Reflector Star Test Question

[Maged]

Hi,

I was doing a star test after collimating my reflector (10" F5.7). I noticed that when the star is near focus it's like there are three small circles in a triangular shape formation moving inwards until they intersect and merge to form the image of the star I'm looking at in focus.

Just to clarify its about the star image nearly out of focus is three images merges together in one image when focused.

but when totally out of focus the image is a normal circular unfocused image of a star with the normal airy disk.

 hope my words were accurate enough to describe what I see.

Can anybody help with that? is this only a collimation issue? or a defective optics like pinched mirror for example?

 

thanks

 

[DHEB]

15 minutes ago, Maged said:

Hi,

I was doing a star test after collimating my reflector (10" F5.7). I noticed that when the star is near focus it's like there are three small circles in a triangular shape formation moving inwards until they intersect and merge to form the image of the star I'm looking at in focus.

I hope my words were accurate enough to describe what I see.

Can anybody help with that? is this only a collimation issue? or a defective optics like pinched mirror for example?

 

thanks

 

I do not have experience with pinched optics, but from your description it does not seem to me like a collimation issue. On the other hand, as shown in this page http://www.loptics.com/articles/starshape/starshape.html, pinched optics seems to give raise to triangular patterns (I guess when there are three clamps holding the primary). If this is the case, and if I understand correctly, the problem can be corrected by just loosing a bit the clamps that might be excessively tight. In any case, it will be very interesting to see what others have to say on this. Good luck!

[Maged]

Thanks Cinco,

 

Just to clarify its about the star image nearly out of focus is three images merges together in one image when focused.

but when totally out of focus the image is a normal circular unfocused image of a star with the normal airy disk.

 

thanks

 

[DHEB]

4 minutes ago, Maged said:

Thanks Cinco,

 

Just to clarify its about the star image nearly out of focus is three images merges together in one image when focused.

but when totally out of focus the image is a normal circular unfocused image of a star with the normal airy disk.

 

thanks

 

I see, I am not sure then. May be it was just the manner in which the star image was deformed due to the particular seeing conditions when you did the test? May be others have more useful hints. Good luck!

[PHIL53]

Anyhow, I would advise to check the primary clamps, they might be tightened too much !

They are there just to prevent the primary mirror to fall down when aiming below the horizon (why would you have to do that, I do not know !), and they actually should not touch the primary mirror surface.

As a rule of thumb, you should be able to slide a credit card between each clamp and the mirror...

[DHEB]

3 hours ago, PHIL53 said:

As a rule of thumb, you should be able to slide a credit card between each clamp and the mirror...

Interesting recommendation!

[John]

What is the design of the secondary supports in your scope ?

The style and shape of secondary supports used can make a difference to appearance of the out of focus star image when star testing.

 

 

[Maged]

4 hours ago, John said:

What is the design of the secondary supports in your scope ?

The style and shape of secondary supports used can make a difference to appearance of the out of focus star image when star testing.

 

 

 

Actually the design of my secondary mirror is the weirdest thing about the scope. It is a rectangular mirror instead of an elliptical design and its glued to the secondary mirror holder attached to the spider. 

[Dave In Vermont]

1 hour ago, Maged said:

 

Actually the design of my secondary mirror is the weirdest thing about the scope. It is a rectangular mirror instead of an elliptical design and its glued to the secondary mirror holder attached to the spider. 

Great advice in this thread!

A rectangular secondary?! Now this is very interesting. If you have a camera, I'd like to see this. I can't recall of seeing this before. May I ask what brand of 10" F/5.7 do you have?

You've gotten me quite intrigued,

Dave

[Moonshane]

Is the star shape like this? Could be astigmatism in your secondary mirror if not glued on correctly.  given it is square it seems like a diy repair. Did you buy the scope new or used?

 

[Linda]

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

On the last page, there is a picture of a star showing 3 points. It says that the primary mirror is stressed, like described by others above. Check out of that is what you see.

[brantuk]

Have a read of this link - it's helped several members from my local club and may help you too:

http://www.loptics.com/articles/starshape/starshape.html

[Maged]

8 hours ago, Dave In Vermont said:

Great advice in this thread!

A rectangular secondary?! Now this is very interesting. If you have a camera, I'd like to see this. I can't recall of seeing this before. May I ask what brand of 10" F/5.7 do you have?

You've gotten me quite intrigued,

Dave

Hi Dave,

I have attached two pictures of the secondary mirror looking through the focuser and one through the tube. 

 

 

[Maged]

4 hours ago, Moonshane said:

Is the star shape like this? Could be astigmatism in your secondary mirror if not glued on correctly.  given it is square it seems like a diy repair. Did you buy the scope new or used?

 

Just to confirm are all shapes in the picture you attached are related to defects to the secondary mirror and not the primary right? I'm asking because I maybe having the left shape below in your picture.

 

This scope is a 100% hand made..

[Moonshane]

Yes, they relate to astigmatism in the secondary.

Things to check 

• there should be a gap between primary and clips
• the primary should be free floating or fixed with three large blobs only
• the secondary should be fixed with three blobs only

If not then I'd re-seat them. If you need advice just ask.

[Moonshane]

ps the odd shape of the secondary would not really matter as such but it's optimal as an ellipse.

[Dave In Vermont]

From your images (for which I thank you!), I can't figure out why someone decided to try a rectangle for a secondary. Bizarre. If it were me, I'd invest in another 'spider' and an elliptical secondary with standard fittings allowing normal collimation.

I'm still curious about what brand this telescope is. It sounds home-made.

I'll be following this soon-to-be saga,

Dave

[Peter Drew]

I strongly suspect the rectangular secondary. There's nothing wrong about the shape other than it's not the most efficient by area, some very large secondaries have been made octagonal due to the difficulty of figuring a large ellipse. It looks to me as though the rectangular mirror is a replacement "fix" it has rough unworked edges. Glueing directly to the holder with possibly non flexing glue could easily provide the described distortion. It might even have a second surface reflection. Do check the primary clips as suggested though.

[laudropb]

I have to agree your problem is most likely to be down to the weird secondary. I would certainly replace it.