MnAstro Forums

Celestron C5 1250mm f10

First time doing a collimation... YouTube made it look easy.

I used a normal camera tripod and DSLR on video mode for quick setup & digitally zoomed in to see out of focus star better.

I have a red dot scope on my DSLR (basically a finder scope) and it's always been pretty accurate. I noticed that after I was finished with collimation the target star is no longer in frame when red dot is centered on target.

My question is this... will stars on edges be screwed up? I had no way to test since 1250mm @ f10 I need longer exposures to get any stars even at higher ISO / gain. Without tracking any long exposure will look bad regardless.

Would rather know if I have more adjustments to make, or if it should be okay for imaging next time I go out.


Sorry it's late & I'm exhausted. Hopefully that was understandable.


Thanks!

When you make collimation adjustments, the target star will typically move out of the view (whether by eye or camera - I’ve never tried with a camera). Then you have to re-center it and make your next adjustment. Assuming your finder was originally aimed at the same place as the main scope, it will no longer be after collimation. You would have to adjust the finder again when you’re done.

Thanks! I was worried I made it worse. (On the edges anyway)

If it’s good in the center, it should be good anywhere in the field, barring any inherent flaws in the optical design or manufacturing.
For example, some telescope designs have some inherent coma, like you might see in a fast (f/5 or shorter) Dob (which is where coma correctors come in handy). And some designs have field curvature, which is why people use field flatteners. But even if a telescope has these issues, you can still collimate just fine by keeping the star in the center of the field. Any coma or field curvature you see should be symmetrical around the center, but not visible AT the center.

Hi Nick,

I'm pretty old school when it comes to collimating Newtonians. I have a laser collimator, but I prefer to just use a star. I typically point the scope at Polaris, the North Star (since it doesn't move in real time) and use that as my point source. Since you mentioned that you don't have tracking, definitely use Polaris...

Slightly unfocus Polaris and you'll start to see a doughnut. The dark center is actually caused by the shadow from the secondary mirror. The collimation process is to essentially put that central dark circle dead center in the outer doughnut, which is light from the primary mirror.

As Deane mentioned, as you turn the collimating screws under your primary mirror, you'll notice that the star will move. So you may need to re-center it a few times. I generally start with a medium power eyepiece (my 12mm Ortho). Once I have it pretty well centered, I fine-collimate with a high power eyepiece (my 3.8mm lense) till the secondary's shadow is perfectly centered in the doughnut. I then unfocus to the opposite side of focus and check the doughnut... just to be sure it still looks correct.

I'm assuming that the secondary mirror is centered here. If not, that's another set of instructions, and that has to be done before the primary is collimated.

We help people learn how to collimate at the BSIG sessions at Metcalf Field. The next one is scheduled for Sat. Oct. 15th, if you're interested. I'll post more about that as we get closer.

I hope this helps!

Suresh

Hopefully, centering the secondary isn’t a problem in a C5 Schmidt-Cassegrain.

Thanks for the tips! I think I got it decent, just wasn't expecting it to move. I'll try to swing out there the 15th! I'm sure I'd learn more than I'll remember. I am planning on buying a c9.25 soon, so it doesn't need to be perfect, but would be nice to know if I got it close, so you're certainly welcome to check it out!

Does Metcalf field have power outlets?

Also I see Suresh you have a c9.25 would you recommend that or something else? I like the longer focal length, but realize there are drawbacks. I suppose my logic is I have some decent camera lenses @ 300-600mm f2.8 - 6 so I can achieve those FOVs and speeds. I realize DSLR lenses might not be ideal, I am happy with my Andromeda and Orion nebula pictures I took with them. Any strong opinions for or against the 9.25?

Hi Nick,

Yes, there is AC power available at Metcalf Field.

Come check out the BSIG on 10/15. We typically have 20-30 telescopes of various types and sizes. You can see which ones may fit you. It will also be easier to answer your questions there.

Best,

Suresh

Sounds good thanks!