Collimating fast newtonian???

Hi Nerro,

I'll be at Sandymount next Friday (provided the weather is decent) and will bring a Cheshire along. I find the Cheshire easy to use, though there can be problems with very fast scopes and for which shorter Cheshires are needed in order to see the whole of the secondary. You're welcome to borrow mine, if you don't get hold of one beforehand.

As regards the star test, I've not had much luck with a 200mm f/5 Newtonian. The seeing here in the city is generally so poor that the Airy disc flickers like crazy and I can never make it out well enough to check the collimation. (On the other hand, maybe I just don't know what I'm doing?). Have a look at the MetaGuide web site for a discussion of this problem ( www.astrogeeks.com/Bliss/MetaGuide/ ).

Mike

martinus wrote: There is another option though which is the barlowed laser. You can take a cheap laser and place it in a barlow, you then put a paper disc over the end of the barlow with a hole punched dead-centre. It's simple to make and is very well tested. Here's a how-to:
www.cameraconcepts.com/barlowed%20laser%20collimation.pdf

Now that is deadly!!!probably the most useful article i have read so far about collimating the fella.Now one of my friends have a laser collimator which he is not using at all!i don't think its going to be the case after i will show this article...anyways hope its going to be all good knowing my DIY skills but definitely worth trying.Thanks a million for that!

mykc wrote: Hi Nerro,

I'll be at Sandymount next Friday (provided the weather is decent) and will bring a Cheshire along. I find the Cheshire easy to use, though there can be problems with very fast scopes and for which shorter Cheshires are needed in order to see the whole of the secondary. You're welcome to borrow mine, if you don't get hold of one beforehand.

As regards the star test, I've not had much luck with a 200mm f/5 Newtonian. The seeing here in the city is generally so poor that the Airy disc flickers like crazy and I can never make it out well enough to check the collimation. (On the other hand, maybe I just don't know what I'm doing?). Have a look at the MetaGuide web site for a discussion of this problem ( www.astrogeeks.com/Bliss/MetaGuide/ ).

Mike

I am planing to be there myself and if you could bring it along i would really appreciate it.
Regarding star tests i was actually collaminating my old 6" meade SCT only by star testing from my back garden.And to be honest was always getting good results (oh man how i miss those days when you needed to align only the secondary and you only needed to do that once a mount or so!). Never experienced problems which i have with this fella.
Anyways thanks guys for your help!really appreciate it.

martinus wrote:
It won't self-centre but that's going to happen with a cheshire as well if the focus-tube is poorly constructed.

Food for thought.

Now as far as i know there always going to be some slack in the focuser?As far as i know the best way to over come that is collimate with the tube in horizontal position??

There are two separate issues that can occur with a focuser depending on quality and design. The first is backlash which you may notice on a rack and pinion focuser - as you focus and the gears mesh you may see the draw-tube tilting slightly within the focuser housing/base. The second is when a locking screw is tightened up where the eyepiece and the eyepiece position deviates from the centre of the drawtube or tilts within the drawtube, this is slop.

Even if you collimate with the OTA horizontal you will still see both in a poor focuser. A crayford can get rid of much of the backlash by virtue of the design but poorly made focusers of any design may have too large an opening for the eyepiece.

Self-centring collimators have a series of bands that expand under compression and ensure the collimator housing stays in the centre of the drawtube.