poor eyesight vs poor collimation

[astronymonkey]

Hi All,

After a recent visit to my opticians I was told my astigmatism has got slightly worse and noticed that the updated prescription sharpened up the letters in the sight board quite nicely.

I told the optician I did astronomy and he said my slight astigmatism may give stars a fuzzy feel without glasses , contacts or I suppose a dioptrix or similar.

Now this got me thinking about how much obsessing I do over getting colimation right on my f4.5 dob but I suppose it will be impossible to get a 100% clean star image if my eyes themselves are going to add some flare the the final image.

So in summary, I wonder how much of the poor collimation worries we have are actually down to slight astigmatism or a glasses which arnt quite corrected enough. In this case getting pin point stars will prove impossible.

Cheers

[alan potts]

I think if like me you use a laser to collimate a Dob or a secondary shadow centred in a de-focused star to collimate Rc, Sc etc then these would not be affected by astigmatism though not having this I don't know.

The laser is a question of getting a dot of light in the right place and as said the shadow in the middle. I feel in any case one has to ensure colliamtion is as spot on as it can be. I was recently a bit bored with the view from my SC 12inch, so checked the collimation at high power, it was out a tad, what a difference that made, it was like having a new scope. There was me thinking I knew what I was doing, I gave myself a kick up the behind.

edit, I missed a line, we have to give our eyes every chance we can.

Alan

[astronymonkey]

Hi alan,

Yes ive always fiddled:with collimation in order to get it bang on using a laser first then with a cheshire and finally a defocussed star. The trouble is though, without my glasses on, even with mild astigmatism there is zero percent chance of me getting sharp stars when I try to focus.

As I know I have astigmatism and know it cant be resolved by winding in the focusser I wonder how many people take their glasses of while observing and notice they just cant quite get a sharp star .... but dont realise that the problem is with their eyes and not the optics.

Cheers

[Moonshane]

I get a slight smearing with my contacts (more at lower powers) presumably caused by the tear wave of the lenses. I continue to use them though as the benefits outweigh the negatives. As with any aspect of the optical train it's a compromise.

[astronymonkey]

I'm considering a diotrix to see if that clears the image up. Ive heard some decent things about them but there doesn't seem to be that many users.

I agree with the tear wave having an effect though shane, I see the same thing which seems to drift in and out of focus a little with my contacts. In addittion to this the toric lenses for astigmatism rely in gravity to have them sit at the right angle but as they only come in 20 degree incrememnts its not always possible to get the orientation bang on whcih is something the dioptric makes easier.

Cheers

Ian

[dph1nm]

Observing with an eyepiece with a small exit pupil can sometimes help, as the centre of the eye is often better optical quality than the edges - rather like stopping down a dodgy telescope lens!

NigelM

[Scooot]

I use a dioptrx Ian. It will certainly show the effect of the astigmatism as you rotate it. I've been meaning to get another 1 sometime, because it's a bit annoying having to keep removing it to put it on another eyepiece when switching. The process I go through is focus first, then rotate the dioptrx, and repeat as necessary.

They work very well with the Delos, can have its eyeguard up with these because of their clever twist eye relief adjustment. With my Nagler and Ethos I have to have the eyeguard down to get the full view. The dioptrx does use up a bit of the eye relief so they might not be so good with some eyepieces. It's probably better than glasses because I suspect the quality of the lens and coatings are better, and I think by rotating it whilst viewing I can get it set at a more precise axis.

Ps

I have contacts as well, and I know many get on very well with these. I find mine sometimes spin in my eye and I have to wait for them to settle again to focus which I find annoying. So I prefer to use the dioptrx or my specs. Thats probably just me though

[alan potts]

Hi alan,

Yes ive always fiddled:with collimation in order to get it bang on using a laser first then with a cheshire and finally a defocussed star. The trouble is though, without my glasses on, even with mild astigmatism there is zero percent chance of me getting sharp stars when I try to focus.

As I know I have astigmatism and know it cant be resolved by winding in the focusser I wonder how many people take their glasses of while observing and notice they just cant quite get a sharp star .... but dont realise that the problem is with their eyes and not the optics.

Cheers

Ah now I get your point, sorry I sort of missed the plot. I never understood that you can't focus properly with this eye defect, sorry if that sounds harsh. I know they are costly but are TeleVue with the Dioptex system any good to you?

Alan.

[Saganite]

Hi Ian,

I think that your take on this is very interesting, and valid. I have always gone to great lengths to get collimation spot on, to the extent that I can achieve it fairly quickly. However, whenever I do a star test, it always appears as shown by Astrobaby's collimation guide, when atmospheric turbulence is present. Now it could be that the atmosphere above my village is always poor, but just today, I was sitting in my south facing conservatory, lazily watching the sunlight dance on the surface of my cup of coffee, and allowing my eyes to defocus, I saw the same Airy disc pattern, so is this the reason I never get a good pattern on star test ?

[Scooot]

My astigmatism shows itself by me seeing a slight double of everything. So if I shut one eye and look at something bright, say the moon, I'd see two moons, one on top of the other but overlapping it. I'm not sure how typical this is.

[Peter Drew]

Very, I hope, as I have a similar problem. 

[astronymonkey]

Hi richard,

I agree about seeing things in double without glasses, a sort of elongated star that looks like a tube vibration.

As for the diotrix, do you get pin point stars ?

Alan - My eyepiece collection is very much optimisied for glasses, with long eye relief delos and nagler t4 being the mainstay of my set. The addition of a diotrix might be my next addition followed by a 2inch oiii ive been wanting for some time.

Cheers

[Scooot]

Hi richard,

As for the diotrix, do you get pin point stars ?

Cheers

Haven't used the dioptrx with tak but yes with the dobs when they're cooled properly and there isn't a major problem with seeing. Some are a bit big in the 16" to describe them as pin point but they're certainly round. Both mirrors are 1/10 wave so I presume this helps but I'd say the dioptrx does a good job of correcting it. My astigmatism in my viewing eye is1.25.

[Charic]

I've looked at Tele Vue Dioptrix in the past with the assumption that if I bought a faster 10" or 12" telescope These would work well with the Delos EPs that I may purchase?

Knowing what's wrong with your eyes in the first place helps to understand the  issues.
I  think that sometime back, it was  verbally explained to me that I have compound myopic astigmatism?

I have just come across this link http://www.eyeprescription.com/PrescriptionsProcess.php?msg=insert&act=myanalysis popped in my prescription details, and yes, It says  I have compound myopic astigmatism!

I don't wear any correction at the eyepiece, I just don't like the gap it produces between spectacles and the eye lens, but when I wear my prescriptions, I can actually see more Stars without an eyepiece, so at the telescope,  maybe I should consider some correction.   

[Scooot]

I've looked at Tele Vue Dioptrix in the past with the assumption that if I bought a faster 10" or 12" telescope These would work well with the Delos EPs that I may purchase?

Dioptrx works very well with Delos eyepieces, you can have the eyeguard up and see the full fov easily.