Mewlon 250 Collimation

[ebeyonder]

Hi all,

I am having great difficulty collimating my Mewlon 250. The problem is that I do not know what I am doing - I do not know what the turn of each screw in each direction does to the secondary mirror.

This is unlike collimating my SCT, for which I used this very useful guide:

http://www.astromart.com/articles/article.asp?article_id=548

Basically the guide says to locate a 12 o'clock screw - which controls the up-and-down motion - and the other two screws control the sideway motion. Thus, by tightening say, the left screw, I know I am shifting the secondary shadow leftwards.

The problem with the Mewlon 250 is that I do not know this. However, I notice that the Mewlon 250 has also got 3 collimation screws. I am told that only the push screws (the silver ones) do any collimation. Can I therefore transpose what I learned from the SCT collimation guide to the Mewlon 250 push screws? Do they function on the same principle? If not, is there any reliable way to predict what the screws do?

Thank you.

PS: I have tried the daytime collimation method recommended but it does not work for me...

[brian nordstrom]

:) I feel your pain , I had the same problem when I first got my M210 , I spent hours , back and forth , back and forth ,, frustrating as all I seemed to be doing was chaseing the off centre image round and round the eyepiece .
Then I discovered that its way easier to do , straight thru , eg. without the diagonal fitted .
And once it was very close I used the pictures in the Takahashi manual to fine tune it , these work but you have to realise only a tiny amount of a turn is required at this point <1/20th a turn .
Once I got it I was rewarded with the best views I have ever seen on the moon and planets that I have ever seen in any scope , ever ! perfect optics ! .
That was over a year ago and I have not had to touch it again , this even after being taken out to our dark sky site many times in a 100 letre bin on the back of my ute , holds it very well :D .
Keep at it and you will get there , remember,,,, Small steps .
Brian.

[ebeyonder]

Thanks very much! Did you ever work out how each turn in a particular direction affected the movement of the shadow? Or was it just trial and error?

[brian nordstrom]

:lol: Good old fashioned trial and error it took me a while , one more thing its way easier with 2 people , you looking at the unfocused ( about 5-8 rings worth) stars image first and someone else turning the columination bolts for you .

Then once you see a concentric donut , de-focus to the other side of focus the same amount and the donut should look the same , if so focus torwards true focus to about 1-2 diffraction rings and as this point is reached you should se the 'Comet' shape as shown in the manual , at this point only tiny turns < 1/20th a turn is needed to get the comet tail to dissappare .
You will know when you have it :eyepop: .

Remember tiny movements at this stage .
Once the comet tail is gone , focus again closer to true focus and do it again , as many times a you want , getting closer and closer to true focus and a point will come when its perfect , you are now at perfect columination , tiny movements !! dont forget ;) .

Lock up the centre bolt , not to tight , check and you will be rewarded with views only a Takahashi Mewlon can give ,, Perfect ! .
Brian.

[icytailmark]

http://www.youtube.com/watch?v=bZV_g57pto4

this is a nice little video showing what the star should look like. Keep playing around with the screws in small adjustments.

good luck

[Den]

Hi,

This may help as well.

http://www.iceinspace.com.au/forum/showthread.php?p=951590

[brian nordstrom]

:thumbsup: That's good info Dennis , I forgot to mention as shown in this that you need very high power 200x plus .
One more thing is that the Mewlon is a DK Cassegrain design and the primary is not collimatable ,t's all done with the secondary only .
Brian.