. What happens is,* I THINK* as the air flows into the tube, the whole column of air starts to rotate as it gets closer to the primary.*caused by the the rotating action of the fan perhaps??* You can actually see it in the EP if you de-focus a fair bit. I THINK what I'm seeing is perhaps not a column of air as such....but the actual boundary layer rotating on the mirror?:confuse3: Food for thought.....

[fringe_dweller]

Gday John, thats very interesting!! - and weird!! some sort of vortex developing :) I think i will stick with the old fashioned cool down for now :( - If i want a quick look I usually use the 8.3" takes no time at all to cool down - in fact sadly that is the scope i use the most these days - just get the 10" out on special occasions these days - which is a waste of course. I really really like Birds set up - but I subscribe to the KISS principal and I would probably only mess up using that system :confuse2: plus I really HATE BATTERIES AND POWER SUPPLIES!! LOL
Cheers
Kearn

[ballaratdragons]

The fan on GS and most others blows up the tube away from the Primary. When left running it also slows or eliminates dew build up on the secondary. Does on mine anyway.

[syzygy]

I've seen a variation of the rotating column of air effect when using a dew heater as well as a cap on my SCT. The air warmed by the heater seems to cling to the inside of the dew cap and slowly spiral to the top and out. This is clearly seen in the out of focus star pattern. In this case though, probably because the rotation is so slow, it seems to have no discernable effect on the in focus image.

Regards,
Chris

[ausastronomer]

I am 80% of the way through making a few more mods to my GS scope. One of those includes fitting an "air pump" to remove the boundary layer. I will let you all know the results when I get the job completed. I just need my wife to stop finding "non-existent" jobs around the house that she thinks need doing.

CS-John B

[ausastronomer]

I drilled a 6mm hole through the side to feed in an air hose but I haven't finished the job so not sure if it will work but I am pretty hopefull.

I have used 6mm silicon air hose with holes drilled every 25mm and connected this to a 12V live bait pump. I have test run it in the garage and it definately gets a little air flowing across the telescope tube about 10mm above the primary which should be all it needs. Once I have tested it and make sure it works how I want it to I will post some pics of the whole setup showing how the hose runs around 50% of the circumference of the tube and is fitted to little clips on the bottom of my primary mirror baffle.

CS-John B

[iceman]

I wanna see a full and comprehensive how-to with pics, for the site John! :)

[slice of heaven]

I read of using injected air on CN but I shivered at the thought of their single nozzle blasting air over the primary. Your gentle multi-nozzled approach sounds more acceptable John B. I'm anxious to hear how it performs.
The baffle above your primary :Was it for masking the edge or reducing the boundary layer or something else ? Have you tried removing the layer with the fan and baffle setup? I'd like to get rid of that layer, another worthwhile tweak to improve performance.

[ausastronomer]

Slice,

Its an annular light baffle made from 10mm plywood. It doesn't mask the primary at all.

CS-John B

[Orion]

Interesting idea John. Would the pump have to run continuous or do you tune it on for a few minutes? I've used these pumps before and some tend to be a bit on the noisy side. Love to see how it turns out.

[[1ponders]]

Not being a newt user I may be way out here, but what about birds temp modifications to his mirror? Wouldn't that work as well. If the problem is turbulance created by the temperature differentail of the mirror boundary layer and the OTA air column, wouldn't chilling the mirror help to releave this problem???

[slice of heaven]

Yes your right Paul, Birds setup is ideal, but its out of my league. I'm looking for a simple way of removing the layer, something low tech.The fan is already fitted so I'd like to incorporate that in removing the layer. If that fails then something like Johns method might have to be used, but like Orion stated the air pumps are noisy and they vibrate .

[ausastronomer]

Ed,

Your right, the pump is a little noisy, and it vibrates a little. My intention was to only run the pump for 15 mins or so prior to observing whilst I also had the cooling fan blowing onto the primary and up the tube. I think in conjunction with the fan I will only have to "stir the air" above the mirror for a short period to get rid of the boundary layer and tube currents. I have mounted the pump firmly to the side of the rocker box so vibration and noise should be dampened a little by this.

CS-John B

[[1ponders]]

I know you guys with your newts don't suffer as much from dew problems as us lens users, but if your game enough to put a larger hole in the side of the tube, how about a 12V hair dryer. Use just the fan setting initially to help remove the boundary and if it starts to fog up during the night, switch to heating to remove dew. ?????

[Orion]

I seen a set up for a trust tube telescope which a small fan was suspended from the corners of the mirror box above the mirror with thin cable. The positive and negative wires for the fan ran along two of the cables to a power source. The cables coincided with the diagonal veins.

[bytor666]

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/1,2,3,4,5,8,9,10/Number/458453/page/2/view/collapsed/sb/5/o/all/fpart/1 For Optimum performance..check out this thread. this next thread is great too..check it out..it'll help immensely. http://www.cloudynights.com/ubbthreads/showflat.php/Cat/1,2,3,4,5,8,9,10/Number/360923/page/5/view/collapsed/sb/5/o/all/fpart/1
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Mark
12" Gso Scope