6 Diffraction Spikes?

[Tandum]

Following on from Collimating a Fast Newt (http://www.iceinspace.com.au/forum/showthread.php?t=81845) and with all the help I got with this 10" Astrograph from you guys, I just could not get it right. I had misshaped stars no matter what I did. Even with excellent results from CCDI the stars where still wrong. So much for software alignment. By pure accident I found that lifting the tube off the bench caused alignment to be lost. I tracked this down to the weight of the secondary, twisting the spider and pushing the prostar tube out of shape at the business end of the scope :rolleyes:

I got a spare 10" ring and wrapped it around the scope near the spider to force the tube to retain it's shape and now I have round stars.

However I still have artifacts on stars as shown in this image. It looks like an extra diffraction spike. Any tips on what is causing this one? I'm sure it's the stuffed spider but would like another opinion.

This is a single 1 sec sub of achernar.

[brian nordstrom]

:) Hi Robin , I wondered this more than 20 years ago after I finished my first telescope , a 10 inch f/10 newtonion witha 3/4 inch secondary and 3 vains , I seen the 6 spikes as well , and back then there was no internet , so I read lots of books.
The reason for 6 splkes from 3 vains is as you know the light path is crossed over at the focal plain ( thats why views are up side down)
but the 3 vains dont quite come to the same focus so they are op side down and doubled . Its a law of physics eg The wave of light .Like throwing 2 rocks into a pond and watching the waves .
I now have a curved 2 vain spider in my 10 inch f/10 and with the 3/4 inch secondary the 2 vain dont become 4 spikes .
They cancel each out , the wave theory of light . and it works, like a 10 inch refractor on the planets .:thumbsup:
:D Whew ! .
Brian.

[Tandum]

It's a normal 4 vane spider Brian.

[bmitchell82]

just like this robin?

http://i700.photobucket.com/albums/ww3/brendanmitchell1982/Project%20Messier/Orion-Final-HQ-AFC.jpg

Do you have a star field with a exposure of say 5-10 minutes that have small stars, I have always found that bigger stars like that don't show how round your actual star is. There is a great double star in the running man that gives you a very very good indication on roundness it also shows how good your resolving power is. the better your scope is collimated and the better its guided the better these stars split.

Alternitively CCDIS will give you a aspect ratio that is how round your star are, ill normally sit around the 10 mark with guided 10 min photos.

[Tandum]

Yep It does, but mine is not a stacked image, it's a single 1sec sub.

I only discovered the tube deformation this arvo and the tube ring I have only gets tight around the rolled edge on the end of the tube, it needs packing to go around the tube where the secondary is. But this looks like the problem with the scope, images tonight are a lot better than before although as altitude goes up, roundness goes down.

Peter is back and is organising another spider. I had to pack one leg of this spider to centralise it as the opposing leg had been stretched by 2mm. That's bound to make the whole assembly slack.

It's gone from this (http://starshed.celticcomputers.com.au/temp/24-10.jpg) to this (http://starshed.celticcomputers.com.au/temp/02-11.jpg) and tonights is straight off the bench, no tweeking. The twisting in the secondary has been throwing it off axis :(

[bmitchell82]

Well that my friend was the primary mirror being warped by the mirror cell. Ive seen it in another scope and it was exactly the same issue, I think just because yours is a conical it doesn't discount it from being warped because it takes absolutely bugger all to do it!!!

But its good to see that you found another one of the annoyances! :D

[Tandum]

How can a primary mirror with a single bolt and nothing else be warped? There is nothing to warp it?

Here's a very very rough M42 (http://starshed.celticcomputers.com.au/temp/02-11-M42-60.jpg) from tonight. 60sec subs.

[bmitchell82]

like crap there isn't!

the bolt isn't cast into the mirror its glued in, Glue contracts!
where you bolt it unless that hole is perfect then youll pull in the middle it takes soo little pressure you would go wtf.... Im not saying that it is 100% definate but yeah have you actually relieved the mirror aka, taken it off that primary yet and re tensioned the central bolt?

Mine was due to my collimation screws slightly warping the mirror cell and touching the rubber stops that i had spaced with a tissue so they wernt touching but after they where It took me 6 months to realise this.......

It cant be the spider vanes being out of alignment as that throws a totally different star diffraction spike.

It could be the secondary mirror not being flat or being pinched as well if you have a spare secondary mirror i would suggest you give it a try! When hunting these things down remove 1 part at a time and replace it with something that is known to work as it should. you will quickly find the culprit part.

[Tandum]

I don't have a spare prostar 3" secondary. I don't own a newt anymore. The primary is from Sachmo on this forum and it's not over tightened but I'm urging Peter to replace this new cell with the original.

[bmitchell82]

not doubting the quality of the mirror, but have you actually removed the primary from the tube and just let everything loose again then redo it? Im just asking the stupid questions because 9 times out of 10 we all get smart and forget the basics :D I am guilty of this

[Tandum]

yes

[bmitchell82]

how is the protostar held to the secondary hub? Also thinking about that primary that you said you didn't like, that could be flexing and putting strain on the glass too if you have tightend it up decently like I would to hold the colimation properly.

[Tandum]

I'll wait for the new spider I think. I believe that has been the problem all along.

[pmrid]

It's coming from here:
http://www.fpi-protostar.com/s4vmnts.htm
I ordered for 290mm ID, 3" secondary ninor axis with the heater, 3mm offset and finger collimating screws. That should do it.

Peter

[ZeroID]

You have 8 diffraction spikes on the Achernar shot, not 6. Misaligned spider vanes. You can misalign 4 just as easily as 3.

[Moon]

Anything in the light path can cause extra diffraction spikes. Common examples:
- Focuser tube extends down inside the OTA
- Clip on the secondary overhangs the mirror
The angle of the spike is a good clue where to look.
James

[Moon]

Brendan,
This is an interesting comment - if I understand correctly you are saying astigmatism in the primary produces extra diffraction spikes for infocus images? If so, this should be easy to reproduce in a test scenario...
James

[Shiraz]

Robin, maybe try something like this - http://www.iceinspace.com.au/forum/showthread.php?t=82230
makes it really easy to see in one glance anything that is intruding into the light column and causing stray diffraction spikes - can also check spider alignment, twist etc.
Regards Ray

[brian nordstrom]

:) As I said Robin , 3 vain or 4 it makes no differance ,the wave theory of light , it will double at the focal point but 2 will always cancel each other out , 4 times 2 = 8 minus the two that cancelled each out = 6 spikes. :thumbsup:

[Satchmo]

Guys- as maker of the primary I can assure you the mirror was astigmatism free and the original owner reported ( as I would expect ) very nice high power images with round extra focal stars and good planetary detail.

I assume you have rotated the primary to ensure that the astigmatism doesn't rotate . The centre bolt need only be finger tight- any tighter could potentially warp the mirror.

[Moon]

I sure we were only talking about the possibility of the optics being pinched somehow, well at least I was :-)

[bmitchell82]

Mark as your name is attached to the mirror I have not a hesitation in the world to say that that mirror was perfect when it left your skilled hands!

Like you just said if its more than finger tight it will introduce stress to the mirror itself and this is what i was trying to explain earlier with my budgo Skywatcher dobsonian, the aluminium cell was warping ever so slightly which inturn pushed the mirror up into the rubber stops (and if you have seen the sky watcher system there is no metal its all rubber and soft) this was enough to produce the astigmatisim

James im unsure what your talking about. a infocus image is a infocus image wheather it be on the bench top or at a star they are all "testing". As soon as you release the mirror it will change or like I was saying previously as soon as i pulled the primary mirror cell out..... the cell would return to its equilibrium state as it is an elastic material and i was non the wiser for 6 months. I was at the brink of chucking the scope at a un desireable while driving fast i was that frustrated...

[Tandum]

I checked and the bolt is only finger tight and the mirror has definitely been rotated. It's some other issue which Peter will have to solve, I'm putting my scopes back in the dome over the week-end.