My Brand New 130pds - hmm...

[Thalestris24]

Hi

So I've just got round to checking out the 130pds properly. Thought I'd check collimation. But when I came to look at it, it looks wayyy out. On further investigation, it actually looks like the focuser is fitted crooked . Here are a couple of pics of the focuser mounting and the view down the focuser tube:

Any thoughts? Any recommendations as too how best to adjust the focuser - there are numerous allen key bolts/grub screws and I'm not sure which does what... My 150pds has a slightly different focuser fitting but looks nice and true and I've never had to touch it.

Thanks!

Louise

[Starlight 1]

Leave as it is and do a Star test.

[DaveS]

That does look a bit wonky. I'll check mine when I get home, but I think it's squarer than that. Have you checked out Astro-Baby's site for collimation tips?

[Thalestris24]

Leave as it is and do a Star test.

But it's so far out! The mirror collimation itself may actually be ok but I'd say the focuser is tilted.

Louise

[Starlight 1]

p-ds are off set.

[Thalestris24]

That does look a bit wonky. I'll check mine when I get home, but I think it's squarer than that. Have you checked out Astro-Baby's site for collimation tips?

Hi

Yeah, I have Astrobaby's guides. They don't cover focuser adjustments though

Louise

[Thalestris24]

p-ds are off set.

My 150pds doesn't have this problem.

Louise

[Starlight 1]

Yes it do look off by the photo my 250p-ds focuser is tilted and did try ajusting it but some allen key bolts/grub screws are under the nobs so leave this bit for  now.

[Starlight 1]

Once you on a Star by moving the focuser in and out the Star will move side ways or up and down it out of adjustment.

[Skyline]

I can see the tilt, your right side (shadow side) is lower, the grub screws on the focuser bottom rim help adjustment of the tilt. As I remember there should be three allen screws paired with grub screws.

[Thalestris24]

I've adjusted two of the allen key screws/grub screws and improved the tilt considerably. I noticed one of the grub screws was screwed very far in. Poor quality control and that takes away my trust in the scope. Although I've improved it I'm not sure how to optimise the adjustment. It looks like I'd have to remove the Crayford mechanism in order to access the 3rd allen screw/grub screw. It doesn't seem right that I should have to do that on a brand new scope...

Thanks

Louise

[kalasinman]

Sent to me be another member. BTW your f/5 is offset. :

The secondary-mirror offset is no doubt the most misunderstood aspect of collimation. Luckily you don't need to understand it to collimate your instrument, but given the level of discussion the subject generates, it's worth delving into.

If you place the secondary centered in the telescope tube, with the primary mirror's optical axis at the center of the diagonal mirror's elliptic face, diagram A (greatly exaggerated for clarity) shows what will happen. The shaded area shows where light from the whole primary can be seen reflected in the secondary — at the focal plane, this defines the fully illuminated field. Inside it, you catch light from the whole mirror; outside of it, some light is lost. You would want the fully illuminated field to be centered in the eyepiece, but as you see here, it isn't — it is shifted away from the primary. This situation is known as nonoffset collimation. Although this condition doesn't cause any great problems, it is easily avoided.

In diagram B, the offset is achieved by sliding the secondary away from the focuser and toward the primary. Now the fully illuminated field is centered in the eyepiece, but the secondary is no longer centered in the telescope tube. This is known as fully offset collimation.

But what if you want the fully illuminated field centered in the eyepiece, but must leave the secondary mirror centered in the telescope tube? It can be done, as shown in diagram C, by slightly adjusting the tilt of both mirrors. Now the optical axis is slightly tilted within the telescope tube. In practice, this is not a problem because the tilt is never more than a small fraction of a degree. Since the secondary is offset down the tube, this is known as partially offset collimation. It is no doubt the most common situation, even among telescope owners who may not even realize that their scope's secondary is offset at all.

If you use a sight tube to center the secondary as described in Step 1, you have automatically offset the secondary toward the primary mirror, thus ensuring that the fully illuminated field is centered. (When using the sight tube, you make the near and far edges of the secondary appear to have the same angular size. This means that the distance from the far edge of the secondary to the optical axis is greater than from the near edge to the axis. This constitutes an offset.) Both partial and full offset conditions give good collimation.

If you want to calculate the offset, use this simple formula:

Offset = (secondary size)/(4*focal ratio).

This is how much the secondary is offset toward the primary mirror and also how much it should be offset away from the focuser for fully offset collimation.

- See more at: http://www.skyandtelescope.com/astronomy-equipment/offsetting-your-secondary-mirror/#sthash.d8gLWZ0p.dpuf

[Skyline]

Don't worry, its a small fix & an easy one. Re-Collimate & check with a star test.

Al.

[Thalestris24]

Hi

Thanks, but as I said previously, at this stage I don't think it's a collimation problem as such, but a focus tube tilt that needs to be adjusted first. I can visually compare the view down the 130pds and my 150pds and there's an obvious difference. The focus tube is orthogonal on the 150pds but isn't on the 130pds.

Louise

[kalasinman]

With the uneven lighting, and no reference point in the background , it's difficult to tell if it's square, at least to me. I tried measuring the pic, but the lighting is too uneven. When I got my GSO, I tried at first to assume I could short cut the collimation process, but for me I had to go right down to the cell. Probably wouldn't have had to take the cell out except that I shortened it. Good to check for tight clips, mirror stuck to the cell, etc. anyway. Vanes off adjustment and central screw out too. Do all the steps in Asto Baby's guide, would be my thought.

[Starlight 1]

Try puting it on the mount and put the camera in and take a pic see how it looks from this end.

[Alkaid]

I was under the impression that although the F5 is offset, the offset itself is slight, just a few mm.  You should be able to see the entire primary mirror reflected in the secondary.  Louise's shot down the focuser only shows half of the primary in the secondary.....effectively this means a loss of aperture....as only half of the light coming from the primary is hitting the secondary...

I think that the secondary mirror position with regard to up / down the tube is probably ok.  But the mirror is incorrectly tilted, thus not showing all of the primary.  Try tilting the mirror just using the tilt screws until the whole primary is visible.

[Peter Drew]

Louise. Alkaid has the correct diagnosis, trust me! 

[Thalestris24]

Hi

Well, I've undone the Crayford mechanism and adjusted the allen key nuts/grubs as best I can by eye. Now I'm fairly sure the focus tube part is orthogonal .

This is the best view I can get through a collimation cap:

Clearly something else needs adjusting!

So you guys think I need to adjust the tilt of the secondary?

If so, I think I'll do that tomorrow - my eyes are squiffy now!

Thanks

Louise

ps I'm a collimation virgin - I've never had to touch the 150pds!

[Thalestris24]

Actually, with paper between the secondary and primary, my secondary now looks like Astrobaby's example:

(courtesy Astrobaby)

Which I guess is ok.

Louise

Edit: I noticed there is a chip in the glass on the left hand side which is annoying but I guess shouldn't affect the mirror side

[Uranium235]

Now youve adjusted the focuser tilt screws, you may need to actually take it apart (removing the secondary) and properly square the focuser by taking the appropriate measurements and marking the tube wall opposite the focuser. For all we know, its inital position may have been correct for that particular tube. I had to do it to my 130, and needed to make a fair bit of adjustment.

Watch this, its for the Quattro - but the principle is exactly the same:

For now, get it collimated as best you can, put a coma corrector on it with a large chip camera - then we will see what the corners bring. I had to collimate and tinker a lot before it did what I wanted it to do (ie: give me good stars in all four corners of a big, heavy camera). If turns out you now have a tilted field, you will need to do as described in the above video.

[Thalestris24]

Hi Rob

Thanks for that. But eeks! I don't think I can do that level of stripdown and rebuild - I don't have the facilities really. I don't have the confidence either and it would probably end up worse than when I started! I'll check the collimation with the primary tomorrow and if all seems ok after any adjustment, I'll give it a star test (when I can see some stars, lol). If that's ok, next step will be to transfer the autofocuser over from the ST80. I need to order a coma corrector. Once I've got that I'll fit my 1100d and see how it performs imaging wise...

Cheers

Louise

[Alkaid]

Hi Louise,

Did things work out for you with the problem?

[Thalestris24]

Hi Louise,

Did things work out for you with the problem?

Hiya

I've not really done any more with it. I'm just ordering a coma corrector. It's still not really had a first imaging light - rotten weather up here! Thanks for asking though. I'll post an update as soon as I'm able to try it out properly

Louise

[Thalestris24]

Well, having spent ages fiddling with secondary adjustment (wish I hadn't touched it but it's all good experience, I suppose...) this is the best I can do without actually tearing my hair out:

I know it's not perfectly round and may not be perfectly centred but, the question is, is this good enough??

Also am I right in thinking that it's just the tension of the central bolt spring that holds it in place against the allen keys? There doesn't appear to be any other mechanism that could lock the secondary.

Thanks for any advice or help

Louise