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Advise on Catseye Collimation instructions

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#1 bluesilver


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Posted 06 July 2020 - 06:20 PM

Hi,  I have just received the Catseye Auto collimating kit from Catseye

I just have one interesting question in regards to the instructions they mention about the secondary mirror on the Dobsonians.

Firstly,   my Dobsonian is the Skywatcher 16' collapsible version with goto 


In the instructions provided by Catseye,  they talk and recommend moving the secondary mirror away from centre,  they talk about moving it away from the eyepiece.

They give you a table that has the the size of your secondary mirror and the f/no of your scope,  find these numbers and then it tells you how far to more the secondary away from the eyepiece,  ( tells you how far to move it off centre )  in my case it works out to be 0.224"

So going by that i need to move the secondary 0.224" off centre away from the eyepiece.


So to make that a bit clearer,  the instructions say:


For optimal "Axial" positioning of the secondary using the TLETUBE XLS/TELECAT XLS I highly recommend that you consider first setting the corresponding ( and aqual ) vertical offset away from the focuser.

Refer to the Offset cross-reference table on the last page and locate the nearest value to your secondary mirror size ( minor axis length ) in the left hand column; then read across to the f/no of your telescope listed across the top.  The appropriate offset value is in inches.

The proceed to to set the vertical offset in a direction AWAY FROM THE FOCUSER by adjusting the spider attachment screws.  Ultimately the difference from the focuser side to the secondary will be longer than the opposite side distance to the Secondary by twice the offset vaLue,  The side to side distance should remain equal.


So after all that,  for those that have used the Caytseye equipment,  dose it read a seam correct to actually move the secondary away from the focuser.

I did some measurements and if i did this,  although it isn't far,  i don't have much thread left on the focuser side of the spider vanes.

Is it worth moving the secondary away from the focuser?  will i gain much by doing this?


Just to me reads a tad odd as everything i have read about collimating the Dobsonians,  everything says to make sure the secondary is dead centre of the scope and make sure the foucuser is square with the tube.

Where Catseye are saying to move the secondary off centre.


Any advice or information on this would be greatly appreciated.










#2 Vic Menard

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Posted 06 July 2020 - 06:42 PM

I think what the instructions are asking you to do is to mechanically offset the secondary mirror away from the focuser side of the tube. This can be useful if you're looking for precise digital setting circle performance. That said, it's possible that SkyWatcher has already done the offset for you (remember--it can be done at the spider and/or the primary mirror).


My advice--don't worry about it. I have a 22-inch f/4 with a 4-inch minor axis secondary mirror that is not offset (0.25-inch) away from the focuser side of the tube assembly. I get excellent DSC/GoTo performance (Argo Navis/ServoCAT) and my collimation is also quite good. 

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#3 ButterFly



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Posted 06 July 2020 - 08:35 PM

You really won't notice the difference in the eyepiece.  The only effect of offsetting, either full offset or new model offset, is to get a fully illuminated field.  See Don's article, the figures on pages 3 and 4, for the difference (seriously, read Don's article).  With the new model, the secondary is not exactly 45 degrees to the telescope tube so the secondary is not perfectly circular when viewed through the focuser.  The new model is much easier to set up and the downsides are just not relevant for visual viewing.  If you are tracking while using a huge sensor to take pictures, maybe the differences becomes relevant.


With the sight tube, if the secondary is not fully offset, you may notice a slight difference between the spacing of the secondary edge and the sight tube in different directions.  This happens because the secondary is tilted slightly toward the focuser instead of at exactly 45 degrees to the tube.  It's a tiny amount even when the secondary is directly centered in the tube.


When using the sight tube, align the crosshairs so that one is in line with the focuser drawtube as seen in the mirrors.  Make the spacing on each side of the secondary uniform with respect to each crosshair.  The spacing can be different from each other between each of the pair of crosshairs and the secondary can still be positioned well. 


Getting a perfect 0.2" offset away from the focuser is rather difficult so don't bother and just trust the sighttube.  The secondary position lasts months if not years but you should check it often.  You can't really fix it in the field easily, but make a note of it to fix it at home if you need to.

#4 bluesilver


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Posted 06 July 2020 - 08:52 PM

Thanks for the replies and information,  appreciated.


I was only really asking as i kind of skipped this first step and went ahead to square the focuser to the tube.

Not sure if i should be stating a new topic on this but will ask anyway,  apologise if i have gone about it the wrong way here.


I did it a little different by using paper inside the tube rather than actually drilling a hole in the tube as they mention in the instructions.

(Might have to go back and do it like they mentioned)

I then  put a rod in the secondary holder in place of the centre bolt to get the secondary spider centre.

But when you go back and measure with a ruler,  the centre of the spider is not centre to the tube,  it is way off.


I took the centre bolt out of the secondary mirror ( removed the secondary mirror ) placed a rod in where the bolt came out of,  placed the laser in the eye piece and adjusted the spider vanes to bring the rod to align with the laser dot.

( laser is fine and checked for collimation before all this )


So once that was done i decided to run the tape over the spider vanes to see how they look and that was when i found out that they were way off centre in relation to the tube.


So my question now is and i guess it is a little of the original topic,  but should i just go back,  centre the spider vanes in relation to the tube by the ruler method,  then adjust the focuser to met the centre of the spider?


As something seams to be way off,  I am guessing i have not centred the focuser correctly to start with.

Edited by bluesilver, 06 July 2020 - 08:54 PM.

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#5 ButterFly



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Posted 06 July 2020 - 09:28 PM

Undrilling is hard, so good call on that.


The toward and away from the focuser direction can't be determined by just pointing a laser at the secondary bolt.  That you have to measure if you want it precise, but again, you don't need that for visual.  The sighttube will take care of the up and down the tube direction as well as the "left and right" direction.  Start with it centered to make the required corrections as seen through the sighttube smaller.


Squaring the focuser to the tube is another "close enough is good enough for visual" thing.  As long as it's close enough, the secondary tilt will make up for the differences.  "Close enough" has a vary wide margin.  That should be done before a sightube is put in so that the secondary can be positioned properly under the focuser.  The secondary tilt is then adjusted to match the focuser axis.  The primary tilt is then adjusted to match both the secondary and the focuser.  The tilt adjustments are repeated until they all agree at the same time.  It's quick once you get the hang of it.


Don't be afraid to mess around with all these things and see the effects they have, especially tilts. That can help you determine what the view looks like when it is time for a touch-up to collimation during a session.  That minute spent on touch-up can make a huge difference when the air is very steady.  You'll get the hang of it rather quickly.  Always remember that the whole point of collimation is to get the optical axis of the primary to be parallel and on top of the optical axis of the eyepiece (tilts) in a way that lets the whole light cone into the eyepiece (positions).

#6 bluesilver


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Posted 06 July 2020 - 10:43 PM

Thanks again for the replies.

I can see now what you are saying,

I think i will have to go back ,  remove the spider assembly again,  square up the focuser to the tube again just to make sure that part is spot on.

Then i am guessing the best thing to do is to put the spider back in and just measure using a ruler or something similar to get it spot on centre again.

As it sits now,   it is way of left hand side of the focuser,  not to and from the focuser ( sorry about that confusion )

I did try to get it centred left and right of the focuser using the laser and rod method,  the end result was that the spider was 5 mm

(.19" ) off centre to the left hand side of the focuser.


An odd problem to have,  but if i go back, start by squaring the focuser,  then forget the laser a rod method,  just measure the spider by ruler and get it spot on centre that way might be the best option.

#7 SteveG



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Posted 06 July 2020 - 10:55 PM

I hate those instructions, and they are wrong IMO. The only way to do that properly is to have the secondary mirror glued to its support offset - the 2 or 3 mm that is needed. Even then, it doesn't do you much good with most standard dob's as mentioned. Bending the vanes is just plain wrong.


The new model works flawlessly and you don't even have to think about it. CENTER your spider hub, making sure the vanes draw a straight line across the opening of your tube. Then center the secondary under the focuser with your site tube. That's it. If you put a laser in there, you will see that it hits the secondary that same 2-3 mm off from the longitudinal center. That is your offset! Proceed with aligning the secondary axis to the primary, then align the primary before moving on the the autocollimator.

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#8 bluesilver


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Posted 06 July 2020 - 11:06 PM

Thanks again,  I thought i must me over reading, overthinking it.

But as i have originally played around trying to square up the focuser,  i guess i am best to go back over that part just to make sure i haven't done anything too silly with that,

Pull it back off, measure everything back up and re install it.

Put everything back together and as you just mentioned,  Centre the spider hub and away you go from there.

Everything else is fairly straight forward.

Great advise on here,  should of got on here ages ago.


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#9 sixela



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Posted 07 July 2020 - 02:37 AM

Yes — either the spider vanes have been sized taking offset into consideration (i.e. for a 4 vane spider they are in parallel pairs with the holder offset) or the secondary holder is centred and the secondary offset.

If not, don't try to offset the secondary away from the focuser. When setting tilt of the two mirrors, you'll automatically slightly tilt the optical axis towards the focuser to compensate, and that'll be the end of it.

Even DSCs these days can model the resulting cone error, so there really is no reason to obsess over it.

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