Jump to content

740427863_Terminatorchallenge.jpg.2f4cb93182b2ce715fac5aa75b0503c8.jpg

Collimation- grateful for some advice


Recommended Posts

Arrg, I've been imaging for nearly a year now with my Skywatcher 250 (the blue OTA) but not getting any where near the in focus images I expect.

I have read all on collimation and have started to use software to make it easier to collimate at high magnifications.

My images however seem to stay the same out of focus fuzzy things.

Back to basics then. I placed a collimation tool in the EP and looked at the reflections of the mirrors and things are not right. I did try to sort this a while ago but obviously i'm missing something with the process.

I will try to describe the problem as best I can:

In order to see the mirror clips I have to use a home-made cap with small hole in center and rack the focuser right in. All clips are only just visible if I move my eye about the hole, does look like there centered.

Placing a shop bought collimation tool in the EP and looking at the secondary I see a black cresent on the side nearest to the primary.

I understand that my secondary alignment is off. In order to adjust the secondary to remove the cresent I now see my image of the primary mirror has shifted towards the primary and showing all the clips furthest away from the primary.

This leads me to belive that the secondary needs to move towards the spider. Trouble is it is bang against it already.

Being befuddled with the knowledge that fast scopes have secondary offset, some built into the secondary mount, some not I am wondering if my secondary needs to be moved away from the focuser. At present the spider assembly is centered.

I would appreciate any advise or knowledge of the SW250 as I am at a loss what to try next, and also nursing a really bad back from last nights collimation attempts.

Cheers

John

Link to comment
Share on other sites

  • Replies 104
  • Created
  • Last Reply

Top Posters In This Topic

Is your secondary directly below the focuser? Recently Astro baby advised against trying to install an offset to a fast scope secondary, I'm inclined to agree, unless you can be certain as to the amount of offset required, if any.

Perhaps if the secondary collimation is too far out, it is giving the appearance of requiring to be moved further up the OTA. When I did the 10" SNT I was trying for ages to get it right, but not getting anywhere near, until I started again and concentrated purely on the secondary until I was happy with that. Only then did I work the primary and everything lined up rather bonny.

Link to comment
Share on other sites

I do secondary first and I 100% go for Astro baby's thing with a brown envelope of some sort and a piece of white card for doing secondary. I don't hold much with the being able to see the clips on the primary as with a colli cap in, no matter where I pull focus to I never see my clips.

Edited by TopHouse
Link to comment
Share on other sites

Hi Yeti Monster, Yes I agree this offset thingy has side tracked me.

I had the same thoughts, concentrate on getting the secondary right.

In order to get the secondary to show no cresents off to one side and to get the reflection of the crosshairs in the colli tool to align with the reflection of the engraved cross on the sloped baffle of the colli tool and to see that all is centered in the secondary my veiw of the primary is offset toward the primary by a large amount, so much so that I can see the edge of the primary nearest the spider an a lot of blank space.

cheers

John

Link to comment
Share on other sites

Don't worry about offset, this is factory built into the secondary.

I always see my 3 mirror clips when looking through a collicap, only time I don't see them is if I look down the barral of a sight tube.

I move my secondary around so it sits perfectly centered (has the same distance all around the mirror) and the three clips are fully shown.

Link to comment
Share on other sites

I am confused a little by your mention of a dark crescent and wonder mightiliy if what you are seeing is in fact the natural effect of a fast refelector which shows non-concentric circles by its nature.

Does the view you are seeing with the collimator look like this ?

post-14805-133877431296_thumb.jpg

Link to comment
Share on other sites

I always see my 3 mirror clips when looking through a collicap, only time I don't see them is if I look down the barral of a sight tube.

Thanks for the replys, That's one off the list of "This isn't right" that I have here.

I'm thinking of starting right from scratch as the secondary has been misaligned from when I purchased it and make sure the focuser is square to the tube and go from there, I'm not sure but that's the only bit I havn't checked yet.

Remembering the last time I tried sorting it, the only way to get all the reflections to line up and centered I still get a cresent shadow (which looks like the side of the secondary showing) nearest the primary side of the secondary.

Could the focuser being out of square give this problem?

Cheers

John

Link to comment
Share on other sites

If it does and that dark crescent is whats shown on the pic than all is ok. There are two versions of offset in a fast newtonian and the terms get mixed up.

What you are seeing in that pic is that the dark crescent is the offset -ie the circles are not all concentric but the two critical ones which are the two pale colored circles are in fact concentric. The crescent is actually the secondary being offset and it allways will be.

You can opt to do it the other way and move the secondary away from the primaryt mirror so that you acheive a fully concentrc set of inner circles in which vcase there will be an offset to the focuser drawtube.

Theres a good reason not to do it that way - the firts is that the field illumination will suffer. You can argue that with oversized secondaries on modern scopes thats not an issue but theres another reason also - its a damn site harder to do because you have to continuoulsy move the secondary up the tube and redo the collimation each time until you have concentric inner circles. Once you have found how far up the tube you need to go its no harder to collimate but a lot more confusing.

So in a nutshell with a fast newt you can have EITHER concentric inner circles and be offset from the focuser OR be in line with the focuser and have the collimation pattern shown in my post above. You can have both.

Thats not just me by the way - the fast collimation thing completely messed me up as well used as I was to speherical mirrored long tube newts years ago. In the end a couple of genuine collimation gurus on CN took pity on me and explained it all in words of less tham two syllables :headbang:

If you are seing a different shape to what I think your seeing is there anyway you can get a happy snap of it - my onw above was taken with a happy snappy cam down the Cheshire.

Edited by Astro_Baby
Link to comment
Share on other sites

Hi Astro-Baby, Great collimation info, I read your page to help me get started.

To explain my view through the OTA, If I look through with the primary mirror to my left, I have a similar view to your picture, crosses centered and secondary centered to the focuser tube but at the side to the left of the secondary mirror I have a darker cresent shape which when I look closer is the side unmirrored part of the secondary.

Hope this helps

Cheers

John

Link to comment
Share on other sites

Astro-Babe,

I have a pretty fast (f3.6) newtonian and the various circles are all "circles" and concentric IF the secondary off-set is set correctly.

I don't want to muddy the water with the current problem.

Ken

Link to comment
Share on other sites

astromerlin - it sounds to me from that as if the secondary is not square to the focuser. That woulkd be my best guess anyway.

Merlin - I cant comment. I only know from hard bitten experience you cant get a SW200P to do that. At least I cant and Vic Menard agrees in his book on collimation. I spent a ton of time trying to make it conccentric from drawtube to centrespot and got very frustrated in the process. Jason Khaddar who lent me the drawings from my guide agreed and gave me the clues that with a fast scope its not possible. I wouldnt lay claim to being expert on the theory so I cant say why its possible in yours but not in mine - just plain dont know - sorry

Link to comment
Share on other sites

I know what he's talking about, he's seeing the 'edge' of the secondary, ie, the part that some people paint black. I see that too, it's fine, as long as the actual 'mirror' bit you see is presenting as a perfect circle and it's in the center then it's OK.

Edited by TopHouse
Link to comment
Share on other sites

I know what he's talking about, he's seeing the 'edge' of the secondary, ie, the part that some people paint black. I see that too, it's fine, as long as the actual 'mirror' bit you see is presenting as a perfect circle and it's in the center then it's OK.

I've spent all day today trying to get this sorted. Still not right though.

I have found that the cresent shape I was on about is actually the edge of the focuser, I found this as I loosened the focuser mount screws to see if there was any movement to move the focuser. As I slid the focuser along the OTA (there is only about 4-5mm to do this) I found I could clearly see the edge of the focuser "my cresent shape" in the secondary reflection is somewhat offset.

No amount of fiddling will bring the focuser to align in the secondary reflection without upsetting other adjustments.

Any idea's..

Link to comment
Share on other sites

John,

Go back to basics... if the alignment of the focuser to the secondary is not correct, then any other "adjustments" you may have will be off.

I'd assume that the inside edge of the focuser tube would be concentric to the secondary... there shouldn't be any edge cut-off..

Always work away from the focuser...the secondary....the primary...

get one step right before even looking at the rest....

Ken

Link to comment
Share on other sites

Sounds like the focuser tube isn't square to the optical axis.....

Not a show stopper.... just align the secondary to the end of the focus tube BEFORE you start looking at the primary or reflections....

Link to comment
Share on other sites

Sounds like the focuser tube isn't square to the optical axis.....

This is what I was thinking so this is what I've done today, I removed the secondary completly. I had a while ago made a laser collimator although I found it is limited in getting all alignments correct.

I placed the laser in the focuser and placed a sheet of paper in the ota opposite the focuser. I loosened the adapter in the focuser slightly and rotated the laser to make sure the spot stayed put, I then measured the distance from the spot to the edge of the ota. I then removed the laser and placed a colli cap in and measured from the hole in the cap to the edge of the ota. Measurement was the same within 1mm or so.

I replaced the secondary and aligned it so that it is centered with my view of the end of the focuser. I adjusted the tilt of the secondary to bring the primary concentric in the secondary with all clips just showing.

At this point I can see in the secondary reflection the focuser is offset.

At this point I know if I move the secondary along the ota axis then it will not be concentric with my view of the bottom of the focuser.

If I tilt the secondary then I lose the concentricity of the primary.

I then made some small adjustments to the tilt of the focuser but noticed that the reflection of the hole in the cheshire was starting to run away from the reflection of the crosshairs.

So this is where i'm at, if I try to get the reflection of the focuser concentric the primary isn't and vise versa.

Link to comment
Share on other sites

At this point I know if I move the secondary along the ota axis then it will not be concentric with my view of the bottom of the focuser.

If I tilt the secondary then I lose the concentricity of the primary.

John,

Don't worry about getting the secondary reflection concentric

just the the crosswire central.... then you're 99.9% there...

It could well be that the design offset built into the secondary is not 100% correct. If the secondary is large enough it won't make any difference to the final collimation.

Link to comment
Share on other sites

If I may, let me clarify why the secondary mirror reflection via the primary mirror (the dark silhouette) will typically look shifted towards the primary mirror.

Refer to attachment.

When we center/round the secondary mirror under the focuser, our eye-axis intercepts the secondary mirror above its geometric center. We might think we are staring directly at the geometric center of the secondary mirror but we are really staring at a point right above it. It is an eye illusion. This view is represented by what the “gray” eye sees in the attached illustration.

Then when we view the secondary mirror reflection via the primary mirror, what we view is represented by what the “red” eye sees in the attached illustration (think of the primary mirror as a lens.) Since we are looking at the secondary mirror from a different angle, what seemed to have been centered from the “gray” eye’s perspective, now will look shifted from the “red” eye’s perspective – the eye illusion fell apart.

The larger the secondary mirror offset, the larger the amount of secondary mirror reflection shift from the “red” eye’s perspective. Typically, faster scopes use relatively larger secondary mirrors and require larger offset, hence, the secondary mirror reflection shift will be more pronounced.

Note that the secondary mirror reflection (the dark silhouette) should look round even though it is shifted. In addition, the shift should be towards the primary mirror center. The secondary mirror photo uploaded in an earlier post can be further adjusted because the secondary mirror silhouette looks oval and it is not pointing towards the primary mirror.

Be careful with the spider vanes reflections. They could be shifted as much as the secondary mirror silhouette. This tends to be confusing. Many assume the spider vanes reflections are centered for all scopes. They can look centered but only for scopes with their secondary mirrors mounted with a full-offset – shifted away from the focuser.

Finally, I just wanted to say “Hi” to Melanie (astro_baby).

Jason

post-17988-133877432012_thumb.jpg

Edited by Jason D
Link to comment
Share on other sites

Jason - whew - thanks for that. I still struggle with that one. The pic of mine above is hardly perfect by the way- To the eyeball it is but it was snapped with a happy snappy cam so not exactly scientific :p

You suddenly appeared when I mentioned your name - its a weird feeling. Brrrr ;)

I hope your well - I dont get on CN much these days as time is a bit tight for me.

I'm begging to think in terms of buying either a long tube newt with a speherical mirror (cos I could understand that) or but myself the new TAL 125 APO and sell the reflector and solve any further collimation problems that way :headbang:

Edited by Astro_Baby
Link to comment
Share on other sites

Hi, Some great advice here. Many thanks to you all. I think I've cracked it.

I had another go today after reading many webpages and the good info you all have provided. It all seemed to align well in an hour of fiddling. I suppose like most newbys I was a bit heavy handed with the adjustment screws on the secondary and they kept settling into the grooves that I had gouged in the secondary holder so down tools and into the workshop to make a thin shim to rest the adjustment screws on.

No time now for anodising but I want it to look good so out with the black marker pen

and it was fitted in minutes. Re-collimation went well and I now have all the dots and crosses aligned and no strange focuser offset in the secondary reflection.

I do however have the secondary reflection offset towards the primary as a fast newt should, mentioned in the posts you have made.

I think because everything from the focuser, secondary and primary was mis-adjusted in the first place, being my first time at doing this I got very confused.

That said I now have a better understanding of how it all goes together.

Thanks again for all your kind advice.

Cheers

John

Link to comment
Share on other sites

Well thats that then - it can be VERY confusing the firts time you do it - all those reflections and sometimes I STILL get confused when I look at pictures unless I am peering down the Cheshire first hand.

Glad its all sorted out though - and that remind me I must get round to making a shim for my secondary as well.

Link to comment
Share on other sites

"You suddenly appeared when I mentioned your name - its a weird feeling. Brrrr :)"

Maybe you should have mentioned a "million dollars" instead of my name :headbang:

"I'm begging to think in terms of buying either a long tube newt with a speherical mirror (cos I could understand that) or but myself the new TAL 125 APO and sell the reflector and solve any further collimation problems that way :D"

But collimation is half of the fun :p

By the way, here is a photo of my aligned secondary mirror.

post-17988-133877432018_thumb.jpg

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.