Refractor collimation check with a Cheshire EP

[Drakester]

So, after owning my Meade 127mm ED APO for approx ten years (purchased second-hand), I’ve decided to have a think about getting it serviced (I.e. cleaned and potentially collimated)

This is my main imaging scope  

I purchased a Cheshire eyepiece as a quick way of checking collimation, and here is the result (taken afocal with an iPhone, so not overly scientifically done..)

What do you guys think? This looks ok-ish to me?

The donuts are slightly out of sync, but look ok compared to some of the examples on Google that I’ve seen.

Any thoughts appreciated. Cheers . 
 

 

 

 

 

 

 

[celestron8g8]

If that was a SCT I’d definitely would recheck it with a star test . I’m not familiar with a Cheshire EP so personally I can’t say mainly cause i was always under the impression that refractors were factory aligned and not to be messed with but time has passed and I don’t know that a fact now . But when i zoom in on your image to the star image i see a couple glare rings that seem offset . I’ll be checking back in for followups here to satisfy my curiosity !  

[John]

Have you checked that your focuser optical axis is accurately aligned with the optical axis of the objects lens ?

Thats the first check to make. I use an accurately collimated laser collimator to do this check. Put it in the focuser without a diagonal in place and see where the laser beam exits the objective lens. If its right in the centre, all is OK. If it is off to one side or other, the focuser alignment needs adjusting.

When you are confident that the focuser and objective optical axis are aligned, then you can use the cheshire to check objective tilt.

 

[Drakester]

Thanks guys. 
 

John - I’ll have a go at checking the focuser alignment. I’ve got a laser I use for collimating my Dob. What is the best way of seeing whether the laser is exiting the objective in the centre? Just line of sight?

 

Thanks. 

[johninderby]

Print out a bullseye on thin paper and place it over the objective. Bit of experimenting needed to get it printed at the right size though. Then the laser will show a dot on the bullseye and you can then see what adjustment is needed if any to centre the dot.

Edited January 7, 2020 by johninderby

[ollypenrice]

2 minutes ago, Drakester said:

Thanks guys. 
 

John - I’ll have a go at checking the focuser alignment. I’ve got a laser I use for collimating my Dob. What is the best way of seeing whether the laser is exiting the objective in the centre? Just line of sight?

 

Thanks. 

Make a vee block to hold the laser. Don't panic, this is ludicrously easy! 😁 You need a flat bit of wood and four six inch nails. Bang in two nails crossing each other to make an X. Bang in a second pair to make a second X separated by the length of the smooth body of the laser. Lay the laser in the cradle formed by the two pairs of nails and point it at a wall as far away as conveniently possible. Say ten feet or more. Fire the laser at the wall and rotate it in your high tech vee blocks. The beam from a mis-collimated laser will describe a circle on the wall when rotated. A perfectly collimated laser will produce a dot on the wall when rotated.

Olly

[johninderby]

A lathe is even better to hold the laser. Less error than a V block. Of course I have heard that not eveyone owns a lathe though. 🙀

[andrew s]

A Cheshire does not need to be square on to the optic axis to work as it has a "rough" reflective surface. I collimated a dublet refractor using a Cheshire eyepiece and aimed for concentric reflections. Yours look out to me.

Regards Andrew 

[WanderingEye]

13 minutes ago, ollypenrice said:

Make a vee block to hold the laser. Don't panic, this is ludicrously easy! 😁 You need a flat bit of wood and four six inch nails. Bang in two nails crossing each other to make an X. Bang in a second pair to make a second X separated by the length of the smooth body of the laser. Lay the laser in the cradle formed by the two pairs of nails and point it at a wall as far away as conveniently possible. Say ten feet or more. Fire the laser at the wall and rotate it in your high tech vee blocks. The beam from a mis-collimated laser will describe a circle on the wall when rotated. A perfectly collimated laser will produce a dot on the wall when rotated.

Olly

Why do this, surely you put the laser in the eyepiece, and the circle of paper fixed on the front of the objective with the centre marked, and then laser should hit that centre spot....? If it does not then focuser tuba adjustment is needed....Or am I missing something here....?

edit, you are talking about collimation of the laser itself, not the scope....sorry .. 😀👍

Edited January 7, 2020 by WanderingEye

[Merlin66]

The laser and a centre spotted target on the objective will confirm the alignment of the focuser to the OTA.

A Cheshire Eyepiece allows the objective elements to be confirmed as collimated with no tilt evident.

Suiter in his "Star Testing Astronomical Telescopes", p122/123 shows the Cheshire reflection patterns for a 152mm f12 scope before and after collimation and states "Neither alignment gave noticeably different images at the eyepiece"

Based on this the laser/ focuser alignment seems to be more critical.

 

[John]

Not wanting to question Suiter of course but I have seen a noticable improvement in the star test before and after getting the tilt of the objective lens correct. Most of my refractors have been slower than F12 though. Usually F/10 or faster.