Little dimond stars = pinched optics Help please

[stanlite]

hi all i believe i have pinched optics as all my stars in my astrophoto work look like little dimonds i have loosened the plates holding the primary mirror of my newt in place but hasn't helped any idea's helpful hints will be much appreciated.

[DavidU]

Hi Grady, can you post an image.

[pgc hunter]

Make sure your primary mirror clips are just a hair off the mirror surface.

[stanlite]

i find that the mirror shifts when i do this (can't see it shift but hear it i can). have tightened till there as light as i can make them without shifting shound.

[multiweb]

The clips must only touch the surface of the mirror. If the mirror slides sideways in the cell pack the sides with cork shims but do not tighten the clips.

You say you have 4 pinched points (diamond), usually there are only 3 clips at 120 degrees. How many clips do you have?

[astro744]

Diamond shaped stars could indicate possible astigmatism in the system. You could have a defective primary and/or secondary mirror. De-focuse inside and outside of focus. Do you get oval diffraction rings that turn 90deg when going through ideal focus?

[stanlite]

ok got loads of data from tonight (tuc47, NGC6744, NGC55) will look at the Raws and see if loosening the clips has helped hoping its not astigmatism will post a star (field) if it is still there as per some requests. Damn, and i have to make the tube shorter too so i can focus with my Coma corrector in sigh will this never end. At least PHD and guiding isn't giving me grief set up and walked away for 3 hours well NGC55 was imaged and not issues by the look.

[stanlite]

ok so here is one from tonight and one from the photo someone mentioned had triangle/dimond stars. Note the first image is slightly more croped/magnified. Also there is no Coma correction in either image.

http://i944.photobucket.com/albums/ad281/stanlite88/NGC6744close.jpg old setup

http://i944.photobucket.com/albums/ad281/stanlite88/NGC55close.jpg after todays play with the mirror

[astro744]

Your 'diamond' stars could be diffraction spikes from the spider although your second 'cropped' image clearly shows square stars. This would indicate possible astigmatism. Do a star test inside and outside of focus to confirm.

[stanlite]

had some inside focus shots from last night think it looks squareish ... "Grady buys shot gun tells jimmy that he is taking the telescope for a walk 5mins later two shots ring out of the semi rural area."

http://i944.photobucket.com/albums/ad281/stanlite88/IMG_4222.jpg

[astro744]

Difficult to tell with mask on. Remove the mask and observe visually with an eyepiece of mod to high power. See http://www.willbell.com/TM/tm5.htm and look at the first set of images. Note these are for an unobstructed system. An obstructed system will have some more of the energy spread to the outer rings. You need to observe inside and outside focus to see if you have oval shaped diffractions rings and if the axis shifts 90 deg.

[bmitchell82]

I might be silly, but what telescope are you using?

The only thing that i see there is guide error, tilt or possibly flexure.

If you are using a Newt, your stars will have slight pins on the small stars because the stars aren't bright enough to throw fully formed diffraction spikes. The big stars should show off the spikes and look quite nice. Looking at these spikes if they are RED GREEN BLUE rainbow spikes and are inline with each other EG + then you don't have a problem.

Is your scope collimated? this is one of the main reasons for things going skew-if, Astrophotography requires a much much higher level of collimation than visual, If you don't have a set of high end collimation tools. get some if you are serious as they will change everything!

it doesn't matter if PHD is sitting on the money your shots can still wonder around. ive seen it too many times! easiest way is to keep your eye ball on it until you are truly confident with your equipment!

If you are using a VC200L, square/triangle stars are the norm unless you modify the vanes.

Your mask shot means nothing. If you are using a newt and your star isn't 100% in the dead set middle of the sensor then it will never be round no matter how much you pay the hethan gods! as the tube will cut a little bit of the light off! Plus it just means nothing ! :D

Astigmatism i am very familiar with If you have it, you will get a shadow of your secondary coming in round until your just about to approach the focal plane when it will go to a oblong once you go though focus it will flip 90 Degrees and come out the way it went in. Very easy to pick.

If you have 4 mirror clips and your getting astigmatism from that. dyam you have perfectly tightened them all up together so the astigmatism is concentric... this is virtually impossible.

I hate mirror clips. they cause so much grief it isn't funny. Do yourself a favor and chuck them as far away from your telescope as you possibly can and use Silicone or high quality velcro to adhere your primary to the Cell it is the only way to achieve a truly pinch less system.

But before we can tailor our advice can you give us some insight to your equipment.! :D

Brendan

[gbeal]

I'm with Brendan, a bit more info on gear would help.
The diamond star issue is something that comes and goes with the VC200L for example. There is has been suggested collimation and/or thicker than normal spider vanes.
Gary

[stanlite]

ok my gear now lets see (don't laugh now :P ) i have a 8" newt produced by konusky it is about 8 years old (although little used till about six months ago. the primary is probably approaching the end of its coatings life so there is that to figure in aswell. I collimate atm with a laser collimater although i think i am hearing i might need better then that sigh more money. I will have to test the defraction spikes theroy i tried for the astigmatism and couldn't see any major problems although i might not have had the mag up high enough. As for clips on the primary i have rectified that earlier today although only temp till i shorten the tube (again) so i can focus with the coma corrector in. Not sure what else i should mention.

[bmitchell82]

Your laser collimator, is it collimated this is a fundamental error every newbie makes... its a laser it must be straight. wrong :( if the laser isn't centered within the holder game over.

If you are deadly serious about astrophotography cut the !$#@!rap and go buy yourself Cats eye collimation kit - Autocollimator + Collimated Cheshire. Job done instant greatness no stuffing around!

I would say that 99% of your issues are collimation and tracking. not pinched optics or focuser alignment to the tube or any other myriad of things. Get the collimation perfect then you can eliminate that as a cause for anything!

The next step is your Focuser you could have focuser slop causing A. Tilt where the image is focusing at differently at different parts of the sensor. B Flexure... of the two of these flexure is the most frustrating and hardest to eliminate it takes patience time and understanding to figure it out!

Keep up your questions and be sure to give every single bit of detail other wise its like walking into Repco and asking for a part to fix your car without knowing what part/car or year model.

Brendan:thumbsup:

[stanlite]

would this be the right sort of collimating tool ...

http://www.myastroshop.com.au/products/details.asp?id=MAS-050A

been looking at some random stuff today and building up the belief that it might be collimation ... made a gradient map using my flats from yesterday (think that is what it was) ... well either way the center of the image(brightess point) was off to the right and down the bottom rather then concentric circles radiating out from the center.

[bmitchell82]

like i said if you want to do it properly go and get yourself something better than that

http://www.catseyecollimation.com/xlcombos.html

Second row first coulomb

you may baulk at the price but yeah if your mirrors are not aligned correctly you might as well pack it away and enjoy the evening with SWMBO. :D

Taking images will capture fine fine details if your mirrors are bluring these details straight up you'll loose out big time.

If you cant afford this type of tool for now, just collimate your laser and use that for the time being. Look up on the forum about collimating your laser there is a lot of knowledge already in print so to speak

[stanlite]

how long would it take to ship from the US does anyone know?

[bmitchell82]

depends on your postage option you choose. normal about 2-3 weeks. express within the week. Put it this way i generally can get things from over seas faster than from eastern states to here.... go figure! :)

[stanlite]

ok given that $200 plus is a bit more then i can afford atm i have done the next best thing and brought a cheshire collimator and decided to use it in conjuction with my laser and my barlow lens (as a stop gap till said catseye becomes affordable :P ) i have shorten my tube, taken out the clips secured the primary centered the secondary and collimation appears good (nothing is perfect). Last night i worked though my PHD guiding and was averageing an oscilliation idex value of about .30 -.35 (without PEC training) and a RMS of about .40 -.45 (not sure what RMS is for if someone could help me out) graph stays withing the one pixel range for as long as i ran it (about 4 hours). now just need another clear night to test the new tube length and collimation together. will keep you all posted.