funny focus patterns

[bmitchell82]

Just wanting to pick some of the more experienced noodles out here, on the weekend i had problems with focusing.

Collmination was correct and has been verified with 3 different methods one with a Astro systems Auto collmination system (Laser for the secondary angle and barlow laser for the primary), cheshire, and my own laser collminator which itself has been collminated to 8 meters with a 5mm circle traced.

Problem.?

- the star pattern looks like a clover leaf (which i think i remember reading is astigmatisim) but where? (I released the primary to collminate only on the collmination thumb screws (not what i normally do i collminate with the locking screws generally) to alliviate any warping of the primary mirror cell. the Secondary mirror is attached by double sided tape, no induced warping of the secondary.). This is the first time i have seen this in the 10".

- Critical focus isn't achived the star comes in and then goes out, this is what the more experienced might be able to decipher. on the way in to focus the star is slightly elongated, coming into a circle (or close enough) once focal point has been reached, but on the other side of focus the elongated star rotates 90 deg then follows the same pattern as coming in.

What has changed?

- i milled a small fraction off the inside of the top collar of the focus tube,
- i milled the standard 2" adaption piece down.(see my post about SW focuser upgrade)
- the focuser has been out and serviced then repalced.

My thoughts

- The focus tube is not Normal to the telescope tube?
- the focuser is skewed to the mounting plate.

The milling that had been done should not of changed anything as it was done in a metal lathe of the prototyping lab for university physics dept.
I havn't had a chance to sit down and have a look yet but im looking for ideas that might help me problem solve this.

I look forward to your thoughts:thanx:

[gmbfilter]

Scary, when stuff like this happens.
Have you re checked collimation of the focuser.
Howie G is a good source of info http://www.collimator.com/coltext.htm

Did you do anything to the primary?
Rotating the mirror can check for astigmatism, the astigmatism rotates.

Do you have artificial star?

Good luck

[DavidU]

Trilobular astigmatism is almost always the mirror clips or the collimation bolts are so tight that it is distorting the primary cell.
rotate the OTA and have a look

[bmitchell82]

I know they arnt tight at all, i use the method of tissues, put the tissue under the mirror clip if i cant pull it out without ripping the tissue and leaving some there, then the clip is too tight. i had this problem the first time i cleaned my mirror... i had a distortion on one side of my picture

whats more worrying is the focus pattern coming in and coming out its a oblong star that rotates 90 deg on the other side of focus.

[multiweb]

That's astigmatism. You have two good focus points on each side of the sweet spot (that will give you the best of both worlds: a cross). An everything is blurry in the center of your field. In my experience that's a misalignment between the tube axis and the axis going from the center of your primary to the point where your focuser axis intersect the secondary diagonal. Get the tilt and centering of your secondary spot on and that will go.

Now if you have three pinches in your star, it's your primary cell that's holding too tight. That's easily fixed by releasing the clamps. They must only touch the mirror not squeeze it.

[DavidU]

star test

[bmitchell82]

see the problem isn't steming from not being collminated. its unlikely that my normal method that has been tried and tested many many times and a Astrosystems collminator that one of the guys that was out there uses on his big dob couldn't get it right. there has to be something else at play here.

Thanks DavidAU for those star patterns, unfortunately its not like any of them really. as when you go from one side of focus to the other the star is oblong not round and it changes in direction on either side.

multiweb, just so im getting this correct the astrosystems laser collminator put the spot right on the primary center. would this not correct for any miss alignment, or is the focusing tube itself skew and not normal to the axis of the tube? If i was to put a laser in there and rack it out and in would this show any error in the tube?

[DavidU]

Have you done anything at all to the primary or cell?
Years ago I had a similar prob with my Meade, it was as simple as leaving off the 3 cell mount screw washers (the ones that go through the side of the OTA to mount the cell) the lack of washers let the screws go another .30" into the cell and pinch the mirror although the clips were nice and loose.

[bmitchell82]

havn't touched the primary apart from loosening the locking screws off to see if it changed the star pattern (which it did not). the only thing that has moved/changed was the modifications to the focuser and the removal/service of the focusing unit.

[multiweb]

Don't bet on that yet. ;) I always assumed I could collimate my scope and it wasn't a collimation error. More than once it turned out to be. :)

The laser alone can give you "many good alignments" for different positions of the secondary. So you think you're collimated then you stick a cheshire in the focuser and you go :eyepop:. The laser alone will get you into a whole world of troubles. You need to use a combination of tools to collimate, the cheshire or cross tube being a bit part of it.

What I try to do is go on a bright star or light source and look for reflections in the glass of the eyepiece or barlow. Checking the alignment of the multiple reflections when the star is dead set in the center is a good indication of which way to shift tings too.

[bmitchell82]

the reson why im quite sure of my collmination is 3 different people had a crack at it, using

Laser and Barlow laser method
Cheshire eye piece method
and pure laser method

all three ways where so close it wasn't funny. This is the first time i have ever seen this and mind you i have taken alot of shots with nice round stars, my latest shot of 47 tuc had round stars and that was less than a month ago, and when i was seeing if i could reach focus everything was perfect, its only just changed so .... it hsa to be something on the mechanical side of things is all i can see that has changed. very confusing!

so what am i going to try.

see if the focus tube is parallel when rising from the main axis first, i could understand how the egg stars would arise if the dslr chip plane was tilted by a miniscule amount.

put my laser in to see if it is moving around on the primary with the focuser being racked in and out.

failing this i will see if the focuser is square to the secondary, then check if the mirror is crimped at one of the 8 mirror clamps.

would there be anything else to try after this do you think?

[Screwdriverone]

Hi Brendan,

After reading all this, I suspect its the alignment of the focuser. The reason I think this is that I had the SAME issue with my 130mm newt with the same in/out of focus shift in the shape of the astigmatism and I ended up fixing it by putting a VERY thin nylon washer between the focuser body and the tube on ONE screw.

To find the problem, I loosened all the screws on the focuser assembly, and while looking at the funny shape, tilted the focuser until it looked right. Rinse/repeat until I had narrowed down which way the focuser was out, then marked with pencil which one needed packing.

Once this was done, everything was spot on, by the way, it ALWAYS looked collimated before this, but the star images were CRAP!

I was tearing my hair out until I tried this...

See how you go.

Good luck

Chris

[multiweb]

You might want to start from scratch and not worry about what's been done as it's going to confuse you even more. Here's what I would suggest. First thing, might sound stupid but check that your laser collimator is not out of wack and is collimated itself (been there... :doh:)

1_ Don't worry about the primary yet. Take the cell off and sticky tape a white piece of paper at the end of the tube in place. Measure & make a cross so you know where the center of the tube is.

2_ Remove the secondary and the spider vanes. Mark accurately the intersection of the focuser axis on the opposite side of the tube. To do this you'll have to remove your focuser (keep the base in) then wrap a piece of paper inside or outside the tube all around. Mark the center of the focuser on one side, fold the paper ring in half, make a dot on the second fold and mark the tube opposite the focuser (you get the idea).

3_ place the focuser back on with the laser dot and make sure it lights up the dot you did on the other side. Square and shim if necessary. It is critical that you square the focuser with a square along the length of the tube. If it doesn't hit the dot move it sideways but DO NOT tilt it. (remember those oval stars at 90degree in & out of focus?)

4_ Set the 3 collimation screws on the secondary so they're all the same length (i.e. secondary square with the spider vanes).

5_ Put the secondary back in with the spider vane. Measure accurately the centering with a digital caliper so you have something to start from.

6_ Use a cross tube to center the secondary under the focuser. DO NOT TILT IT at this stage. Adjust the roll so you see a circle.

7_ Stick the laser in the focuser then check that the reflected beam ends-up in your paper template at the end of the tube. (it should be pretty close)

8_ Put the primary back in. Have a look from the front and look at the first laser reflection in the secondary. It should hit it pretty much in the middle (sideways - don't worry about up/down).

9_ You can then start the collimation routine by tilting the primary and the secondary.

Always use both laser and cheshire to confirm.
Sounds a bit tedious but that's the only way to have a "clean" start.
If your secondary is off to start with it makes it very hard to close in onto the sweet spot. HTH.

[bmitchell82]

Thankyou very much for the details in how to start a fresh.

Chris, i will do what you said, as i have removed the focuser, and it could be tilted. Failing that i will move on to the fresh start method! ill try the easiest thing first then move onto the more complex things.

[DavidU]

Brendan, I had a look at your focuser photos. Where the body of the focuser goes onto the square mount plate it looks like collimation hex screws next to the phillips head screws.
No need to shim to collimate.

[bmitchell82]

there is indeed. the only other thing that im going to do before all of this is undo the whole focuser, and spin the base plate around. why? well theres a 50 50 chance of getting it right, as i havn't moved anything apart from pulling the focuser off twisting it around might help.

I will use my digital inclometer and put the scope on the eq6 to rotate the scope into a horizontal position, then put the inclometer ontop of the focuser this will tell me instantly if the tube is parallel with the OTA axis. then like you said david, the shimming hex heads to do the rest.

Something so simple to kill your DSO night

[DavidU]

Those focus tubes are never concentric to the body !:D

[bmitchell82]

ahh well ill iron this baby out pronto as it looks like i might be going out this weekend.! the most promising cloud free weekend yet :)

[bmitchell82]

Well...

yesterday during a break from doing study i thought why not have a crack at the focuser and see what i can see. well what did i see... thankyou screwdriver one. apon racking the focuser with the cheshire ep in and out things moved. not only that the secondary was tilted so i could see on the outside of the drawtube on one side... (that will teach me to do my collmination in the dark :) )

so i loosend everything off and by hand got it roughly in the right spot, then used the collmination screws from the focuser to align the tube so the image wasn't moving when i racked it in and out, then aligned the primary back to the focuser..

Result:-

Near consentric circles in the eye piece, defocused image shifts VERY MINIMUMLY, so a tiny bit more work needs to be done on it, but all in all im happy with the results.

So thank you once again all that helped.:thanx:

[Screwdriverone]

Hi Brendan,

Glad I could help, as I said, the problem sounded identical to the images I saw in my scope, and its great the advice seems to have fixed the problem.

See? the "One" "Screwdriver" is all you needed :)

Sorry for the bad pun. :rolleyes:

Now, If I could just teach you how to spell Collimation, my work here would be done. :lol:

Cheers

Chris

[RobF]

Fascinating thread.
Glad to hear you're back in business too Brendan :thumbsup:

[DavidU]

:thumbsup:

[bmitchell82]

hehehe ill continue to spell it wrong just to be the thorn in your backside chris :)

I wish i could have only used 1 screwdriver :) i had my multi head screwdriver for Flathead, Phillips head, then my arsnal of head heads. But at the end of the day i learned something new about "collminaton" :D and hopefully if anybody else comes up with this problem we can sort it Straight up.

Oh and just for the record, it was Pure Cheshire Eye piece that aligned the scope, i ditched the Laser and trusted my eye. (if they could do it back in the day with this, then i trust it :D )