Following on from Collimating a Fast Newt 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
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.
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 . Whew ! .
Brian.
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.
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 to this and tonights is straight off the bench, no tweeking. The twisting in the secondary has been throwing it off axis
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!
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.
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.
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 I am guilty of this
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.
I'll wait for the new spider I think. I believe that has been the problem all along.
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.
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
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!!!
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
Robin, maybe try something like this - http://www.iceinspace.com.au/forum/s...ad.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
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.
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.
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.
Whew ! . 
