Collimation question

[johninderby]

I have a Concenter and now consider it essential. No other method comes close in getting an accurate secondary setup. It is very easy to use. I use a Hotec laser for adjusting the primary afterwards. Gives me perfect colimation. 

The Concenter on it”s own will still allow you to adjust the primary reasonably close but I find the laser easier. You could skip the laser and fine tune the primary using a star test afterward of course.

APM has a wider range than TS though.

https://www.apm-telescopes.de/en/othernbspaccessories/artificial-stars

 

[blinky]

A bit cheaper as well, shipping is half the price

[blinky]

Ok, had another go at it, hows this?

[blinky]

Right,cleaned primary and put it back in, 2hrs later how does this look? All 3 clips visible, shadow of secondary is round and the centre spot is centered

[alacant]

8 hours ago, blinky said:

how does this look?

If the top of the tube is at 10-o'clock, and you have an f4-f5 telescope, good.

HTH.

[blinky]

37 minutes ago, alacant said:

If the top of the tube is at 10-o'clock, and you have an f4-f5 telescope, good.

HTH.

Cheers! Took all night, what I eventually did was back off the 3 small screws on the secondary right into the body then brought the secondary mirror back so it was also flush against the body.  This meant I had a ‘square’ flush mirror and found it was easier to start from here. I think the root of the problem, was that the secondary was too far down the tube and I needed a lot of tilt in it, bringing it back in, it only took a few minutes to fix as everything just seemed a lot rounder, due to not having massive tilt on the mirror.

[Geoff Barnes]

9 hours ago, blinky said:

Right,cleaned primary and put it back in, 2hrs later how does this look? All 3 clips visible, shadow of secondary is round and the centre spot is centered

I'd be happy with that.

That's how mine looks and then I just finalise with a defocused star test to confirm and everything looks nice and sharp.

[blinky]

Managed to borrow a good collimator and do a final tweak, think its nailed now 

[blinky]

although it does look look perfect the primary mirror far was not on and there were tube currents about

[Skyline]

I to have a 2" concenter and makes collimation a breeze.

[Shooting star]

New to the pursuit and collimating 8" Dobsonian. Following Astro Baby instructions. Great until step 2 complete. Secondary mirror perfect circle in collimator cap view. Then step 3...Do we REALLY than mess with the secondary again!?!! Please tell me I'm reading this wrong. 🤣 Surely once secondary set, we want to align primary to secondary via Cheshire? 

Thanks to anyone who might reply.

 

[Ally8446]

3 hours ago, Shooting star said:

New to the pursuit and collimating 8" Dobsonian. Following Astro Baby instructions. Great until step 2 complete. Secondary mirror perfect circle in collimator cap view. Then step 3...Do we REALLY than mess with the secondary again!?!! Please tell me I'm reading this wrong. 🤣 Surely once secondary set, we want to align primary to secondary via Cheshire? 

Thanks to anyone who might reply.

 

Once you have centred the secondary you then have to adjust it to hopefully get all the primary mirror clips in view (see photo). I've you're happy with that then refer to the primary and adjust accordingly. 

Al

[alacant]

3 hours ago, Shooting star said:

Do we REALLY than mess with the secondary again

Short answer: for a Dobsonian you're only going to look through, no.

Longer? The adjustments you make the second time around are tiny. The secondary mirror is just a flat bit of glass. It has no optical properties apart from reflection. The first time you use a Cheshire with cross-hairs for the secondary may take several hours, during which you may feel like taking a big hammer to destroy the whole telescope (sic). The second time takes less than 2 minutes and you then wonder what all the fuss and frustration was about. Get it close -or spot on if you like- and then stop fiddling with it.

But hey, make sure you get the primary as perfect as you can. That's the easy bit anyway.

Just my €0.02 but hoping it may save you some time.

 

Edited April 17, 2020 by alacant

[Shooting star]

Many thanks. I'll have my wife hide the hammer!

 

[alacant]

3 minutes ago, Shooting star said:

have my wife hide the hammer!

Very wise LOL!

[Shooting star]

48 minutes ago, Ally8446 said:

Once you have centred the secondary you then have to adjust it to hopefully get all the primary mirror clips in view (see photo). I've you're happy with that then refer to the primary and adjust accordingly. 

Al

Many thanks. My brain says the second fiddling will only de center the work initially done. But understand the intent. 

[Shooting star]

Just now, alacant said:

Very wise LOL!

Hopefully whatever wisdom I have will offset total lack of experience! Thank you for your kind help. 

[Shooting star]

On 17/04/2020 at 11:25, Ally8446 said:

Once you have centred the secondary you then have to adjust it to hopefully get all the primary mirror clips in view (see photo). I've you're happy with that then refer to the primary and adjust accordingly. 

Al

Help! So I got the secondary perfectly round and centered to the collimation cap view through focuser. Quite pleased with myself. Than I summoned the courage to remove the white baffle and coloured background cards and view again through the collimation cap...🤔 the view now shows I'm no where near on centre secondary to primary. Is that normal? Why SO far out?! And how am I going to now okay with secondary adjustment screws without undoing all that initial work?!

In all the online guides and YouTube videos, once you get secondary good in the focuser with collimation cap, you simply proceed to align the primary. Why are they saying that if you actually need to do a secondary to primary alignment in between?

So why does Astro Baby have you do two secondary alignments (which in theory results in an infinite loop of secondary adjustments as one will akways throw the other off- you get it right and then undo it)?

Are all the two step: secondary in focuser + primary wrong somehow?

Bemused...

Neill

[alacant]

1 hour ago, Shooting star said:

the view now shows I'm no where near on centre secondary to primary

Correct. You placed the card baffles and background to prevent seeing this and misleading yourself.

2 choices:

1. Call that good enough and go and adjust the primary.

Or

2. Make sure the big hammer is still well hidden. Then...

Get the cross hair intersection in the centre of the primary spot. That should be close. Look only at the primary donut and the cross-hairs. Proceed first slackening one secondary screw and tightening another. Make tiny movements. Hopefully, you'll have thumb adjusters on your secondary. If not, fit them now. Keep doing this until the cross-hairs coincide. If you mess this up, back to the baffles and start again. 

Cheers

Edited April 18, 2020 by alacant

[Shooting star]

26 minutes ago, alacant said:

Correct. You placed the card baffles and background to prevent seeing this and misleading yourself.

2 choices:

1. Call that good enough and go and adjust the primary.

Or

2. Make sure the big hammer is still well hidden. Then...

Get the cross hair intersection in the centre of the primary spot. That should be close. Look only at the primary donut and the cross-hairs. Proceed first slackening one secondary screw and tightening another. Make tiny movements. Hopefully, you'll have thumb adjusters on your secondary. If not, fit them now. Keep doing this until the cross-hairs coincide. If you mess this up, back to the baffles and start again. 

Cheers

LOL Hammer locked away...

No thumb adjusters, but can see great benefit in having them.  Will have a go. Thanks.

[alacant]

36 minutes ago, Shooting star said:

No thumb adjusters

Phew. A hex key makes it that much more difficult. What telescope is it? Hex head m4 screws work fine as adjusters in most spiders.

 

[Shooting star]

6 minutes ago, alacant said:

Phew. A hex key makes it that much more difficult. What telescope is it? Hex head m4 screws work fine as adjusters in most spiders.

 

Your a star (no pun intended). I really appreciate you helping as you are. Sure you have better things to do on a Sat!

It's in Orion XT8  8" mirror; f5.9; 1200mm  focal (most generously gifted to me), but in need of care with spider web in tube and speckeld, dirty primary (but that is phase 2 work for me!).

When I looked after the initial secondary adjustment was made and now without baffles, I see this attached view (representation I found). Pretty close to that, which makes me instinctively think something is wrong...

I'm actively practicing Zen/Jedi ultra-patience mind control where I metaphysically alter the hammer to a silk like material...😂

Cheers.

 

 

 

[alacant]

12 minutes ago, Shooting star said:

silk like material

Good. So It's working. Oh, and f6 is very forgiving anyway:)

Difficult to visualise. Probably best to send one shot through the collimation cap and then another with the sight tube. Try to keep the 'phone central.

Edited April 18, 2020 by alacant

[Shooting star]

26 minutes ago, alacant said:

Good. So It's working. Oh, and f6 is very forgiving anyway:)

Difficult to visualise. Probably best to send one shot through the collimation cap and then another with the sight tube. Try to keep the 'phone central.

That is sensible and thanks. Will try and image and send through.

[Shooting star]

4 hours ago, alacant said:

Good. So It's working. Oh, and f6 is very forgiving anyway:)

Difficult to visualise. Probably best to send one shot through the collimation cap and then another with the sight tube. Try to keep the 'phone central.

Not sure my photos will help (?) Top is with collimation cap.