Having bit of a prob with secondary mirror

[REG78]

Hi all was just setting my secondary but can't seem to get it flush in th middle of primary mirror can see clear rnd th tube & th primary clips but still seems to be offset slightly been tinkering with it for age still cnt get it flush do I need to losen all primary screws aswel any ideas

[Astro Imp]

Have a good look at Astro Baby's excellent guide to collimation. The point you are making is covered under the heading fats/slow scopes. http://www.astro-baby.com/collimation/astro babys collimation guide.htm

HTH and good luck.

[Peter Drew]

The secondary mirror might have to be offset depending on the aperture and focal ratio of the primary.

[REG78]

Has anyone out ther got any idea Wat size th Allen key screws are tht fit th skywatcher 130-900 secondary mirror need help think I rung one of mine 

[Alan64]

4 hours ago, REG78 said:

Has anyone out ther got any idea Wat size th Allen key screws are tht fit th skywatcher 130-900 secondary mirror need help think I rung one of mine 

All of the screws should be metric.  The ones for the secondary on my 6" f/5 were pretty tight.  I couldn't budge them at first.  Did the kit come with a collimation-cap, like this one...

If not, you'll need at least that in order to collimate, per AstroBaby's and others' online instructions, carefully and patiently, and after the primary is center-dotted...

http://www.firstlightoptics.com/other-collimation-tools/rigel-aline-collimation-cap.html

I'd hold off on attempting to adjust the secondary before having that all-important tool in hand.  You may need this in addition, and per said instructions...

http://www.firstlightoptics.com/other-collimation-tools/cheshire-collimating-eyepiece.html

[Alan64]

Incidentally, as I had eventually understood it, off-setting of a fast(f/5) Newtonian's secondary mirror occurs automatically during the process of collimating.  Yours is an f/7, and most probably would not result in an off-setting.  If you're curious as to the aspect...

http://www.skyandtelescope.com/astronomy-equipment/offsetting-your-secondary-mirror/

[REG78]

No Alan never got anythng like tht with mine but got myself a collimating lazer a few weeks bk but can't shift lazer to th centre of prim mirror because th Allen bolt hav striped was gona replace thm tomorrow but can't get thm fully extracted because Allen key is just ringn in it dono wat to do can't use my scope

[Ruud]

Here you can see how the secondary, while in the right position, must be offset with the primary.

1 = centre of primary
2 = centre of secondary

The secondary hangs a bit lower when it fully catches the light cone from the primary.

This is of course an extremely fast primary

[Alan64]

3 minutes ago, REG78 said:

No Alan never got anythng like tht with mine but got myself a collimating lazer a few weeks bk but can't shift lazer to th centre of prim mirror because th Allen bolt hav striped was gona replace thm tomorrow but can't get thm fully extracted because Allen key is just ringn in it dono wat to do can't use my scope

Is or has the laser collimator itself been collimated first?

[REG78]

No nothn like tht how do you collimate th lazer 

[Alan64]

In the absence of a center-dot, I suppose a laser would help in that regard.  The one you have must be able to be adjusted, and with its grub-screws as shown here...

 

[Dave In Vermont]

I was just about to post that, Alan! LOL! However - you may find some of these helpful to you as well:

https://www.google.com/search?q=Collimating+laser+collimators&ie=utf-8&oe=utf-8

One thing I've found with many laser-collimators is that the 3 little screws that you use to collimate the laser with are hidden. Some may be under a label. Others may be under a little dab of hard putty-like stuff. I always make a point of asking questions about this from the vendor. I've bought and used quite a few of these devices over the years. I'm someone who actually enjoys collimating telescopes!

Don't worry - you'll get there. And from then on it's like riding a bicycle - you will find it very easy after the 'first-time.'

Dave

[Alan64]

The secondary hub may be the same as on mine...

I'm able to use a metric #2 or a standard 5/64" Allen wrench.  Do be careful to avoid stripping the screws' hex-sockets.  I wouldn't be surprised if they were set with superglue at the factory.  To be on the safe side, I'd use pure 100% acetone on a cotton swab and carefully soak each socket, then following that with something akin to WD-40.

 

[REG78]

Thts wat my prob was they wer striped only after getn thm out with much tinkering smorn ther was doin my head in lastnite gona grb new set today ? So happydays

[REG78]

All sortd got new set bolts today bolt are sitn out of spider about half inch I'm able to collimate just we twist here n ther with my fingers no mor Allen keys/screwdrivers etc....Happydays ??

[REG78]

[REG78]

[Alan64]

First, the secondary mirror is aligned to the focusser.  You can eyeball it, I suppose, but a combination Cheshire-sight tube tool is best to use.  Here's a shot through my sight tube, inserted in place of the eyepiece, and showing the secondary mirror aligned, centered, with the drawtube.  Click on the images for a larger view...

Then, the center of the primary mirror is aligned to the center of the secondary, and after the white and orange paper is removed...

That is the view through the collimation-cap illustrated earlier, and as reflected by the secondary mirror. 

Next, I've purposely thrown the collimation off to illustrate and notate what is actually being seen, again, as reflected by the secondary mirror, and what I call the "scene"... 

1. The observing hole of the collimation-cap; the camera's "eye" is just on the other side of the hole, taking the image.

2. The center-dot, or "doughnut" of the primary mirror

3. The reflective surface on the underside of the collimation-cap

4. The secondary mirror itself; its "shadow", and as reflected in the primary mirror.

5.  The spider-vanes

6. The primary mirror

7. The primary mirror's clips, holding it in place

8. The interior of the focusser's drawtube

As you can see here, the secondary mirror rests at a 45° angle...

The flat, horizontal surface of the secondary mirror should be parallel to the flat bottom of the focusser's drawtube...

The mirror will try to twist from side to side whilst adjusting, so be aware of that.  The secondary mirror is actually oval in shape, but when inclined at a 45° angle, and whether looking straight down into the focusser or as the primary mirror "sees" it, it appears round...

 

If you plan to stick with the Newtonian design, I recommend getting a collimation-cap and a Cheshire-sight tube tool.  They don't need collimating before use.

[Dave In Vermont]

Alan's piece is a very good overview of the anxiety-provoking 'art' of collimation, which every owner of a Newtonian telescope must undergo. I was debating whether to go into this myself at this stage of the game. Better him than me! Now that this feline has been liberated of the sack it was in - allow me to add another item that will be likely encountered if you decide to go down the Dobsonian road: A finder that won't send you to a chiropractor or a Yoga instructor.

The finder-scope to be found on most Dob's are straight-through types. This usually is found to be something in need of changing. From the first time you try to look through it with your scope pointing near the zenith (directly overhead). A much better one is a RACI of around 8 X 50mm. An example is here:

http://www.teleskop-express.de/shop/product_info.php/info/p297_TS-8x50-Finder---with-Bracket---black-colour---90--angled.html

Enjoy!

Dave

[REG78]

      That's th original Allen screws I took out of my skywatcher 130-900mm any wonder I was struggling with thm never thought wuda been tht small I'm glad to get thm replaced 

[billyharris72]

On 28/03/2016 at 23:38, REG78 said:

never got anythng like tht with mine but got myself a collimating lazer a few weeks bk

I would definitely recommend getting a collimating cap - only a fiver and less prone to accidentally screwing up collimation, which lasers can be bad for. The main issue for lasers is collimating the primary. If the secondary is not absolutely spot on you won't get accurate collimation with a laser, while with a collimation cap or Cheshire things are simultaneously more accurate and less sensitive to error. I tend to use the laser for collimating the secondary and rough alignment of the primary, and then check my work with the cap just to make sure I haven't made a mistake.

Of course your current problem is with the secondary, which the laser is pretty good for (so long as you follow up with a visual check). It's really important to make sure that the laser is square in the focuser. Loosening the eyepiece holder and rotating the laser should produce no movement in the laser dot; neither should running the focuser in and out. If either of them does then try adjusting the laser to make sure it is properly seated. If it won't behave, you'd be better off using the cap and estimating visually (or better still use a Cheshire).

 

 

[REG78]

Was out lastnite for good 3-4 hours  nice clear sky but my secondary mirror startd steam'n up a bit because it was bitterly cold is ther any way to avoid this

[laudropb]

You can try using a dew shield.

[REG78]

Was thnkn tht wud it make a difference to th secondary see'n it's so close to th front of th reflector 

[Alan64]

4 hours ago, REG78 said:

Was thnkn tht wud it make a difference to th secondary see'n it's so close to th front of th reflector 

Last summer, when it was so humid here, my primary fogged up as well.  Be careful when taking the telescope out from an air-conditioned interior to the outside this summer.  It might be prudent to store it inside an automobile, on a seat, whilst still daylight, with the sun shining into the windows of the auto even, and prior to observing that night.