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Old 25-06-2019, 07:30 AM
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mynameiscd (Andy)
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Setting secondary mirror

Hi all,
Any help on setting the seconary in its corect depth
Perfect night on sunday so I set up and thought i would try my seconary dew heater.
I ended up unscrewing the centre screw to remove the secodary to put the stalk type heater on. I screwed it back to where it was but had a lot of trouble with collimation esp with the depth and twist of the mirror.
I did a check with a laser and it looked ok.
After getting 3 hours of data back and stacking my images are all sharp in the middle, not too bad on the right side, but heaps of paisley comets on the left which means some sort of twist in the mirror.
This part of collimation i have always had trouble with. Ive read lots of info but the secondary offset with f5 scopes goes over my bead a bit.
My scope is a Skywatcher black diamond 150mm f5
Collimating tools are orion 1.25 Collimating eyepiece and 2 lasers which have checked for collimation.
Any help with this exact scope because most info i read is about larger newts or dobs and confusing images of secodary offset etc.
Is there somd sort of tool or method to set the rotation and depth without scratching the mirror?
I thought of using some sort of tube or ruler that touches the mirror but even that could be rotated a little to get a false reading.
Also there's adjustment on the spider to add to my troubles.
Still a bit confused
Cheers
Andy
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  #2  
Old 25-06-2019, 07:49 AM
glend (Glen)
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Read Astrobaby's Collimation Guide, it's all you need to know.
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Old 25-06-2019, 09:27 AM
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mynameiscd (Andy)
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Thanks Glen,
I'll have a look at that site tonight.
Cheers
Andy
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Old 25-06-2019, 12:50 PM
RyanJones
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Hi Andy,

Something I would add and I also found with mine is that over time screwing the secondary collimation screws in and out puts burrs on the secondary holder. This is not a major issue during normal collimation, however, when you remove the secondary and potentially the spider and you go to put it all back together again you may find ( as you have ) that you need to make rotational adjustments. When you try to align the secondary with your focuser it will line up but as you then tighten up the center screw the secondary will walk itself slightly back onto the previous grooves causing poor alignment. Luckily the solution to that is simple. Remove the secondary, remove the old burrs and fit a think fiber or nylon washer to the surface where the collimation screws make contact. Reassemble and problem solved. The other bonus of this is that you can rotate the secondary now while it has a slight tension on it as the washer creates a bit of a bearing surface. This maintains where you set it a lot easier than having it loose to move it.

I hope this helps
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Old 25-06-2019, 02:38 PM
Startrek (Martin)
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I had issues with my 6” f6 secondary and primary a while back , stars looked odd shaped on one side or flaired on one side
Did the following to resolve -
1/ Bought a 1.25 Cheshire , learnt how to use it on Astrobaby, it’s a must to use one when aligning your secondary
2/ Cut a oval piece of plastic out of a 2 litre milk bottle the same size and shape of your secondary , drilled three 5mm holes , placed it between the secondary holder and base , as the secondary screws after time gouge out or pit the base which creates a huge issue when adjusting. The plastic makes your adjusting very smooth and precise
3/ Replace primary springs with new custom springs with more length and tension ( Better Springs at Winston Hills Sydney) The primary mirror now is so easy to adjust and so precise

After doing the above items my 6” f6 is so easy to collimate now and so precise
Stars are correct shape with pin point diffraction spikes
My spider vanes we’re out a bit so used a small metal right angle to square them up 90 deg / 90 deg on the horizontal and vertical


Aligning your secondary is one of the most important part of collimating your telescope !!!

Hope the above helps
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Old 25-06-2019, 07:22 PM
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mynameiscd (Andy)
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Thanks Ryan and Martin,
Just got home.
Will try all the advice you have given me. Just printed out astrobabys guide so if i get time I'll have a go tonight.
Work always gets in the way.
Cheers
Andy
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Old 25-06-2019, 11:10 PM
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Merlin66 (Ken)
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Andy,
You don’t have to worry about secondary mirror off-set, this is designed into the commercial telescopes nowadays.
DIY construction is a different matter......
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Old 26-06-2019, 05:08 PM
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I like the milk carton Martin. My secondary is a bit pitted so adjustments are not very smooth
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Old 26-06-2019, 05:30 PM
Startrek (Martin)
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Chris
Apparently the plastic used in a 2 litre milk bottle works the best, and I can vouch for that , mine is so much easier to micro adjust now
Can I suggest to cut the oval shape a couple of mm less than your secondary but large enough to cover the area of the base
Also the new custom springs fitted to the primary made a huge difference as well ( they weren’t cheap , cost me about $15 each ) but they will outlast the scope and so important as we all know. The springs that came with scope were extremely poor quality ( as you would expect for a GSO $299 Newtonian telescope)
I had my new GSO 8” f5 fitted with new primary springs as well , once again made all the difference when collimating
Cheers
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Old 26-06-2019, 10:01 PM
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mynameiscd (Andy)
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Hi all,
Just done the astrobabys collimation guide.
The light was not the best and I'm pretty tired as well so I might give it a check during the day.
Also did the plastic milk carton thing as well.
The spider was about 15mm off centre so that should make a difference. Its a bit tricky to get the rotation spot on so the mirror is a perfect circle esp with bad light.
Too tired to do a star test tonight.
Thanks everyone for steering me in the right direction.
Cheers
Andy
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Old 27-06-2019, 11:56 AM
Startrek (Martin)
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Apologies as I looked back on my original post and said drill three 5mm holes in the plastic which is incorrect, I meant to say just drill a hole in the centre the size or your centre fixing screw
I assume you picked this up anyway as drilling 3 holes where the adjusting screws are is of no use
Silly me
Cheers
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Old 27-06-2019, 08:42 PM
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mynameiscd (Andy)
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Yeah,
I worked that out Martin but thanks for the tip.
Its far to windy here for a star test tonight so maybe tomorrow
Cheers again
Andy
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Old 28-06-2019, 02:33 AM
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Jason D (Jason)
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Quote:
Originally Posted by Startrek View Post
to cut the oval shape a couple of mm less than your secondary but large enough to cover the area of the base
Just curious, why did you make the oval shape cut that follows the secondary mirror base shape? I put my Milk Jug washers above the stalk holding the secondary mirror which means I make circular cuts -- see attachment.
In addition, for those who do not want to remove the secondary mirror to place the washer, they can make a radial cut then slide the washers in -- again see attachments. Finally, even though drilling a hole gives you a nice circular cut, a small scissors can be used to make the hole cut.
Jason
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Old 28-06-2019, 07:57 AM
Startrek (Martin)
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I did this a while back and I couldn’t remember the exact shape plus everyone’s scope is different so a bit of overhang wouldn’t hurt as long as it’s smaller than the mirror to avoid obstruction
My scopes centre fixing screw which attaches the stalk to the base is only 6.5mm or quarter inch so you can’t cut that out with a pair of scissors. I just used a cordless drill with a 6.5mm bit over a block of wood and drilled a neat hole. This make shift plastic “washer” fitted perfectly and solved the issue
Cheers
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Old 29-06-2019, 02:23 AM
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Jason D (Jason)
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I was curious about why you needed the washers to have an oval shape but based on your response the oval shape was not really needed.
Using the tip of a small scissors is adequate to cut small holes. That is what I did.
I first cut and installed my Milk jug washers 10 years ago and I never had to change them. They are quite durable.
Jason
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