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TopHouse

How To Center Spot A Primary

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I replaced the primary center spot on my primary with one of the new fangled 'hotspot' centers to fit in with some experimentation I'm doing with collimating, however this will also apply if you have a mirror 'without' a center spot and need to add one.

Firstly, these are the 'hotspot' centers, there are a few design feature of this particular type, firstly the 'spokes' point at your 3 collimation bolts when fitted properly, aiding alignment using a collicap. Secondly they are reflective, meaning they project better in multi mirror situations, especially if used with an autocolimater, thirdly they aren't symetrical like the normal ring, which is of benefit again if using an autocollimater.

This is what they look like :-

post-18181-133877463226_thumb.jpg

Edited by TopHouse

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Firstly, you need to remove the mirror cell from the OTA, it's just a case of unscrewing the bolts from around the circumference of the tube and giving the cell a pull, you don't have to worry about orientation with the skywatchers as they will only go back one way, having a cutout to accommodate the joint in the tube.

post-18181-133877463229_thumb.jpg

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The job is made a lot easier if you remove the mirror itself from the cell, at this point MAKE SURE you are using the correct size driver since what you certainly don't want is for the driver to slip out and skate across your lovely mirror surface. This is donr by simply unscrewing the bolts on the mirror clips.

post-18181-133877463235_thumb.jpg

Edited by TopHouse

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Now one of the scary parts if you're doing a replacement, removing the old center spot. You need a solvent that will dissolve the adhesive holding it in place. I first tried Isopropyl which didn't touch it, so the next option was an acetone base, I used nail varnish remover which did work, but it still needs a little effort.

Using a cotton bud soaked in the solvent, dab it onto the old spot, keeping it nice and wet and working it in, try to contain things in as small an area as possible since if you do anything wrong, any mark will be within the center and be technically invisible. Eventually the old ring loosens, you just have to work it a little with the cotton bud.

take note that this process will lead to another scary process as it leaves evaporation marks on the mirror when you've done.

post-18181-133877463248_thumb.jpg

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Next stage is the one that instills fear into the hearts of newt owners the world over, you need to clean the mirror, don't fret over it, there are many tutorials on cleaning mirrors on the net, the following works absolutely fine for me. Firstly, give your hands a GOOD wash, use something like fairy liquid.

Now I have very soft hands, not because I do a lot of dishes, they just are, if you have a job where your hands are rough or you're naturally heavy handed, you may want to get your wife to do it.

Using EITHER mixer taps or shower head, hold the mirror from underneath and let water flow all over it, have it about as hot as if you're washing dishes. (ask the wife) if it's particularly badly soiled, run it for a while to clear any debris.

Keep water flowing over the mirror constantly, then place a small drip of washing up liquid on the mirror, then using a light touch with your fingers, glide them through the washing up liquid and all over the mirror, you are hardly putting any pressure on and you can feel a layer of water between your fingers and the mirror, don't forget that the mirror is kept PERMANENTLY under the flowing water at this point.

You can, if you like, rinse completely and treat again with fairy liquid (or whatever brand you use)

next give it a GOOD flushing just run water over it until all traces of soap have gone, turn off the tap and just hold the mirror edge on to let the water run off, the vast majority of it will go, just give a little 'rattle' to encourage it.

Next, use one of the very soft cushiony brands of kitchen roll, two squares folded to make one double thickness square. Place the mirror face up on a surface and place the kitchen roll over it and just lightly 'touch it down'. Lift the paper and you will probably see a couple of very tiny odd drops of water, just gently 'touch' them off with the paper.

If you get it right, the mirror will now be stunning, if you have any water marks etc, don't worry, just repeat the process.

Leave the mirror alone for half an hour to thoroughly dry.

NOTE: The above works fine for me, if you prefer, do a search on the net or ask other SGL users for their preferred method of mirror cleaning. In other words, I'm not responsible if you screw up!

So, now we have this :-

post-18181-133877463253_thumb.jpg

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No, the mirror is NOT now a crazed finish! That's our artexed kitchen ceiling! I think you'll agree that's a nice clean UNBLEMISHED mirror.

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Next comes the centering and placement of the new spot. In the first pictures above you can see on the table an acetate printed centering template, this can be downloaded from the Jim Fly website, it also gives instructions for using it to center the new spot using holes and pieces of selotape.

I will say at this point that I'm not keen on the template at all as it was just confusing, also, the selotape method just did not work for me!

So, instead, I made my own template in my graphics package, with a single circle drawn at the same diameter as my mirror, and the center marked, the center was then made into a hole.

I placed my template precisely onto my mirror and marked the center with a fine whiteboard marker (these are the wipe off sort)

post-18181-133877463259_thumb.jpg

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Next I peeled off the backing from the new spot and gripped it with tweezers, positioning it directly in the center, then pressing down with a fresh cotton bud, once in place, just ever so slightly dampen the bud and take off the little center dot you drew on, then just dab with the other 'dry' end to clean it up. Eventually you end up with this :-

post-18181-133877463267_thumb.jpg

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Now simply replace the mirror in it's cell, replace the cell in the OTA and recollimate.

That's all there is to it! :eek:

Edited by TopHouse

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My primary mirror has a donut. I'm happy with that. I am not gonna mess with something that aint broken.

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That is a very nice walk through. Thanks for that.

Rik

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im a big fan of mr muscle glass cleaner, living and observing near the sea i have to clean my primary mirror more than most, have used MM three or four times now and it leaves a nice streak free finish

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im a big fan of mr muscle glass cleaner, living and observing near the sea i have to clean my primary mirror more than most, have used MM three or four times now and it leaves a nice streak free finish

That's just SCARY!!! What are you using to apply/polish it?

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What are you using to apply/polish it?

A brillo pad........... NO! I'm only joking, put that wire wool away......

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Next comes the centering and placement of the new spot. In the first pictures above you can see on the table an acetate printed centering template, this can be downloaded from the Jim Fly website....

The template graphic on the website is NOT to scale and is for illustrative purposes only. As such, it is not intended for downloading purposes.

Firstly, these are the 'hotspot' centers, there are a few design feature of this particular type, firstly the 'spokes' point at your 3 mirror clips when fitted properly, aiding alignment using a collicap.

For best intuitive Primary adjustments when using a Cheshire or Barlowed laser, the segments of the HotSpot should be pointed to the collimation bolts - not the holding clips (unless they are in line with the collimation bolts). See and download the CATSEYE spotting procedure: http://www.catseyecollimation.com/Spotting_hotspot.pdf. Also see the HotSpot INFINITY XLK Collimation Procedure: http://www.catseyecollimation.com/XLK-HotSpot-R1.pdf

Regards,

Jim Fly

CATSEYE Collimation

Edited by catseyeman

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The template graphic on the website is NOT to scale and is for illustrative purposes only. As such, it is not intended for downloading purposes.

To be fair Jim it's a PDF download under a heading

" Primary Mirror SPOTTING TEMPLATES"

And the one I downloaded is titled

"4" - 15" Max. Spotting Template

Click for 15" Max. pdf""

here :-

CATSEYE TEMPLATES & CENTER SPOTS

perhaps a disclaimer needs to be added to the links.

For best intuitive Primary adjustments when using a Cheshire or Barlowed laser, the segments of the HotSpot should be pointed to the collimation bolts - not the holding clips (unless they are in line with the collimation bolts).

My bad on that one, going to try and edit the above to correct it.

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... perhaps a disclaimer needs to be added to the links.

Thanks :eek: - Disclaimer has been added below the pdf links.

Regards,

Jim Fly

CATSEYE Collimation

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Originally Posted by Llamanaut viewpost.gif

im a big fan of mr muscle glass cleaner, living and observing near the sea i have to clean my primary mirror more than most, have used MM three or four times now and it leaves a nice streak free finish

That's just SCARY!!! What are you using to apply/polish it?

i use a lens cloth to wipe on and a clean dry one to wipe off... no problems so far

:eek:

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The idea of a nice visible accurately spotted spot sounds nice! Does anyone else find the standard donut quite small? I don't have a big scope but the donut is a long way away at times.

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Yes I find the standard doughnut small, and a long way off. after a re-coat I replaced mine with a pac-man shape, cut from pink card, about the size of a pound coin. The colour helps to work out what you are looking at through a Cheshire eyepiece, and the cut-out in the pac-man locates the center. Gets some odd looks though.

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