Odd shaped stars through Newtonian

[DJScotty (Scott)]

Hi all...

I need some help.
I recently bought a 10inch newt off another IIS member.
There seems to be something odd about the optical train.
I am having great difficulty focusing.
I have colimated using a laser colimator.
The star shape is very odd when very out of focus.
Also when nearing focus, the diffraction spikes don't match up. (ie up and down spikes are close together but the sideways spikes are apart. This tells me that something is not right in the optical train, but I am at a loss as to what
I hope someone can help please.

Cheers
DJ Scotty

[linstar88 (Linda and Guy)]

1st guess, pinched optics.
2nd guess wrong laser collimator used. If it's only 1mW the beam is possibly not strong enough to go the distance through a 10" tube. 5mW laser collimator will do the trick (in my experience) or a laser specifically made for newtonians. It's better to double check collimation with a Cheshire after using a laser to confirm accuracy anyway.
Perhaps it's a good idea to contact the previous owner & ask what he/she used, maybe(?).
Hope this info was helpful or useful.
Note, I find it interesting no one with more experience than myself did not post a help message .
Good luck
Guy

[peter_4059 (Peter)]

Looks like pinched primary to me. Try loosening the screws on the clips that hold the primary mirror onto the mirror cell. You want the clips to be just touching the mirror - not tight.

[CJ (Chris)]

Hi.
I went through something similar recently. There may be some info in my thread that might help.
Good luck.

http://www.iceinspace.com.au/forum/s...d.php?t=130903

[Allan]

Scott, what laser collimator do you have? Cheap collimators often ad their own errors into the system and are best avoided. You can collimate with your eyes closed using a good quality laser collimator like the Glatter collimators, but they aren't cheap unfortunately.

A pinched primary is often the reason for out of round stars. Check that the mirror clips are not touching the surface of the mirror. They should be a fraction of a millimetre above the surface, enough to slide a business card between the gap.

Your secondary mirror vains are not straight, that is giving you the diffraction spikes that don't line up. To see what I mean get a piece of cotton and hold it tight. Now place the ends over the ends of the spider vains where they attach to the tube. You will see where they attach to the hub in the centre that they aren't aligned with the cotton. Sounds like the horizontal vains might be ok, but the vertical ones need adjusting. If so you will have to straighten up the vertical vains by moving the hub in the centre. Do this by loosening and tightening the opposite horizontal vains where they attach into the tube.

[DJScotty (Scott)]

Al, Chris, Peter and Guy,

Thank you for your responses. I am currently reading the mentioned threads and Astrobaby stuff. Taking all of your suggestions into consideration.

I will say this. The focuser that came with the scope was a piece of crap plastic rack and pinion. Very quickly went in the bin. However, It did reach good focus with nice diffraction spikes. The main issue with it was I couldn't attach the camera to it with a filter attached.

So I bought the Bintel generic Crayford 10:1 focuser. The Bintel focuser tube is larger than the original crap focuser, so I have ground the hole bigger in the tube with the trusty Dremel. Not a lot of care was taken and I realised tonight that the focusing tube was not centered over the secondary. Working slowly on that as well. Almost there.

The spider vanes are 3mm thick and cannot be adjusted. There are no knobs or adjustments apart from the screws that keep the whole thing attached to the OTA.

I think once the focuser tube is centered and straight and I have taken all your suggestions into consideration, especially the pinched optics, it should be sorted. But like selling a house, it's not over til the money is in the bank. I will keep you all informed. Currently waiting for the clouds to disappear... YAWWWWWWWNNNNN...

[DJScotty (Scott)]

Al, Chris, Peter and Guy,
The problem is solved but not yet fixed.
After reading the Astrobaby article, I have figured out that the focuser is too high on the OTA. It needs to be dropped down by about 3-4mm.
Whilst viewing through the peephole in the focusing tube, I loosened the secondary mirror until it was moving very freely but had the main mirror centre spot in line with the vanes. I then gently tilted the secondary until it appeared circular instead of elliptical. The distance the centre spot was out from the middle of the peep hole was how much I had to raise the secondary mirror. Unfortunately I can't raise the secondary any more and still have about 2-3 mm to go... So the trusty Dremel will be working overtime tomorrow (today) to make the hole bigger!

Thanks once again

[DJScotty (Scott)]

As far as I can tell, I have fixed it. Just have to wait and see tonight. Hopefully those big white fluffy things disappear...

Thanks one and all
DJ Scotty