Collimating cassegrains like Tal, Intes and Vixen.
I just wanted to know how many owners have a fear in collimating their scopes. This discussion may bring combined experiences in helping others in achieving that magic wow factor in their scope. For example my Tal 200k. After owning different types of scopes from refractors to mirrors each had to be collimated at one stage mostly due to couriers or factory alignment. Over the years you develop your collimating skills through reading and practical sessions.
Telescope owners really rely on optical parts being attached accurately especially on fixed rear mirror cells. If your rear mirror is out then you can only collimate with limited success. The benefits in being able to collimate two mirrors instead of one can mean you have some control in achieving that magic sharp focus. In my case it is the Tal 200k FL 8.5 (1700mm) these scopes have a self centring design so collimation is very simple. Another point in the optical train is the focuser in some scopes they have their focuser attached to their primary mirror support, so you need to align that to the centre of the tube and work back from there.
In the diagrams the red and green bits are self centering.
Has anyone had to realign 3 or more optical paths eg. Centring the focuser, primary mirror, secondary mirror or even having to machine parts.
Regards
Den
Last edited by Den; 07-03-2013 at 09:14 PM.
Reason: Add pic
Hi Dennis, I dont own any of the 3 you mentioned , but I have built and owned many Newts over the years and now have a Takahashi Mewlon 210 and the columination of that scope is a little different to a newt , but it really only took me a few tries , a lot of reading to get it spot on .
I found it is much easier with 2 people , one looking at the defocused star pattern and me turning the secondary screws ( Taks dont have a columinatable primary ) a tiny little bit ( really tiny < 1/8th a turn) to get it perfect , and WOAW! you know when its right , the images are so very good ,APO refractor like it is that good !
Brian.
Yes I started at about 150x and then 250x and finally around 400x , but at that the incruments of adjustement are so tiny as I said < 1/8th a turn on any given bolt , max ! .
OH yes , I dont use the diagonal when columinating , straight thru is the best .
But yes the images when properly columinated are well worth the time taken .
I have only done a proper columination once in 9 months , the Tak holds it very well .
Brian.
To collimate, In my case I put a laser in the 2” focuser and the light passes through the secondary mirror (The tal has a section of mirror uncoated in the centre of secondary) so the light passes through it, I then place clear tape on a centring bolt to see if the focuser is on centre. In my case I placed a thin shim in between focuser flange ( this focuser is separate from the primary mirror attachment ). I then place the TAL on a mini mount and prepare to adjust the secondary mirror by making an artificial star light made up of a pin hole on black paper and shine a led torch through it, place this light box as far as you can from the telescope, in doors. ( With the Tal200k you line up the light using the unmirrored centre and the centre of the focus tube, this brings multi circles of light together, see pictures). . If you do this outside on a sunny day you can use a RED or GREEN xmas ball only instead. Using the light I aim the telescopescope on centre (you can use a illuminated eye piece to help centre it) and use a low power eye piece with a defocused circular star pattern (winding the focuser towards the secondary it is very bright). If collimation is close you will see lots of circular rings like a bulls eye. When not collimated (see below: a picture of diffraction rings) you will see the rings leaning to one side. My scope was only a little out so adjusted the secondary bolts till it was like a bulls eye. Then I winded the focuser outwards to check the primary mirror it needed only a slight adjustment. Then I took the scope outside on a real star and use higher powers to do a fine adjustment.
Den
Last edited by Den; 05-03-2013 at 08:43 PM.
Reason: Add picture and edit notes.
And a artificial star sample with a xmas ball done with camera flash reflection( could not find my red ball so I used this ball only as a sample image): The sun does produce a much better point of light (ONLY use RED or GREEN ball), I would not look at a lighter colour ball because of the suns rays being to bright and may damage your eyes. remember DEFOCUSED pattern only see image below .
Hi Den, as discussed, here's a quick presentation I put together that may be relevant to SCTs collimation. In my view there is no bad scopes. Just scopes needing a bit of mechanical tweaking now and then to get the best performance out of them. What you pay for in premium products is the time and attention spent but the manufacturer in putting the rigs together in a nicely engineered package. I don't think nowadays there are any really bad optics being produced out there but mass produced scopes lack in QA . Sometime you get a bad one.
Excellent work Marc, that is a huge effort .
This will help others in understanding the importance of correctly lined optics.
Thankyou Marc for this link.
Regards
Den
Here is a link to tweaking up of a Celestron 90, it is not complete but has some good images in helping with visual aspect of alignment.
I like how he has marked the secondary mirror! this would be very accurate.
Here is a link to tweaking up of a Celestron 90, it is not complete but has some good images in helping with visual aspect of alignment.
I like how he has marked the secondary mirror! this would be very accurate.
I centered spotted my secondary as well. I reckon it's a must. Otherwise you never know if it's centered in respect to the back focus. Very hard to check by looking at the concentric reflection alone. I could never do it.
I also used a board in the past with the pin hole and laser to check the halfway point for the primary focal point but since found that if you get a laser point source and stick some mate adhesive tape on it you can diffuse the light and illuminate the whole primary.
By looking at the secondary shadow in the reflected image you can pretty much get everything bang on. The latest Hotech tutorial explains how to do this on their website.
Problem with sticking a laser at the back of the scope is that you assume it's aligned mechanicaly. If you shine a diffuse laser in the front aperure and look at the secondary shadow and the reflection of the primary you don't care about anything else but the primary. You take the tube out of the equation. So it's always right to start with.
It was raining last night but I managed to set up under the veranda and do some star test across the street. I did a couple of videos on vimeo.
All videos made with a celestron neximage video camera. Pixel size on sensor is 5.6um. Not very high quality videos with a little turbulence but enough to check a few things.
In retrospect I was setup in the back of the house looking through the garage with both doors open and across the street. Maybe the air in the garage was warmer than the outside.
First star test C11 non collimated in and out of focus. https://vimeo.com/60788825
So this is straight out the collimation with the laser. You can see it's off but pretty close. This tells me that the secondary is still a little offset.
Then C11 collimated out focus to focus. https://vimeo.com/60796558
Out of focus is always better than In focus because the air inside the tube is more turbulent than outside.
It's looking good but I'm sure I'll get better airy patterns looking up guiding on a real star. The distance between the artificial star and the scope is not big enough to get the true picture.
Especially with the In and Out of focus patterns. You get spherical aberrations on short distances that go away when you focus at infinity due to the right spacing between the mirrors.
Here is another project showing parts of optical alignment.
Note how he has used caps on the mirrors with centre marks making it easy to align with the laser. (see picture)
Here is another project showing parts of optical alignment.
Note how he has used caps on the mirrors with centre marks making it easy to align with the laser. (see picture)
It was raining last night but I managed to set up under the veranda and do some star test across the street. I did a couple of videos on vimeo.
All videos made with a celestron neximage video camera. Pixel size on sensor is 5.6um. Not very high quality videos with a little turbulence but enough to check a few things.
In retrospect I was setup in the back of the house looking through the garage with both doors open and across the street. Maybe the air in the garage was warmer than the outside.
First star test C11 non collimated in and out of focus. https://vimeo.com/60788825
So this is straight out the collimation with the laser. You can see it's off but pretty close. This tells me that the secondary is still a little offset.
Then C11 collimated out focus to focus. https://vimeo.com/60796558
Out of focus is always better than In focus because the air inside the tube is more turbulent than outside.
It's looking good but I'm sure I'll get better airy patterns looking up guiding on a real star. The distance between the artificial star and the scope is not big enough to get the true picture.
Especially with the In and Out of focus patterns. You get spherical aberrations on short distances that go away when you focus at infinity due to the right spacing between the mirrors.
Thanks Marc,
That does show up quiet clear.
You can see the heat plumes clearly as well.
This is good footage for people starting out collimating.
Hi Dennis, I dont own any of the 3 you mentioned , but I have built and owned many Newts over the years and now have a Takahashi Mewlon 210 and the columination of that scope is a little different to a newt , but it really only took me a few tries , a lot of reading to get it spot on .
