collimation - mechanical questions on primary mirror adjustment screws

[YahooGroup]

This is a transfered topic from the ASA Yahoo Group.

 

Posted By: konihlav Sun Apr 21, 2013 12:51 pm

Hi group,

I am a bit frustrated with my new ASA Newtonian. The PDF manual is grossly
inadequate and poor on information
(http://www.astrosysteme.at/images/Collimating_ASA-Astrographen_E.pdf). I try to
calm down my emotions now though it's not easy as I wonder if the guy who
invented the mirror holding system (3 allen key screws) was a real genius or an
real ..... :-(

OK, gang, please could some of you shed more information and give tips on how to
operate these so called pull and push and traction and pressure screws (there
are 4 different names for 2 different screws used in the manual which is really
confusing and annoying)? What sequence to use, how many turns to make and any
other practical tips on how to mechanically adjust the primary mirror. What to
do and what never do in order to make the collimation last (I do not have any
fixed place or an observatory so this nightmare will be present every time I
want to use the ASA astrograph for imaging).

The biggest stupidity in the manual is on page #5 above Image 11, I am citing
the original "In order to collimate the main mirror, start by first turning one
of the pressure screws (max. 5 mm) then fasten with the help of the traction
screw while turning the second pressure srew until end stop (see Image 11)."

COULD PLEASE SOMEONE EXPLAIN the meaning of
- the max. 5 mm ??? is it just an information on total travel the screw can
make? or anything else?
- fasten with traction while turning... ? how can I fasten one screw while turn
the other??? should I operate two allen keys at the same time? or in which
sequence should that be done?
- what "until end stop" really means?

if I fasten the middle screw and then the other side screw the remaining side
screw remains loose.

I tried to play with my mirror a bit and it doesn't move at all. I had to loosen
all three screws to make any movement apparent - using a laser collimator (and I
really have the mirror in almost vertical position). BTW my tools are Blackcat,
Infinity and Howie Glatter.

thanks for any tips and howtos. I am not so big idiot as I seem even though I am
Newtonian newbie, because I know how to collimate ordinary simple SkyWatcher
Newtonian, I know how it works, the only thing that I do not understand is HOW
TO MECHANICALLY ADJUST ASA'S SYSTEM holding primary mirror. It seems too
complicated to have 3 screws.

thank you!
best regards
Pavel

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Posted By: bernd_eppinger Sun Apr 21, 2013 6:20 pm

Hi, Pavel,

I don't have an ASA astrograph, but I speak German, and I think the German version of the manual is easier to understand.

Here is how I would understand and translate the text of figures 10 and 11:

Fig. 10:
After that, adjust the primary mirror using the pull- and push-screws. First loosen the Pull-screw by turning it counter-clockwise.

Fig. 11:
For adjusting, turn one of the 2 push-screws (max. 5mm ?!?), then lock it with the pull-screw, and then turn the other push-screw in the same direction until it reaches a stop.


I can't tell you what the "5mm" means, but I have the impression that I understand the rest, although it could be written clearer. The following is my understanding. I think that for locking the mirror holder screws, they use two push-screws that work against one pull-screw for each of the 3 adjustment positions. That avoids inserting too much strain into the mirror holder.

As the telescope is pointing up during adjustment, the weight of the mirror is initially on the push-screws. Therefore, loosening the pull-screw does not move the mirror holder very much. The next step is to adjust the push-screws. In my opinion, turning only one of them only works when the mirror must be pushed up. The general case will be that both must be turned. After that, the mirror holder will usually lie on only one of the 2 push-screws. After the pull-screw is tightened again, one of the push-screws will still be loose and must be tightened, too. Ideally, both push-screws should have a similar force so that a minimum amount of torque is brought onto the mirror holder.

The German manual also says that the collimation screws should not be tightened too much to avoid strain on the primary mirror.

Perhaps an owner of an ASA astrograph can provide practical collimation tips.


Best regards,

Bernd

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Posted By: konihlav Mon Apr 22, 2013 9:28 am

Hi Bernd,

thank you for your insight, it makes sense to me. Your version makes it clear. I
think I understand it now.

From practical point of view (that's what I am looking for) the 5mm most
probably means the overall length (range) the screws can travel and the primary
mirror can be positioned (changes focus). I was surprised that after loosening
the pull screw (center screw) and one of the push screws (side screw) nothing
happened/changed. Therefore I need to play also with the other side (push) screw
to adjust primary and then, when done, fix the center screw and re-tight (hand)
the other side screw and then it's in fixed position I guess.

Gonna be a night mare to collimate the F/3.8 system, I knew I am walking on a
thick ice when I decided to go this way...

Now I go to order the "tuBlug" from Howie (if he's already back home from NEAF)
because I need to see the laser spot from the back of the telescope. I plan on
using simple Howie laser first and probably won't use the Catseye that I already
purchased - or I would have to figure out a way how to project the tringles with
a webcam on LCD screen to see what's happening from the back end of the OTA...

seems hard job to me now (being beginner is nothing I like)

thanks again
Pavel

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Posted By: bernd_eppinger Tue Apr 23, 2013 12:24 am


Hi, Pavel,

I wish you good success for your telescope collimation.

When I bought a laser, I found it made collimation much easier, because I can now see it from the back of the telescope just as you write it. However, my telescope is an f/5 which is more forgiving.

But if the laser is not precise enough for your f/3.8, you could still use your webcam idea with the CatsEye. By the way, I wonder why nobody had your idea before. It could be quite simple: Something like a CatsEye collimator with a simple lens and a T-Thread for a planetary camera. An advanced version could include a software which inspects the image and tells you which screw to turn in which direction.


Best regards,

Bernd

 

 

[si6pack]

Hi Pavel,

I can understand your frustration with the collimation - I share the same pain! (ASA12")

May be I can add some findings, which helped but did not solve my problems:

1) Collimation MUST take place in a (near) zenith position of the telescope, all other positions did not lead to any satisfactory result (at least in hands)

2) Final collimation must take place on a star

3) I tested 4 (!) different lasers - only two were perfectly centered!

4) During collimation, always focus on one of the three adjustment points (consisting of 2 push screws and 1 pull screw)

5) Being desperate with all my collimation efforts  and doubting that I am capable of collimating this telescope I decided to visit an astronomy dealer nearby to test my collimation with an artificial star - it turned out to be perfect...what's the problem now???

6) My observation: I did a perfect collimation with my laser in zenith position, then I moved the telescope in different positions (W and E of the meridian) and had to realize, that depending on which direction I was pointing, the reflected laser beam is slightly moving out of the center, leading (my interpretation) to disrupted stars...may be I am to picky, but if you spend hours of collimating, then I would like to have round stars, especially when everything seems to be perfect...

8) My read-out: I have to remove the mirror from the telescope and check whether the mirror is perfectly fixed in the mirror cell.

 

By the way, I also considered that the ccd equipment might lead to some flexture - I compared a FLI8300 with a FLI 2-7 FW with a Xpress H694 and the Xpress USB FW (heavy vs. moderate) - no if any difference using the flexture tool in Sequence...

 

Hope this helps!? Please keep me posted concerning your progress!

Best,

Christian

[ChristerS]

Hi Christian,

 

I also have a ASA12" and share your frustration. I can not get a good collimation with the glatter laser and tublug (glattter laser making a small cone when rotating it so I tried to collimate it but I dont think I got it perfect, need to send it to Howie), now waiting a catseye infinity kit from Jim to see what it can do. My mirror was lose in the cell so the collimation changed i different directions, tighten the plastic screws in the cell so now that solved. But collimation still bad. When the pull and push srews are tighten I can press on the backside of the mirror and notised it is not stiff it has a spring resistance I asume this is normal. 

Thanks for the collimation tips, have you made any progress in collimation efforts sins last? I will update on my progress later.

 

Regards,

Christer