Need Collimating?

[ZiHao]

Weather has been awful these days and I decide to check out my newtonians collimation. Picture attached is taken through a diy collimation cap. Do I need to collimate the scope? I notice that one of the primary mirror clip looks 'smaller' than the others, means I have to adjust my secondary mirror? If yes, how? I don't have either a Cheshire or a laser collimator. Thanks. 

 

[Demonperformer]

I am no expert on collimation (only just beginning to tackle it myself), but I think the black circle is supposed to be perfectly central and it looks offset to the right to me. I think this means you need to adjust the secondary a little.

Golden rule: whatever you do, do it in small (very small) steps, so that it is easy to reverse it.

[David Levi]

Whether that black circle is central or not depends on the telescope. Here is a link to the document that is very popular as a guide to Newtonian reflector collimation:-

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

Provided those cross hairs in the picture are centred on the primary mirror centre dot then your collimation looks ok to me, for a fast Newtonian reflector e.g. f5. A star test when you get the chance will show you how good it is.

[ZiHao]

Found a guide on internet just now, the references in it are almost identical to mine. So I have to adjust the primary mirror as the black circle is bias to the right, anyway, I will do a star test first before making any adjustments.

http://www.opticalvision.co.uk/documents/203.pdf

Edited March 28, 2019 by ZiHao

[Jason D]

2 hours ago, ZiHao said:

I notice that one of the primary mirror clip looks 'smaller' than the others, means I have to adjust my secondary mirror? If yes, how? I don't have either a Cheshire or a laser collimator. Thanks. 

That does not mean much with respect to collimation. I would ignore that observation if I were in your shoes.

Your photo does not provide enough information to evaluate your collimation. The primary center spot is not showing and the focuser edge is also not showing. 

A good collimation cap will have a reflective (or at leave a white) underneath surface.

[Jason D]

1 hour ago, Demonperformer said:

but I think the black circle is supposed to be perfectly central and it looks offset to the right to me.

It should look like that (with an offset). Here is a photo of my collimated scope

[Jason D]

51 minutes ago, David Levi said:

Whether that black circle is central or not depends on the telescope. 

Provided those cross hairs in the picture are centred on the primary mirror centre dot 

Secondary mirror silhouette will always appear skewed towards the primary mirror. It is just more paramount for fast scopes.

The cross hairs in the photo are the spider vanes. These should be ignored when collimating. Refer to my photo in the pre reply. My scope's spider vanes intersect slightly to the right of the center spot. 

[ZiHao]

Pointed my telescope to the sky through the window, it shows the center spot but not the focuser edge. Second picture was how I attach the cap, is it correct? 

Edited March 28, 2019 by ZiHao

[Demonperformer]

1 hour ago, Jason D said:

It should look like that (with an offset). Here is a photo of my collimated scope

I stand corrected ... proved my first comment ("I am no expert")!

[cjdawson]

The most reliable method for collimation that I've found is to do a star test.  If the collimation is correct, when you defocus the star, the black dot will appear - but it will be very small, that will give a much better clue as to how close you are in final collimation.  Also when you focus the star, it should end up as a point, the dimmer the star that you can do this on the better, as you end up with a smaller, sharper point.

[Jason D]

22 hours ago, ZiHao said:

Pointed my telescope to the sky through the window, it shows the center spot but not the focuser edge. Second picture was how I attach the cap, is it correct? 

 

Your collimation as shown on the above photo looks good

Jason

[Rush]

3 hours ago, Jason D said:

Your collimation as shown on the above photo looks good

Jason

Hi ,

apologise if im gatecrashing

 i do have a simmilar issue, i.e when i look through the cap, Reflection shows, a left bootom clip and the top one , focuser and the 3rd clip refelection on the right is missing. What should i do from this point ? any advice will be helpful.

 thanx man,

CS

Rush

[Rush]

Star test looks like this,

Cs

[alacant]

19 minutes ago, Rush said:

What should i do from this point ?

Hi. Center the secondary so you can see all clips. But be sure to read the myths of collimation before you begin adjusting. HTH

[Adam J]

36 minutes ago, Rush said:

Star test looks like this,

Cs

The star would need to be in the center of the chip for that to work. 

[RolandKol]

Welcome to the collimation BattleField of the Secondary Mirror!

It looks like you have PDS just like me... And I can assure you, even if you would have Cheshire - you would fail to place the secondary correctly from the first time.

Without it, - not even worth a try... Laser will not help with this task....

So order one, - even second hand, try once received.

Just not sure, which one is better for PDS, - short or long one... Short are used for refractors after focuser change, but long ones do not show mirror clips on PDS , - a bit too long.

Maybe someone else is able to advice on it.

 

Edited March 29, 2019 by RolandKol

[glowingturnip]

4 hours ago, RolandKol said:

Just not sure, which one is better for PDS, - short or long one... Short are used for refractors after focuser change, but long ones do not show mirror clips on PDS , - a bit too long.

 

I use a long one with my 200PDS - with the secondary properly positioned I can *just* about see all of the mirror through it, at which point I'd say I'm good.

Just putting the cheshire, or even a laser into a focusser that has the two tightening knobs isn't even going to help either, you really need one of those compression collar style focussers to make sure the collimation tool is square in the tube.

It's a pain - my 200PDS was well out of collimation when it came new, but it was in Spain, maybe came from handling during shipping.  Took me a whole afternoon to get the 2ndry right with the Cheshire, but with that done, it's now just a 5 minute job with the laser, then pop the cheshire in afterwards for a quick pat on the back at how nicely lined up it looks

[RolandKol]

5 minutes ago, glowingturnip said:

I use a long one with my 200PDS - with the secondary properly positioned I can *just* about see all of the mirror through it, at which point I'd say I'm good.

Just putting the cheshire, or even a laser into a focusser that has the two tightening knobs isn't even going to help either, you really need one of those compression collar style focussers to make sure the collimation tool is square in the tube.

It's a pain - my 200PDS was well out of collimation when it came new, but it was in Spain, maybe came from handling during shipping.  Took me a whole afternoon to get the 2ndry right with the Cheshire, but with that done, it's now just a 5 minute job with the laser, then pop the cheshire in afterwards for a quick pat on the back at how nicely lined up it looks

My 130PDS does not want to show all mirror holders via long Cheshire whatever I do!  I do have a special EP holder to make it square, - it helped a lot. And yes,  - without it, it was more luck or guesswork even with cheshire....

However, 3 holders are not visible and it is still a bit of guesswork at the moment....

I do have a cheap laser, - do not trust it anymore... had funny results.

I would be very glad if you  would share your Secondary Collimation routine, - I want to get the secondary to perfect position during summer. Now it's very close, - even afraid to touch

[glowingturnip]

oh, I'd hate to hold myself out as an expert in collimation - took me a lot of trial and error to get my 2ndry into the right position, and probably not by approved means !  But here goes, and ppl more knowledgeable than me please chime in.

As you know, there are four things that have to be right - the distance along the axis of the scope where the 2ndry is so it's properly pointing through the focusser, the rotation of the 2ndry so it's square to the focusser, then 2ndry mirror tilt and primary tilt. 

To get the first bit, the position along the scope axis, you will need to be able to see both the left and right hand edge of the mirror or clips (as you look at it through the cheshire) and they should be equally spaced from the edge as they appear in your cheshire.  If you can't see the whole of the mirror or all of the clips, maybe you do need the short cheshire.  To adjust, slacken off all three of the little screws and then adjust with the big screw in the middle - being very careful not to unscrew it all the way, else the 2ndry will fall out, and being very careful not to touch the glass or let things flop about !  The three surrounding screws push against the mirror base plate and the centre screw pulls it, so if your 2ndry needs to go further inside the tube you would loosen the centre screw, and vice versa.

Unfortunately while you're doing this, the 2ndry is obviously completely out of collimation, so even when you think it's right, once you've re-tightened the 3 screws and adjusted the tilt back, you can find you're way off still, or even worse !  A little trick though, you can kind of 'hold' the secondary in collimation while the three screws are loose - hold the base of the mirror (without touching the glass and without bending anything) then as you're looking through the cheshire, tilt the mirror by hand till the crosshairs on the cheshire line up with the spider veins, that's a rough collimation, and while you've got it lined up like that, then check the left and right hand edges of the primary, readjust, look again, etc. 

Then you need to adjust the rotation of the 2ndry in the tube so that it's square to the focusser.  Looking through the cheshire, the mirror should appear round, not oval, and the mirror edges, or clips, should be same distance from top and bottom and the same distance as the left and right ones are.  When the three screws are slack, the secondary is free to be rotated around the centre screw.  You don't need to adjust the centre screw any more, since you've got the distance right, but now you will be slackening the 3 screws, rotating the mirror by tiny amounts, checking in the cheshire while holding it in collimation again, retightening the 3 screws, finding you've disturbed the rotation whilst tightening them, starting again, repeat, repeat. 

These two steps are definitely the hardest part, but once done once, it'll hardly ever need doing again.  As I said, it took me a whole afternoon, and I started over several times.

The two mirror tilts probably need doing every time, and are much quicker with a laser than by eye - it does need to be a properly collimated laser though, and it sounds like you've got a dud one.  The Hotech ones are good.

 

Just reading that back, it sounds like a nightmare doesn't it ! and we've not even mentioned coma correction yet.  It takes a real masochist to use a fast Newtonian I reckon, mind you, I love mine.

[RolandKol]

Thanks a lot for your explanation.

And yes, the main problem in my case with 130PDS, - the position of the secondary along the scope axis...  As I simply not able to see the Primary Mirror Clips, so I constantly swap Cheshire with my EP Cap  And it is the main pain...

Once more or less in center, - I move the secondary in circles using 3 adj.bolts and once once clip starts to show, I move mirror other direction, -  to hunt for the second clip, - later for the 3rd. Once I see that these adjustments are really minor,

I check via the EP cap if the secondary mirror looks co-centric... if all looks nice, wait a week or even two for the clear skies to test!!  

 Nightmare!  

I even tried adjusting the Primary into Top or Lower end of the OTA by adjusting it's collimation bolts to the end limit or loosening, - no use... Clips not visible in the Cheshire anyway...

I even think of the artificial star and try the collimation using it.

P.S. your images and Alnitak tells me all!  you do all correctly!

Edited March 29, 2019 by RolandKol

[glowingturnip]

3 hours ago, RolandKol said:

P.S. your images and Alnitak tells me all!  you do all correctly!

Thanks for the kind words, and yes, I guess that image was a bit of an acid-test wasn't it !

This, in case anyone's wondering:

[Rush]

Well Well, Thank you great people for the precise detailed explanations and helping spirits.

Right now Im walking the front line of secondary collimating battlefield, but  reaching  somewhere near the shimmering winning path?.

I don’t have a Ceshire  C T., but have only a self made primitive collimating cap with which I used to collimate my old GSO Newton. But this Quattro beast is quiet weird as all the primary screws are extrem sensitive and the Offset secondary.  Here is bahtinov focus test which looks promising. Will update my progression, Thanks to all, great week end, 

CS

Rush

 

[Rush]

On 29/03/2019 at 11:41, RolandKol said:

Welcome to the collimation BattleField of the Secondary Mirror!

It looks like you have PDS just like me... And I can assure you, even if you would have Cheshire - you would fail to place the secondary correctly from the first time.

Without it, - not even worth a try... Laser will not help with this task....

So order one, - even second hand, try once received.

Just not sure, which one is better for PDS, - short or long one... Short are used for refractors after focuser change, but long ones do not show mirror clips on PDS , - a bit too long.

Maybe someone else is able to advice on it.

 

 

On 29/03/2019 at 11:41, RolandKol said:

Welcome to the collimation BattleField of the Secondary Mirror!

It looks like you have PDS just like me... And I can assure you, even if you would have Cheshire - you would fail to place the secondary correctly from the first time.

Without it, - not even worth a try... Laser will not help with this task....

So order one, - even second hand, try once received.

Just not sure, which one is better for PDS, - short or long one... Short are used for refractors after focuser change, but long ones do not show mirror clips on PDS , - a bit too long.

Maybe someone else is able to advice on it.

 

Thank you for the warm welcome to the crucial warfields?,

ordred a long handle from TS . Meanwhile continue with my self made collimating cap . Read so many articles on collimating , can sing  collimaion blues in C.

Cs

Rush

PS : this scope was send by a gentle person  from UK to Germany. Plus me giving a hot bath to the secondary made it all worse.?

Edited March 30, 2019 by Rush

[RolandKol]

The problem is actually in all those tutorials...

I fell on them, probably just like majority newbies did... I do believe my PDS was quite well collimated, but it was my first scope for AP, and I read a lot and checked youtube tutorials before I touched the secondary, and guess what?

All of the tutorials suggest to adjust the secondary during the usual collimation routine of the primary, and guess what? None of them actually highlight, - "Do not touch the Central Bolt of the secondary!!!".

So I have adjusted even the central bolt as other 3 were quite tight...

And later star test became soo bad... Just like yours... And maybe even worse! And I had only laser at that time and could not figure out why it is so bad, as laser beam was in center and etc

So I ordered Cheshire, while waiting it to come by post, disassembled scope into small peaces, flocked it, centered focuser, placed Autofocuser, put all back together and had a prolonged battle with the secondary... Which is not yet 100% finished

P.S. But I would afraid to do it to Quattro!!!  130PDS is in completely different price range... in fact, thats why I bought it, - to learn the "anatomy" of the Newtonian! 

plus, I guess... Battle with F4 is much more difficult than with F5

Edited April 1, 2019 by RolandKol

[alacant]

3 minutes ago, RolandKol said:

The problem is actually in all those tutorials...

+1. Yeah, there's so much rubbish out there. It's just a flat mirror which apart from reflection does nothing to the incident light falling upon it. Get it close and leave it! I wish I had read nothing before I started. All I needed was someone to say 'make it look like this'. With apologies to @Jason D  for reusing the post.

Cheers and clear skies