How To Guide for Collimating the E160ED Takahashi Epsilon-160ED · Bill Long - Dark Matters Astrophotography · ... · 17 · 3025 · 5

Hey all,

Quick post to separate collimation out from the other thread about full-frame imaging. A lot of people are afraid of collimation in general, and not just with the Epsilon scopes. However, the Epsilons have an even worse reputation from users (and those that do not own the scope) about collimating them. I am not going to post a ton of photos, as those are easily found on the internet, I just want to illuminate a few very important things one needs to keep in mind when collimating the E160ED specifically. This write-up assumes you are using the Takahashi Collimating Tube and Eyepiece suggested by them for the scope. If you are using something else, stop doing that.

This guide, is in essence trying to better explain what the manual states (poorly) so use the photos in the manual along with this guide to help close the gap.

Key Points:

1. This entire sequence can be completed indoors, in the daytime.
2. You do not need to make changes at night. In fact, if you look at an out of focus star and think it is improperly collimated and try to "fix" it, you will throw the scope out of collimation by doing so.
3. Your ultimate goal is to have the center of the crosshairs, secondary blot (aka the football), center of the reflection of the collimating eyepiece, and the primary mirror spot all properly spaced and centered, and any annulus created by the primary marker need to be concentric.
4. Here is what moves the things you see:
1. Crosshair stays in place at the center of the collimating tube, as it is not a reflection at all.
2. Secondary blot moves small amounts with the adjustment of the secondary mirror knobs, with respect to the crosshairs. You are only looking at the reflection of this spot, from the secondary mirror. That is why it takes on a slight football like shape as the mirror is angled. It is indeed a round spot though.
3. Primary circle moves with adjustments to either mirror (as it is a reflection) however, and this is important, it will move independently of the secondary mirror blot (with respect to the crosshairs) only with adjustments to the primary mirror. If you nudge the secondary bolt around it will move, but so will the secondary blot. This is important to know, as at some points you will want both to move, and at others you will only want one of them to move (the primary) and knowing how to get that result is important.
4. Center of the collimation eyepiece moves with adjustments to the primary mirror or by rotation of the secondary mirror (technically, you can think it moves with both mirrors, as it's a reflection). You will find that influence by the primary or rotation of the secondary mirror is the more influential way to alter its position.

Clear as mud, right? 

High level steps to collimate:

1. Center the secondary blot on the crosshairs using the secondary collimation screws.
1. Note that if the difference is large enough, you may need to completely loosen the secondary mirror and manually adjust it until the spot is close, then lock it down. After that, you can use the secondary mirror screws to dial it in. This video shows this in more detail (Credit: Ben's Astrophotography) (1) Collimating Epsilon 180 (Secondary mirror ONLY) - YouTube. Note that the secondary mirror adjustment screws are older design in that video, than the ones on the E160ED.


2. Rotate the secondary to place the reflection of the center of the collimating eyepiece close to center of the crosshairs, note it will not be perfectly aligned, just close enough. It may already be close, if you followed the video linked above, since he basically does both of these steps at the same time. You do not need to do that though, you can do them seperately.
3. Adjust the primary to properly center the reflection of the collimating eyepiece under the crosshair and the primary mirror marker.
1. Note, tiny back and forth tweaks on the primary and secondary bolts may be required to adjust the primary mirror marker with respect to the secondary blot. This is fine, just be extra careful not to move the blot from the center of the crosshairs.
2. The blot and crosshair relationship can be extremely difficult to see as you get closer and closer to proper collimation. Some folks use thinner crosshair material, I personally use a Takahashi Collimation Scope which allows me to zoom in and see the relationship of the blot and the crosshair directly.


4. Check that the center of the crosshairs, secondary blot (aka the football), center of the reflection of the collimating eyepiece, and the primary mirror spot is properly spaced and centered, and any annulus created by the primary marker is concentric.
5. Review steps 1-4 until the desired state is met.

A few things to look out for:

1. The primary is adjusted by loosening the large hex bolts, and using the two small grub screws at each of them to make the actual adjustments.
2. Adjustments needed may be in or outward, depending on your situation.
3. Be extremely careful not to loosen the large hex bolts too much, this could cause the cell to shift backwards, without you knowing it. Also do not run the grub screws too far in, as they can pop out. Don't ask me how I know this. 
4. Ensure that all 3 large primary hex bolts and the center locking screw on the secondary are tight when finished, and that there is no change to the collimation when doing so. You will probably find that it does have a minor impact, thus you will need to carefully move toward a fully locked and snug state while adjusting near the end.
5. You may need to go through this a few times to really get your head wrapped around it. If you plan on removing the stock focuser for a Leo, this would be good to practice while you are waiting for the Leo, as you will most definitely need to collimate after replacing the focuser.

What should this look like at the end, you ask? Well, like this (Credit: @Ram Viswanathan):



Feel free to share experiences, corrections to what I wrote here (totally possible), questions, tears from frustration, or even tears of joy and elation from conquering the process. All are welcome!

Thanks,
Bill

1. Remove the rear "cooling port" cover from the scope.
2. Take a photo of the markings on the backside of the primary mirror, this is very important as you need to know how to place the mirror back inside the cell.
1. Note: The mirror cell has markings on it, on the edge, but I have no clue what they mean.


3. Remove the 3 large hex bolts on the back of the primary mirror cell.
4. Pull the primary mirror cell out.
5. Retract the grub screws so they are not poking out.
6. Reinstall the mirror cell in the right orientation, using the 3 large hex bolts.
7. Tighten down the grub screws until they just make contact.

Yuexiao Shen:
Use an adapter to connect the video to the eyepiece?

Aygen:
Just stumbled upon your write-up, Bill. What else to say other than a BIG thank you. I will have to go through this sonner or later so, yeah, it brings clarity compared to the very poor explanations of the manual. Bookmarked it !

How crucial is the centering of the secondary blot for correction?

I had read a yearish ago that the secondary centering was only going to influence the  illumination of the sensor - the primary mark, crosshairs, and reflection were the key elements to collimation.

I've never been super serious about centering the secondary blot and I've never had great corner stars on an APS-C sensor. I've spent hours fiddling with tilt and backfocus and I haven't had good results.

Paul Wilson:
Well, as a prospective buyer, this is of course hard to follow.  

I am still not clear on exactly what tools I would need to purchase with the scope in order to be able to be most efficient and effective.

I do "like" that the collimation can be done inside in the daytime.  I am not at my best out in the dark with a hex wrench.  Ha.  Much better to be doing it at 10am with full tank of coffee on board.   

Now that I see this particular thread, I also think I read that the 160, although smaller than the 8's that I have owned, is a newer design than the 180, is that correct?
I also would feel that at F3.something, it might be inherently a bit more stable than my F2 scope.  The few 160 images I have seen looked good.  However, it looks like the primary poster himself is using some other scope in his astrobin photos?