Collimation of a Newt at night with a Cheshire EP?

[kirkster501]

...how????  Needs to be done in light, or at least, thats how I have always done it.  But it is not always possible to set up in light, collimate and then leave until dark so whats the methd used?  I find colliamtion with standard 12" springs needs a tweak every time its set up.

Appreciate your thoughts.

Thanks, Steve

[bambuko]

red torch - takes few seconds to do

[kirkster501]

red torch - takes few seconds to do

You mean shine the torch into the front of the OTA?  You must have long arms to do that on a 12" Dob...

[wookie1965]

Shine the red light on the reflective surface of the Cheshire  keep making slight adjustments going backwards and forwards and checking unless you have someone help you and you can guide them.

[RikM]

That's exactly the way I do it too. I have a single red LED headlamp that is to bright for wandering around with, but works perfectly if you shine it on the reflective bit of the Cheshire.

[Moonshane]

yep, what they said. in some ways this is easier than in daylight as it lights things up a bit like an illuminated finder cross hair.

[moffer]

Is it possible to adjust a mirror in the dark with a simple collimation cap with a reflective inner surface ?   I'm sure I can recall reading an old post by someone saying it was, but not explaining how to do it.  The only thing I can think of that might work is to point the OTA at a car headlight with a white sheet draped over it. Thoughts much appreciated.

[Gib007]

You can shine a bright light at the Cheshire Eyepiece's reflective surface or into the primary mirror from the front. That will give you all the light you need to perform a collimation in the dark. I actually use my phone's extremely bright, white flash as my light. You can tell I don't observe stuff directly, just image!

[moffer]

Apologies if I haven't made myself clear... it's a collimation cap and not a cheshire combination tool that I'm asking about. The one I have cost £5 from FLO and has a reflective inner surface around the peephole unlike most of the home made jobbies. Might be no need, but I'm thinking of draping a white sheet over the car headlight which should give adequate illumination rather than using a naked headlight, which might be too bright.  Anyway, I'll be giving it a go tomorrow night if the predicted forecast of a clear night holds good.  If it doesn't work then I'll have to buy a cheshire combi tool because the mirror on my 200p never fails to move after driving a short distance to a darkish site, which means I have to arrive there while it's still light.

[RikM]

I haven't tried using car headlights to illuminate the primary. It's got to be worth giving it a go though.

[jezhughes]

Apologies if I haven't made myself clear... it's a collimation cap and not a cheshire combination tool that I'm asking about. The one I have cost £5 from FLO and has a reflective inner surface around the peephole unlike most of the home made jobbies. Might be no need, but I'm thinking of draping a white sheet over the car headlight which should give adequate illumination rather than using a naked headlight, which might be too bright.  Anyway, I'll be giving it a go tomorrow night if the predicted forecast of a clear night holds good.  If it doesn't work then I'll have to buy a cheshire combi tool because the mirror on my 200p never fails to move after driving a short distance to a darkish site, which means I have to arrive there while it's still light.

Not sure why you'd need to use one of these in the dark? They're for positioning the secondary mirror correctly, which is one of the first steps of collimation. The only tweaks you should need to make in the dark are fine adjustments to your primary mirror. Unless i've misunderstood you?

[moffer]

Not sure what makes you think you can't adjust the primary with a collimation cap? I use a cap for the whole collimation process - it works just fine for me as star tests prove. No doubt it would be easier with a cheshire combi tool, but guess I'm a tight old beggar and won't spend money if I can avoid it.   

[jezhughes]

Not sure what makes you think you can't adjust the primary with a collimation cap? I use a cap for the whole collimation process - it works just fine for me as star tests prove. No doubt it would be easier with a cheshire combi tool, but guess I'm a tight old beggar and won't spend money if I can avoid it.   

Ah i see what you mean, i only use it for the secondary positioning. Then use a cheshire/laser. I don't know how you can achieve much accuracy with just a cap, the crosshairs on a cheshire seem pretty important 

[Moonshane]

the position of the secondary is not as critical as accurate collimation of the primary. to me it's all important but if it's not to the owner then it won't make a massive difference.

[moffer]

I don't know how you can achieve much accuracy with just a cap, the crosshairs on a cheshire seem pretty important 

It takes a bit of practice (I had plenty during the 70s when cheshires were not available, or if they were I wasn't aware of them) but I'm quite confident that I'm getting accurate alignment, though it would be easier to do with crosshairs. I believe there are a number of members on here who also just use a colli cap.

One thing I definitely could have done with back in the 70s was astro-babys collimation guide. I had to rely on the slightly ho-hum collimation instructions in the Fullerscopes catalogue . I eventually got it sussed, but It sure was a struggle for a beginner trying to understand that little lot on their lonesome.   

[jetstream]

I'm just curious how the secondary can get pointed at the primary dot without the cross hairs,if there is a way I am interested.My Catseye Chesire is really just a good collimation cap,but I always check my secondary allignment with the sight tube....

[scarp15]

I use the collimation cap just to check the alignment of the secondary, but use the cheshire to collimate the secondary. I must say that I prefer to collimate in day light (but I am still a bit of a novice). The secondary though is usually fine and the primary only ever slightly out. If necessary, I will check collimation on an out of focus star, once the scope has cooled and I can use a high powered e.p. 

[jetstream]

I wonder how accurate collimation needs to be before it really affects the view.My scope holds collimation well,but at times needs a tweak at both mirrors,according to sight tube & then cheshire.However being new to collimation my ability to read the star test is limited,if the "hole" is centered in the defocused star,I'm happy.Maybe at this level of collimation a cap is all that is needed?I also wonder what the result would be if the secondary isn't exactly centered under the focuser?My scope provides great views of all objects so far and the other morning Jupiter was fantastic@ 150x given the conditions.At dark sites I do use my truck lights or just a flash light set up on the tailgate,my truck lights are kind of hard on the eyes though.I find I do like to have the mirrors aligned as perfectly as possible and I do check during extended sessions...just my nature

[Damo636]

I use a magic torch that hangs in mid air (look, no hands!!) to illuminate the primary and the reflective surface of my Farpoint cheshire.....

Seriously though, its a little magnetic LED work lamp attached to the spider, simples 

[RikM]

.However being new to collimation my ability to read the star test is limited,if the "hole" is centered in the defocused star,I'm happy.Maybe at this level of collimation a cap is all that is needed?I also wonder what the result would be if the secondary isn't exactly centered under the focuser?

A star test is so much more than 'it looks like a polo mint with the hole in the centre'. It is about getting the fine diffraction rings concentrically spaced. You need a very high magnification and a perfectly centred star otherwise coma can skew things. If you are happy with the views you get using your method, that's all hat really matters though

I had a poorly centred secondary for a while but didn't bother to correct it because the detail I was getting was quite acceptable. When I did align it properly, I found no change in detail but I did get a very slightly brighter image. I can only put this down to the misalignment clipping the light cone. I have no idea what other things may have contributed to this, but it was quite a welcome surprise.

[Moonshane]

the secondary alignment is important to get about right but if it's out is affects the volume of light collected by the syste rather than the sharpness and contrast. small losses of gathered light are not usually visible at the eyepiece of course.

the primary adjustment is the critical one and affects the sharpness and contrast much more so if you want to spend a lot of time collimating, ensure your primary gets the lion's share (although it's the easiest adjustment of course). as the secondary adjustments are effectively fixed and hardly need adjusting ever once correct collimation really is a simple thing.

[moffer]

Tried tweaking the primary in the dark with car headlight & white sheet last night and it worked a treat.

Also tried without the sheet (actually an old white nylon shower curtain I use for moth trapping) but foundthe headlight a bit bright - it can be done but was easier with the sheet draped over the headlight.No more waiting an hour or so for it to get dark and drinking most of my tea before I've started observing.

BTW, had a good night looking for DSO's after the moon went down. Quite a few meteors seen too.

[jetstream]

the secondary alignment is important to get about right but if it's out is affects the volume of light collected by the syste rather than the sharpness and contrast. small losses of gathered light are not usually visible at the eyepiece of course.

the primary adjustment is the critical one and affects the sharpness and contrast much more so if you want to spend a lot of time collimating, ensure your primary gets the lion's share (although it's the easiest adjustment of course). as the secondary adjustments are effectively fixed and hardly need adjusting ever once correct collimation really is a simple thing.

Thanks Shane,I was wondering about how critical the secondary adjustment is,when I carry my telescope to set up at dark sites the collimation sometimes goes out a bit.Not much,but I like things in line.My star tests show the hole centered in the donut,with fine little rings evenly spaced on both sides of focus....but they are not EXACTLY the same.Here is a pic borrowed from Astro Baby that shows how my collimation looks when done.Even with the crosshairs off a bit I really have not noticed much of a difference...

[Moonshane]

that is actually a spot on collimation. the cross hair/spider vane reflections will not all line up generally as there is secondary offset (correct and normal) and this is the reason for the black 'bulge' on the right of the central section.

[bellavia]

Hi,

Regarding Astrobaby's treatise on collimation, it is very good.  In her last photo she says "near perfect collimation" for the fast Newtonian.  And it is.  But she has a rotation of her secondary mirror, which is evident in the photo as you can see the secondary mirror looks elliptical, and not in line with the optical axis. (Though it is perfect in the sketch).

I did too, but fixed it.  Of course while observing you are not likely to see this effect, which might only be to cause some very slight vignetting in the off-axis portion of the field of view, that is the outer edges, since the primary would not be "filling" the secondary uniformly all the way around.

I have images of Astrobaby's photo where I superimposed a circle to show the effect the rotation causes.  I also have actual photographs where I had the same exact situation but corrected it by rotating the secondary mirror until its elliptical axes were lined up correctly with the telescope's optical axis.  But I can't figure out how to post or attach images on this forum  :/