Crosshairs on cheshire?

[AlexB67]

Well I was going to buy a Cheshire this Week , but then I spied  the new (for me anyway ) "Starguider" lens bag from Sky's!

I needed something  better to keep my optics and tools in( presently using two plastic biscuit tubs ?)  and about £95 cheaper than a TeleVue version, it seems great value, and should be here next Week.

ps. Just ordered  the Cheshire 'Long Version' from  Sky's (had to be done)  I'll completely loosen the spider and align, because it appears to be 1.5mm offset? Then once all collimated, I`ll let you know, if at all, it was any easier using the tool over the 35mm cannister? Another extension to the bad weather no doubt!

That offset may be deliberate. I would not assume it is wrong because it was like shipped that, though in a scope of that ratio whether you collimate with or without offset will hardly make a difference to the views.  See this article below

For your scope it should just come under roughly 2mm I think for the offset if you want to collimate it with an offset.

See here

http://www.lcas-astronomy.org/articles/display.php?filename=newtonian_secondary_offset&category=telescopes

and here.

http://www.skyandtelescope.com/howto/diy/3306996.html

I hope the long sight tube will allow you to see all of the secondary. Not sure you would have needed a long one for an 8 inch Dob since you may be getting very close to the secondary ?  You can work it out more or less from the focuser length and knowing the diameter of the tube if it might even touch. My standard length one will not work in a 5 inch scope without some extensions in the focuser. I am thinking you got this one ?

http://www.ebay.co.uk/itm/Skys-the-Limit-Cheshire-Collimator-for-Newtonian-Telescopes-long-/380801248654

In any case there is always some improvisation with extension tubes that can be done if need be, make sure you get a good snug fit that gives consistent results with whatever improvisation you use. 

Good luck

[Charic]

AlexB67 Cheers for that......... Sorry, I was being a little facetious maybe? I wouldn't  normally advise to play about with such small adjustments, because the mirrors will correct the issue!

Everything I've seen for the 200P says the spider assembly should be on centre axis with the tube, the secondary mirror off-set is built in.  When my telescope first arrived there was a gap between the OTA and the end cap perimeter ring? The bit that holds the dust-cap in place. I made a few adjustments, and put it all back together ok. But just this Week, during a re-measure, I noted the dead centre of the bolt on the secondary was about 1.5mm (ish) off possibly less, which I will correct when the new toy arrives.  

This URL mentions offset at 2:10 Ralph doesn't explain too well except to say the edges of the mirror "should be different" ?

www.youtube.com/watch?v=N1asF3m2Mh0&list=PLE85BBCD2DE59F779

Mine does look similar, so not overly worried.

Your assumption is correct. I spoke with Alan about the issue. I have the option, if it doesn't work for me, I can have the smaller one? The longer one allows for a more critical, tighter focus of the secondary. If at some stage I need/want to see more, I could always collar the tube, so that it doesn't  go into the focuser so far. At least that way I have options. 

Note. The short one is 6cm, the long one is 16cm ,  Catseye Teletube suggest 24.5cm   One of them should work?  I chose the one in the middle.

[AlexB67]

You may well be right about the offset for the spider having to be central, thing is there are different ways to achieve it and how the mirror is mounted or is it down to the spider. SW never make it clear AFAIK, there again I've never looked into it in much detail since I doubt it matters for visual use, a mm here or there up or down, as long as the secondary appears central and the everything is collimated with respect to it I reckon is 99% of the battle for optimal views. 

I don't have the tools like calipers to even make measurement that accurate on my scope for it to be worth it to worry about the offset.  AFAIK anything where the offset is  from 0 to maximum it should be for full offset collimation, the results will hardly matter for visual use, unless you get down to really fast scopes like f/4 perhaps and also imaging stuff where these things need to be more seriously considered.

[Jason D]

, the results will hardly matter for visual use, unless you get down to really fast scopes like f/4 perhaps and also imaging stuff where these things need to be more seriously considered.

There are two secondary mirror offsets: Towards-the-primary mirror offset and away-from-focuser offset. The first will have an impact on imaging. The latter has no impact on imaging or views or collimation. It is mainly to:

1- Add "little" improvement to DSC accuracy

2- Avoids front-aperture vigentting when the OTA opening is too tight compared to the primary mirror size.

3- Is definitely needed for reflectors with front corrector lens.

Jason

[AlexB67]

There are two secondary mirror offsets: Towards-the-primary mirror offset and away-from-focuser offset. The first will have an impact on imaging. The latter has no impact on imaging or views or collimation. It is mainly to:

1- Add "little" improvement to DSC accuracy

2- Avoids front-aperture vigentting when the OTA opening is too tight compared to the primary mirror size.

3- Is definitely needed for reflectors with front corrector lens.

Jason

Thanks for a more clear explanation. Would you agree though that for a scope in the range discussed it is hardly worth worrying about in terms of impact ?, that was my understanding anyway for visual use. 

[Jason D]

Hi Alex,

Regardless of F-ratio, away-from-focuser offset has no impact on visual views or imaging -- assuming no corrective lens and no aperture vignetting.

Well, for imaging it might have little impact since tracking is important and away-from-focuser improves DSC readings.

Jason

[jetstream]

I love my Catseye adj. sight tube(Teletube XL) and also the handy little Blackcat XL cheshire.Simple and easy to use.My collimation is never off much and I like this!

[Charic]

jetstream......The Catseye is the only one I've seen that's adjustable. It was second on my list against making my own?  Also could not decide over the 2" version or just stick with a 1.25" tube. The rest is history.

[Charic]

post #25 I mentioned the Starguider bag? That report is in Equipment, Discussions - Eyepieces, But this post is about my new Sky's the Limit Cheshire.

My "Sky's The Limit" Cheshire - Long 16cm has arrived and here are my thoughts.

Upon initial observation through the sight tube, there appeared to be some mis-alignment of the secondary mirror towards the primary. I decided to loosen the secondary completley and start over.

Because the tube is long, it gives a much tighter view in which to centrally align the secondary mirror. The cross hairs dont work too well for me as I can't focus on them very well. I need to replace the Mk1 eyeball, or use the less dominant left eye, be he`s no better either?

Ok, the secondary is now centralised, I  remove the light blocking sheet in the OTA and I'm now adjusting the tilt to align with the primary mirror. I can just make out the three mirror clips, but its tight!

Now to align the primary mirror. But first I modify the Cheshire by adding another cross-hair behind the peep-hole, to allow for a more accurate eye alignment. This again is no good for me, as its almost impossible to focus on it.

So back in with the Cheshire, but I cant reach the primary mirror screws, whilst looking through the Cheshire, so out comes the Laser.

On first check, the Laser is inside the centre spot, but carefull seating of the Laser and a final tweek on a secondary grub screw, and the Laser is dead centre! 

Out with the Laser, double check again with the Cheshire, all looks ok. In with the Barlow and Laser, and a slight adjustment at the primary, and all looking good.

Now back in with the Cheshire, final checks,  and there appears to be  some secondary mirror misalignment of the secondary ( towards the primary?) Rotate the Cheshire, look again, now eveything looks ok, so not going to adjust any further from the present setup. I`ll Star test when possible.

All said and done, the Cheshire allows me to get the secondary mirror concentric in the sight tube, more accurately than just my 35mm film cannister or drilled dust cap. The cross hairs dont work for me (  only two wires are needed for a cross hair, Im seeing double due to my old eyes). same reason I sold on my Telrad! Yet through the eyepieces and telescope, all appears well corrected.

Final thoughts.

1. I think the Cheshire is more accurate over the DIY 35mm or Coli-cap, due to the tighter crop.

2. There is no fear of the 'Long' version Cheshire hitting the secondary mirror, on the Skywatcher, Skyliner 8"

3. I further checked with the Laser and everything is as accurate as I can get it at present (during the day) so if you believe in your Collimated laser to work properly, then I think the laser is the quickest and most visual method to use, for me, because from the last time I laser collimated,  to today's check, there really wasn't much adjustment needed in the first place.

4.  A  Cheshire sight tube with a Barlowed laser combination, should get me the best set-up possible, without being heavy handed with the adjusters. their not micron perfect? you often see the effect when tightening the lock screws on a primary, only needs the slightest touch and its off, so  take caution, handle with care,  and with the options to cross check using the tools available, its the best I can offer myself.

The only thing failing me now, are my eyes, but I'm sure there's a few Years left, or even start wearing spectacles full time, instead of part time, but today my prescriptions made no difference to the cross hair focussing at such a short distance.

[Charic]

........and the Star test appears to be spot on!