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Unusual star image

Started by NGC88888, 08/26/2002 11:48AM
Posted 08/26/2002 11:48AM Opening Post
When I put a 1st mag star in the center of the FOV with a 25mm plossl and a 10" f/5 Dob, I see a couple of lines shooting off the star to the "south". If I try to focus to get rid of them, I get new lines shooting off to the "east". I have looked at some examples on the web of what a star looks like with astigmatism, turned down edge, etc., but do not see pictures that look similar to what I see. Any ideas? I attached a picture of what it looks like.

Attached Image:

NGC88888's attachment for post 1402
Posted 08/26/2002 01:58PM #1
My first guess is your scope needs to be recollimated. When was the last time you collimated your scope?
Posted 08/26/2002 03:37PM #2
Ryan
get scope collimated. Then re startest.

If the images reappear I would guess that it is astigmatism caused by the mirror cooling. Perhaps the mirror is thin and not annealed??

It could also be astigmatism in your eye with the large exit pupil.

Kevin B
Posted 08/26/2002 03:53PM #3
Ryan
I think Kevin is correct about astigmatism in your eye.
The exit pupil for the scope/eyepiece combination is 5mm. If you Barlow the eyepiece or go to a very short focal length eyepiece such as a 5 to 7 mm I bet the effect goes away. If not then your collimation is probably the issue.
Defects in your own eye show up at low magnifications and large exit pupils. Defects in your optics show up at high magnifications and small exit pupils.
Posted 09/22/2020 01:07AM #4
Originally Posted by Kevin Barker
Ryan<br>get scope collimated.  Then re startest.<br><br>If the images reappear I would guess that it is astigmatism caused by the mirror cooling.  Perhaps the mirror is thin and not annealed??<br><br>It could also be astigmatism in your eye with the large exit pupil.<br><br>Kevin B

Totally random reply, 18 years later.  I was looking at old posts and saw this one.  You told me that the problem might be my eye.  It was.  I was 29 at the time and had never been to an eye doctor - my first visit was not until 2009, 7 years after this post.

I use the dioptrx 150 now to deal with this issue.

So - thanks!
Posted 09/24/2020 09:24PM #5
LOL....18 years later smile

Dont Let Trump Cheat!! Demand Voter ID Now!
20" MidnighTelescopes f/5
8" f/16 Muffoletto Achro
Meade 6" AR6 f/8.0 Achro
Celestron C11
Parallax PI250 10" F/5 Newt
Vixen FL102S,NA140SS,R200SS
80mm Stellarvue Nighthawk with WO APOupgrade lens, 90mm f/16.67 Parallax Achro
LXD75/LX200, Mini-tower, GP-DX, CG5 ASGT, LXD650, GM-8, G11, GM-100, GM-150EX, GM-200 and a Gemini G40
4" portable AP convertable 44" or 70" pier
Stellarvue M7 Alt-Az, TSL7 Pier/Tripod
12.5" f/5 MidnighTelescopes DOB Swayze optics
Statham GA
Posted 09/25/2020 08:47PM | Edited 09/25/2020 08:49PM #6
Originally Posted by Greg Shaffer
LOL....18 years later smile
Greg maybe this might help someone else.  I have left eye that is Myopic can not see far away W/0 glasses and I never had eye exam as a kid and when I went to get my drivers permit they check eyesight and found I had my left eye did not pass so I had to get glasses and go in again. Our School in Md. did not check kids eyesight at that time.  I just never noticed my left eye problem  I guess because my right eye is 20/15+.
Posted 10/01/2020 05:37PM #7
Originally Posted by Steven Moore

Greg maybe this might help someone else.  I have left eye that is Myopic can not see far away W/0 glasses and I never had eye exam as a kid and when I went to get my drivers permit they check eyesight and found I had my left eye did not pass so I had to get glasses and go in again. Our School in Md. did not check kids eyesight at that time.  I just never noticed my left eye problem  I guess because my right eye is 20/15+.

Ohhh I agree.....I just found it humorous that the topic is 18 years old smile

Dont Let Trump Cheat!! Demand Voter ID Now!
20" MidnighTelescopes f/5
8" f/16 Muffoletto Achro
Meade 6" AR6 f/8.0 Achro
Celestron C11
Parallax PI250 10" F/5 Newt
Vixen FL102S,NA140SS,R200SS
80mm Stellarvue Nighthawk with WO APOupgrade lens, 90mm f/16.67 Parallax Achro
LXD75/LX200, Mini-tower, GP-DX, CG5 ASGT, LXD650, GM-8, G11, GM-100, GM-150EX, GM-200 and a Gemini G40
4" portable AP convertable 44" or 70" pier
Stellarvue M7 Alt-Az, TSL7 Pier/Tripod
12.5" f/5 MidnighTelescopes DOB Swayze optics
Statham GA
Posted 10/01/2020 05:57PM #8
Yeah totally - kind of a bummer these forums are hardly used.  

When I saw the old post and my drawings, I showed my wife and said "hey look at this, someone diagnosed my astigmatism 5 years before an eye doctor did!"

I remember the first day I wore glasses after that visit - it was like the world had become High Def.

Anyway, agreed, hope this helps someone.
Posted 10/30/2020 12:18AM #9
This was a fun read...thanks for coming back and confirming the diagnosis! I bought the Dioptrx soon after Televue offered them and have used them ever since. They are a welcome alternative to wearing eyeglasses when observing.

I was at NEAF when they were introduced. Al was manning the booth when I stopped by to inquire about them. When he asked if I knew the cylinder value for my prescription, I had to confess I did not. He asked to take a look at my eyeglasses. He held them at arms length, rotated them 90°, and handed them back to me and confidently stated I needed a 1.5 Dioptrx. He was right on the money!

-Gary
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