Boren-Simon collimation

[ayoussef]

That is one beast I cannot tame.

Even after I upgraded the focuser to one of starlight instruments heavy duty focusers, I keep getting comet shaped stars, 95% of the time, only one time that I imaged the Hercules cluster and it didn't happen.

I use Catseye collimation set, and after I am done I can get an image like this :

Then once I start my imaging session, right away I get stars shaped like that :

I tightened the knobs for both the primary and the secondary mirror, not too tight but just firm, I test my work by using a laser collimator, I put it in the focuser and slew the telescope from east to west, if the laser dot stays at the same point then non of the mirrors are shifting around.

The image is taken using QSI6120 with ASA reducer at F/2.8, but I still get the same issue if I used Baader coma corrector at F/4, I also took a sample image using my lodestar x2 which weighs only few ounces, and still got a similar result.

Should I buy a new newtonian with better performance and just use the ASA reducer with it?

[SimonfromSussex]

I'm no imaging expert but your collimation looks ok.

When I used the Asa in a 10" ts newt I remember the spacing was key, I.e. Distance from ccd chip to the reducer, have you had a play changing the spacing?

[ayoussef]

I'm no imaging expert but your collimation looks ok.

When I used the Asa in a 10" ts newt I remember the spacing was key, I.e. Distance from ccd chip to the reducer, have you had a play changing the spacing?

That is correct Simon and yes I did. when the spacing was incorrect, the stars in the corners were pointing outward, in this one they are pointing toward one direction, down left, and I just realized that the stars in the up right corner  looks better than the rest of them.

[ayoussef]

Just an update in case if someone faced the same issue and searched the forum for a solution, it seems this article http://astropixels.com/bifrost/tak_e180-collimate.html describes the same issue I am having, its a focus gradient due to misalignment somewhere in the imaging train. 

[Tim]

Is that the whole frame pictured?  When you took the shot of the defocused star, whereabouts in the field was it? Central?  When you have it like that in the centre, how does it look if oyu move the star to the edge of the frame?

Are you sure the secondary mirror isn't rotated a little bit? The Cats eye should help solve that if so, are you using the twin pupil auto collimator with the set?

At f2.8 the collimation is devilishly difficult to maintain.

If you can work out where the issue stems from, if it is the primary cell not holding in place nicely, you may be able to upgrade the cell.

Once you have achieved collimation with the auto collimator, if you rotate it 180° it will give you an added stage of accuracy.

Finally, when focusing, don't focus on a central star, choose one 1/3 in from either edge.

[Tim]

Some great points in here :  http://www.catseyecollimation.com/mccluneytext.html

Also check out Vic Menards book 

http://www.firstlightoptics.com/books/new-perspectives-on-newtonian-collimation-vic-menard.html

HTH

Tim

[Astroscot2]

One of the issues you may have is with the tube itself.  A good focuser is fine however if the tube sags with the weight of the focuser and imaging train then

it may appearl like a collimation issue.  I had the same problem with my OO newtonian and had to strengthen the area around the focuser with an inner tube.

Just a thought- hope you get it resolved

Mark

[ayoussef]

Is that the whole frame pictured?  When you took the shot of the defocused star, whereabouts in the field was it? Central?  When you have it like that in the centre, how does it look if oyu move the star to the edge of the frame?

Are you sure the secondary mirror isn't rotated a little bit? The Cats eye should help solve that if so, are you using the twin pupil auto collimator with the set?

At f2.8 the collimation is devilishly difficult to maintain.

If you can work out where the issue stems from, if it is the primary cell not holding in place nicely, you may be able to upgrade the cell.

Once you have achieved collimation with the auto collimator, if you rotate it 180° it will give you an added stage of accuracy.

Finally, when focusing, don't focus on a central star, choose one 1/3 in from either edge.

Hi Tim, yes both images are as is from the ccd, no cropping, so the defoucesed star is in the middle, defoucesed stars away from the center will look oval instead of circular but I thought this is normal ?

The catseye autocollimater make it look like the secondery is in the right orientation, but I still can double check just to be 100% sure.

I will try focusing with star 1/3 from the center as you said, I have a feeling this will lead to something.

BTW thanks for the links!

[ayoussef]

One of the issues you may have is with the tube itself.  A good focuser is fine however if the tube sags with the weight of the focuser and imaging train then

it may appearl like a collimation issue.  I had the same problem with my OO newtonian and had to strengthen the area around the focuser with an inner tube.

Just a thought- hope you get it resolved

Mark

Thanks Mark that is a good point, the tube is made of carbon fiber, it can bend with some pressure.

I have no idea how to fix that other than getting another ota with stronger body.

[Astroscot2]

My own CF tube is only 2 - 3 mm thick, I had  to have an CF ring made and inserted behind the focuser to prevent sag.  no need for a whole new tube.

[ayoussef]

My own CF tube is only 2 - 3 mm thick, I had  to have an CF ring made and inserted behind the focuser to prevent sag.  no need for a whole new tube.

Thanks Scott, I think I gave up already, I had 2 clear nights in row and I wasted most of them trying to collimate then test it with 3 different cameras , and I kept getting the same result over and over.

The moment I gave up was when I read on CN that the inventor of this newtonian sometimes had to collimate it after each slew to image at 2.8F , That is unrealistic.

I should have listened to people who advised me to get a Tak epsilon 180 or 130, most people say they hold their collimation pretty well, so now I am on a journey to find a used one.

By the second night I had only 3 hours left, I unmounted the newtonian  and mounted my Esprit 100, ten minutes later it was already imaging the crescent nebula, no issues, no pain.

[famax]

Your collimation is wrong.

As a fast newtonian, you should notice  an off center secondary on the image you posted.

This is not the case so your newton is com)letly out of collimation.

What to do : take of the secondary , and mark the off seted secondary center (something around4 to 6 mm in your case , up to the geometric center of your mirror)

Put back the secondary and set it til a laser beam in the focuser  reaches the off center spot.

Then collimate your secondary, then primary with your laser.

Finish with catseye collilmation tools.

[opticalpath]

Just one point about collimation:

If you are using a defocused star to fine-tune collimation, it must be defocused only very slightly and showing just two or three diffraction rings.  If it's so out of focus that many rings are visible and you can see the shadow of the secondary - as in your posted image - you may be able to see gross misalignment problems but you cannot use such a view for the final fine adjustments.

Adrian