Takahashi Epsilon OCAL Collimation
We all know the reputation these scopes have when it comes to collimation. As a former RC owner I always wondered if collimation was really that difficult on an Epsilon or if people just aren't using the correct method. I've owned an OCAL v2 for a while now and used it to collimate my 10" F4 ONTC and it does a fantastic job. Not only is it more precise than I could ever be with a laser and cheshire, but the convenience of not having to look down a drawtube is pretty sweet especially when you're tall like I me (6'3"). I posted a pretty high level overview on that experience HERE for anyone interested.
I'm going to try and keep this as brief as possible. Anytime someone brings up collimating an Epsilon with anything other than the Tak tool they are quickly jumped on and told they are wrong. I've found a few posts from individuals saying they collimate their Epsilon with the OCAL and the result is always the same.. "DONT USE THAT!". But I never understood why not. The Epsilon is a Newtonian telescope. Yes its fast. Yes it has a hyperbolic primary mirror. Yes it has an offset secondary, but do does my ONTC. I don't see why the OCAL wouldn't work if used correctly.
When it came time to purchase my own Epsilon I took the safe route and also purchased the Tak collimation tools just in case. I used it for about 2 weeks. I didn't find collimation to be very difficult with the tool. Between the Tak manual and @Bill Long - Dark Matters Astrophotography 's E160 collimation guide (LINK) I felt like I had a pretty good understanding on what needed to be done and how to do it. I got the scope, rotated the focuser to the position I intend it to stay at permanently, collimated as best I could with the Tak tool, and tuned out the sensor tilt as best I could with the ASG tilt cage. Overall I was happy with the results I've been getting but the corners still had more tilt than I liked. BlurX does a pretty good job at cleaning that up but I would prefer to not have to rely on software to fix hardware issues when possible. Eventually I found myself back to thinking the OCAL should work. So I took a chance, ordered an OCAL v3 MAX and set out to find out once and for all.
The result... The OCAL 100% works and can be used to collimate an Epsilon 130. So I would imagine it can also be used on the 160 and 180.
Full disclosure: All results here are with an IMX571 APS-C sensor. If someone wants to send me a full frame camera for me to test I'd be more than happy to 😂.
First of all I want to show the result I was able to get with the Tak collimation tool. Perfect? Probably not 100% but I think it is within tolerance. The stars I was getting in my data definitely suggest so for the most part.
Setup: I installed the device using its 1.25" nosepiece adapter and inserting that into the Tak eyepiece compression ring. The sight tube was not installed. If able, the ocal should be threaded into the focuser instead of using a compression ring. M60 male to M42 male adapters aren't very hard to find.
I used a large drawing pad on a music stand as my light source to be able to see what I'm doing. Collimation was performed on my office desk during the day. The OCAL is sensitive enough that pretty much any daylight will be enough if outside. Even a completely overcast sky.
The OCAL:First we need to talk about how the OCAL works. It is literally just an electronic sight tube / cheshire. It gives you 3 digital rings which you can change the size and color of each to match it to whatever part of the optical system you need. It also gives you a digital crosshair. What color you use is a personal choice. I like yellow and light blue because they are the easiest to see contrasting against the surface I'm trying to align. The goal here is the same as with any other Newtonian telescope which is to:
a) Align the optical center of the secondary mirror to the center of the focuser.
b) Align the optical center of the primary mirror to the center of the focuser.
The Mechanics:Now we need to understand exactly how the OCAL interacts with the mirror spots. Since with the OCAL your crosshair is "inside" the camera instead of physically in the drawtube as it is with the Tak sight tube, things move differently. It should also be noted that per the OCAL manual every unit has a different serial number. That serial number corresponds to a code on a spreadsheet that OCAL provides that you need to entire in order to calibrate the device.
As seen on the OCAL display...
Secondary mirror screws - Moves the secondary spot AND the primary mirror circle.
Primary mirror screws - Moves the reflection of the eyepiece that is inserted into the focuser (what's holding the OCAL sensor). You can see the camera sensor at the middle of the eyepiece.
Step 1: First we focus the camera so that the end of the drawtube is in focus. Then using the offset sliders to align the first circle with the end of the drawtube. This is a crucial step! If you don't take time to ensure the OCAL display is centered in the focuser no adjustment you make going forward will be valid. Here you can see my first ring is concentric with the focuser drawtube. Is it absolutely perfect? No. But I am zoomed in as far as the camera will allow so the size of the error is very small. So small it would be undetectable by the naked eye looking through an pinhole eyepiece. Now that we have confidence that all the OCAL rings and crosshair are centered in the focuser, we can start adjusting the mirrors.
Step 2: As with the Tak tool our first step is to center the secondary spot. Enable the OCAL crosshair then focus the OCAL on the secondary spot then loosen the secondary locking screw and collimation screws and by hand center the secondary spot on the crosshair. Once you're there snug up the locking screw and collimation screws so that the mirror stays put.
Step 3: Enable circle #2 , size it to be a little larger than the primary mirror circle. We are using the gap between the blue circle and the outside edge of the primary mirror circle as our reference points. Using the 3 secondary collimation screws, adjust the secondary mirror's tip and tilt until the primary mirror circle is concentric with the blue circle.
Step 4: Lastly, enable the third OCAL circle (red) and size it so that there is a small gap between it and the inside black edge of the eyepiece. Adjust the primary mirror push/pull screws to make the black ring concentric with the red circle. Make sure the mirror is locked down when done without throwing off collimation.
Some may find it easier to use tips of the crosshair to judge alignment instead of a circle. Here's the result of the crosshair being used to align the primary mirror instead of a circle:
At this point collimation is done. We have ensured that the OCAL circles and crosshair are concentric with the focuser. The secondary spot is centered on the crosshair. The primary mirror circle is concentric with its circle (blue). The OCAL body/Tak adapter in the focuser is concentric with its circle (red). Thats it. I would consider this to be a pretty simple process. Getting your computer set up and the focuser circle placed right takes more time than the mirror adjustments. As with the Tak tool this can be done indoors during the day or even out in the field. I have performed this task in my backyard, in the field, and in my remote observatory.
