TAK Collimating scope and RC issues......

[E_ri_k]

Hey all I started a thread here a while ago, probably close to 12 months ago. I was having conflicting results from my TAK collimating scope and star testing my GSO RC10. I took a few measurements and put it down to the centre spot on the secondary mirror not being placed correctly (now found to be incorrect!)

I proceded to collimate off the star, and imaged away, things seemed ok, been getting good images, but I wasn't 100% happy. 

Today I have confirmed that the centre spot has been placed correctly on the secondary. I am a bit lost as what to test next?? When the star test shows concentric circles, the TAK scope shows the centre spot incorrectly, and vice versa.

I'm not sure how to explain this and have it make sense to you guys! I'm hopeless when it comes to putting things into words lol! Let me know if I need to clarify something.

1.) I have been playing around with the moonlite focuser. I found, that when looking through the TAK scope, and rotating the focuser around, 360 Deg, the FOV I can see in the primary, moves around as if an axis somewhere is not in the centre? Like watching a wheel turn, but the axle is slightly off centre. Whilst this is happening, the black centre of the TAK scope moves a little in respect to the secondary spot, but only a fraction of a mm, then returns to it's original, "incorrect" position.

2.) Applying pressure to the focuser body, in any direction, also causes the FOV of the primary to shift slightly. I'm pretty sure the focuser and mirror cell will move as one piece, so what is happening is understandable, but should there be any play here? I don't think this is related to my original problem, but just an interesting observation.

Moonlite says that their focusers are perfectly collimated before shipping, so I am a bit reluctant to touch any screws there. I'm not certain, but it feels like the focuser is not 100% centred with the secondary, and I have collimated the secondary to suit the focuser's position, without knowing. Is that possible to do?

I can't think why the TAK scope pattern won't agree on the star test. My thinking is that the star test is really the only accurate way of testing collimation, but Something is not right with what is happening?

If anyone has any thoughts on this it would be greatly appreciated !

[Oddsocks]

When you bought your RC had it been updated by the factory with the focuser decoupler ring?

The reason I ask is because I had tilt issues with my GSO 8" RC, the centre hole bored through the primary mirror was not perfectly aligned with the optical and mechanical axis and therefore the optical axis could never be aligned to the focuser axis.

When imaging with a DSLR I used to get really bad coma like stars in just one corner.

At that time there was no aftermarket solution and after a year of struggle I gave up and sold the scope but since then the manufacturer (I believe) has changed the design by the inclusion of a tilt-able de-coupler ring that sits between the telescope back plate and the focuser.

There are also tilt-able camera "T" mount adaptors now that weren't available when I had the problem so there is more than one way to fix it, but the de-couple ring is best because it solves the collimation tool issue where the tilt-able "T" mount adaptor only fixes the imaging camera problem.

From your description it sounds the same problem, carrying out collimation adjustment by star testing will give adequate collimation but the TAK scope in the focuser will never be aligned perfectly if the focuser axis is not the same as the optical axis.

The de-coupler ring is obtainable for retrofit, you just unscrew the Moonlite focuser from the current tube back, screw on the tilt-able de-coupler and then screw the Moonlite focuser to the front of the de-coupler, the rear and front de-coupler threads are the same.

Here is a link to a UK supplier that stock the ring de-coupler I expect you should be able to find the same in Australia but if not this supplier will ship to you.

http://www.iankingimaging.com/show_product.php?id=1383

William.

[r3i]

Don't the collimation adjustment screws in the MoonLite do the same job as the TS ring? i.e. adjustment of the focuser axis independently of the primary collimation adjustments.  The TS ring is required for the stock focuser because it doesn't have the collimation adjustments that are built into the MoonLite.

[kookoo_gr]

Also i believe that the moonlite focuser covres by a small amount the screws from this ring thus not allowing the user to adjust the focuser. At my scope if i remove the extension rings i have, my moonlite focuser covers the adjustment screws of the primary mirror. I have noticed this at my 8" RC scope. The best way is to collimate the scope using a star but now i am thining of buying a Howie Glatter laser collimator because star collimation takes a lot of time

[E_ri_k]

Thank you for your replies

William, I am not sure what the decoupler ring is supposed to look like, so I can't be certain, but there was something about a redesigned, or upgraded focuser mentioned in the paperwork with it. The only "Ring" it has on it is like an aluminium collar used to screw the focuser to the body of the scope. It doesn't have any type of adjustable screws or anything.

The moonlite however does have four tip/tilt screws, but the recommendation was only to touch them if there was a defect in the OTA's optical alignment. I'm not sure how to test for that. I will attach the stock focuser and see how it behaves with the TAK scope as well maybe?

I agree that the optional decoupler ring may not be necessary, as the moonlite is able to be adjusted. Like I said I am reluctant to turn anything as it is apparently perfectly set in the factory. However the scope may be out of wack? 

I do have a Glatter collimator. It also disagrees with the star test, and again, with the laser in the moonlit, when I rotate the focuser around in a circle, the laser spot follows a non-concentric circle in respect to the secondary spot. Then again, it is only held in by the compression ring.

I can't help think that the focuser must need adjusting, not the secondary, because according to my star tests, the secondary doesn't need touching.

I will see what happens with the stock focuser and keep in touch.

Erik

[r3i]

William, I am not sure what the decoupler ring is supposed to look like, so I can't be certain, but there was something about a redesigned, or upgraded focuser mentioned in the paperwork with it. The only "Ring" it has on it is like an aluminium collar used to screw the focuser to the body of the scope. It doesn't have any type of adjustable screws or anything.

Erik,

This was the one that I bought to use with the stock focuser on my 8" RC:  http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4272_TS-Okularauszug-Justierung---RCs-von-GSO---Astrotech-6--und-8-.html

The corresponding one for the 10" RC from the same supplier is this one: http://www.teleskop-express.de/shop/product_info.php/info/p4043_TS-focuser-collimation---Tilting---with-M117-Thread---for-RC-and-Refractors.html

Will be interested to see how you get on with the MoonLite should you you choose to adjust the collimation settings as getting a MoonLite is high up on my wish list.