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Focus not consistent across the viewing field

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#1 oilmanmojo

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Posted 07 May 2021 - 03:43 PM

I recently got my  primary 16" mirror back from refiguring and recoating services. Steve Dobbs did the refiguring and coordinating the recoating. Overall the mirror looks great. This was the final piece of my 16" dob refurbishment that i have been undertaking. After installing the mirror and making final cuts on the truss tubes, i proceeded to perform a collimation on the scope. I am using the Catseye collimation system and feel i have a pretty decent collimation, though I am not 100% certain as i am learning how to use these collimation eyepieces. I have not conducted a star test yet as the viewing conditions have been marginal in my area. Hopefully tonite, i can look at Polaris and conduct the star test. But I was able to get a view for a short time last nite before some clouds rolled in. What i noticed was that i could get pretty sharp (round) star focus in the middle but on the edges the focus was not as sharp. It was not coma (it tail on the pattern), but just slightly out of focus. I could slightly adjust the focuser to get the outside edges sharp, but then the center was slightly out of focus. I am assuming that my collimation is still off some but was wondering if others have seen something like this before? I was using a 30MM and 35 MM wide angle lens (ie Stratus) as well as another wide angle lens (friend made for me many years ago) as I was looking at M101 and M87 galaxies near the big dipper. My UTA and Focuser boards are new so i am concerned i may not have the focuser board quite right yet. Any thoughts would be helpful. I will try to take some pics during my next Collimation exercise with the Catseye tool to see if there is something i am missing. Thanks as always for feedback

 



#2 dan_h

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Posted 07 May 2021 - 04:30 PM

It was not coma (it tail on the pattern), but just slightly out of focus. I could slightly adjust the focuser to get the outside edges sharp, but then the center was slightly out of focus. I am assuming that my collimation is still off some but was wondering if others have seen something like this before?

Your collimation may still be out a bit but I don't think that is your problem. This sounds exactly like what I would expect from field curvature. The curved field means that the edges are not in the same plane as the center.  Younger eyes can accommodate to see everything in focus but older eyes may not adjust so reliably.   Low power wide field views see a larger portion of the image plane and therefore show defocus due to field curvature much more than high power views. 

 

If the problem was due to tilted focuser or tilted focuser board I would expect that you could not focus both sides of a view at the same time. 

 

hope this helps,

dan


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#3 Jon Isaacs

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Posted 07 May 2021 - 06:25 PM

I am assuming that my collimation is still off some but was wondering if others have seen something like this before? I was using a 30MM and 35 MM wide angle lens (ie Stratus) as well as another wide angle lens (friend made for me many years ago) as I was looking at M101 and M87 galaxies near the big dipper.

 

My two cents:

 

You didn't say what the focal ratio of your scope is but I'd say your problem is the eyepieces.

 

They're not sharp across the field at F/5. The combination of coma which is major and the eyepiece astigmatism makes a messy image. 

 

This is why people pay $650 for a 31mm Nagler and another $500 for a Paracorr.

 

The quality of the mirror is measured by the on-axis views at high magnification. Off-axis is the eyepiece and coma correction.

 

Jon


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#4 CHASLX200

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Posted 07 May 2021 - 06:31 PM

I bet it is coma.  When my 18" F/4.5 is out of collimation the planets will not focus into any kid of sharp image. Stars will have a flare to one side.  With coma the center is sharp and outward stars look like a comet with the tails going outward.


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#5 oilmanmojo

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Posted 07 May 2021 - 08:08 PM

IMG_1605.JPG Thanks for the feedback. A little more info. The Mirror as i related earlier is a 16" diameter 3" thick mirror that was recently refigured by Steve Dobbs. He had related that it had a bad turned edge and a bit of a hole in the center that he resolved as he completed the refiguring. The calculated focal length is 4.97 (ie F5). I do have an older Paracorr that i have not inserted into the optical path yet as i was just doing a quick check on my truss cuts (may actually need to trim another 1/2 inch for the paracorr. As far as coma, I do intend on using the Paracorr, but i did not notice the flaring on the edge stars, just seemed to be a little out of focus. Unfortunately, we got a cloudy night over next 3 nights so i will have to be patient. I will try to take a picture through the lens if the paracorr does not correct the issue. Hopefully I can wake up early enough to get a peek at Jupiter once the clouds leave us! Here is the "semi" finished scope. I still have to paint my truss and add another coat of stain on the boxes.

 


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#6 oilmanmojo

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Posted 07 May 2021 - 08:11 PM

Your collimation may still be out a bit but I don't think that is your problem. This sounds exactly like what I would expect from field curvature. The curved field means that the edges are not in the same plane as the center.  Younger eyes can accommodate to see everything in focus but older eyes may not adjust so reliably.   Low power wide field views see a larger portion of the image plane and therefore show defocus due to field curvature much more than high power views. 

 

If the problem was due to tilted focuser or tilted focuser board I would expect that you could not focus both sides of a view at the same time. 

 

hope this helps,

dan

Dan--i do have those "older" eyes!!! 



#7 oilmanmojo

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Posted 07 May 2021 - 08:17 PM

My two cents:

 

You didn't say what the focal ratio of your scope is but I'd say your problem is the eyepieces.

 

They're not sharp across the field at F/5. The combination of coma which is major and the eyepiece astigmatism makes a messy image. 

 

This is why people pay $650 for a 31mm Nagler and another $500 for a Paracorr.

 

The quality of the mirror is measured by the on-axis views at high magnification. Off-axis is the eyepiece and coma correction.

 

Jon

Jon--thanks for the note. I added a little more info on the scope. I know that the Orion Stratus are not a premium eyepiece but i thought they were decent. I do plan on getting a quality Nagler or two for deep space viewing as that is my passion. I will find out if the Paracorr (i have an older model). 



#8 oilmanmojo

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Posted 07 May 2021 - 08:19 PM

I bet it is coma.  When my 18" F/4.5 is out of collimation the planets will not focus into any kid of sharp image. Stars will have a flare to one side.  With coma the center is sharp and outward stars look like a comet with the tails going outward.

I hope to get up early to check out Jupiter once the skies clear! I did not notice a flare, but if it is coma, the Paracorr should solve that



#9 Jon Isaacs

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Posted 07 May 2021 - 09:08 PM

Jon--thanks for the note. I added a little more info on the scope. I know that the Orion Stratus are not a premium eyepiece but i thought they were decent. I do plan on getting a quality Nagler or two for deep space viewing as that is my passion. I will find out if the Paracorr (i have an older model). 

 

The Stratus eyepieces are decent but a F/5, decent only gets you so far.

 

My real theory on what you're seeing:

 

Concern and worry that the mirror is flawed. It's normal..

 

But if the mirror has issues, you wont see them at 67x off-axis..

 

Your Stratus eyepiece will be "OK" but not perfect perfect.

 

Jon



#10 Inkswitch

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Posted 08 May 2021 - 12:08 PM

Jon--thanks for the note. I added a little more info on the scope. I know that the Orion Stratus are not a premium eyepiece but i thought they were decent. I do plan on getting a quality Nagler or two for deep space viewing as that is my passion. I will find out if the Paracorr (i have an older model). 

I have three of the Stratus eyepieces, the 17, 13, and 8.  They are excellent performers in my 150mm F/8 but are underwhelming in my 300mm F/5.  I don't want to slander the line because I used them in my F/5 for years before I knew better and they do show themselves well in slower focal ratios.


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#11 CHASLX200

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Posted 08 May 2021 - 06:31 PM

attachicon.gifIMG_1605.JPGThanks for the feedback. A little more info. The Mirror as i related earlier is a 16" diameter 3" thick mirror that was recently refigured by Steve Dobbs. He had related that it had a bad turned edge and a bit of a hole in the center that he resolved as he completed the refiguring. The calculated focal length is 4.97 (ie F5). I do have an older Paracorr that i have not inserted into the optical path yet as i was just doing a quick check on my truss cuts (may actually need to trim another 1/2 inch for the paracorr. As far as coma, I do intend on using the Paracorr, but i did not notice the flaring on the edge stars, just seemed to be a little out of focus. Unfortunately, we got a cloudy night over next 3 nights so i will have to be patient. I will try to take a picture through the lens if the paracorr does not correct the issue. Hopefully I can wake up early enough to get a peek at Jupiter once the clouds leave us! Here is the "semi" finished scope. I still have to paint my truss and add another coat of stain on the boxes.

If you got the older Paracorr you can't adjust it should work fine. If not i would trade my Paracorr with the top you can tune.


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#12 dave brock

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Posted 08 May 2021 - 07:18 PM

attachicon.gifIMG_1605.JPG   Here is the "semi" finished scope. 

An off topic note.

Be careful when wheeling your scope around that the ota doesn't slide out of the rocker.

If you have the handles attached in the other direction then the rocker back board will act as a stop.


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#13 Jon Isaacs

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Posted 08 May 2021 - 09:05 PM

An off topic note.

Be careful when wheeling your scope around that the ota doesn't slide out of the rocker.

If you have the handles attached in the other direction then the rocker back board will act as a stop.

 

:waytogo:

 

If the handles are raised too high, the scope slides forward just like dumping a wheel barrow. 

 

The way it is the photo, the entire scope is dumped out.. with the handles behind, the scope slides forward, hits the backboard and then tips forward, the upper cage hits the ground. Still not good but not as bad.

 

I remove the handles when observing.. tripping on the handles in the dark is not a good thing.. another lesson I learned the hard way.

 

Jon


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#14 oilmanmojo

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Posted 08 May 2021 - 11:10 PM

An off topic note.
Be careful when wheeling your scope around that the ota doesn't slide out of the rocker.
If you have the handles attached in the other direction then the rocker back board will act as a stop.

Excellent point. I still need to do some tweaking here but do i do orient the scope so that the back of the rocker box acts as a stop. My terrain is pretty rough so i will not use the the wheel handles to move the scope around much. I will most likely have to build an observatory to house the scope for viewing at home. when I transport to remote sites, i will break down the truss and UTA and reassemble. The scope is easy to break down and reassemble using the Aurora connections on top and the moonlite on the bottom.

#15 oilmanmojo

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Posted 08 May 2021 - 11:18 PM

waytogo.gif
 
If the handles are raised too high, the scope slides forward just like dumping a wheel barrow. 
 
The way it is the photo, the entire scope is dumped out.. with the handles behind, the scope slides forward, hits the backboard and then tips forward, the upper cage hits the ground. Still not good but not as bad.
 
I remove the handles when observing.. tripping on the handles in the dark is not a good thing.. another lesson I learned the hard way.
 
Jon
Agree about removing the handles during viewing. This scope is a little more top heavy than its earlier version so I am thinking about how to modify the wheels and handle to make it easier to move safely. As i said in my earlier reply to Dave, i will build a small observatory to house the scope. Nice thing to have enough land to do that. I do appreciate the feedback as this site has a lot of knowledgable folks on it!



#16 oilmanmojo

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Posted 08 May 2021 - 11:21 PM

If you got the older Paracorr you can't adjust it should work fine. If not i would trade my Paracorr with the top you can tune.

Chas--Thanks, it is the older paracorr and is not adjustable. I got it at a cheap price and my hope was with a f5 scope, it would work fine. Just got to get a clear nite to play! if not, i will ante up to get the new model

#17 oilmanmojo

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Posted 08 May 2021 - 11:27 PM

I have three of the Stratus eyepieces, the 17, 13, and 8.  They are excellent performers in my 150mm F/8 but are underwhelming in my 300mm F/5.  I don't want to slander the line because I used them in my F/5 for years before I knew better and they do show themselves well in slower focal ratios.

Thanks for the feedback. I am just getting back into this hobby after quite a few years so i got some catching up to do. I got a set of Stratus from a friend who was downsizing his collection. But i do plan on getting a couple of quality eyepieces as i get the scope tuned up. I have been looking on the eyepiece blog trying to get some ideas on what to get. Like most hobbies, you get what you pay for!

#18 oilmanmojo

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Posted 12 May 2021 - 11:18 PM

I want to thank all who helped me in with this issue. I finally got a clear sky night
and was able to take a peek for a little while. I reworked my collimation and though not
perfect, I feel it was pretty good. I got to learn how to take some of the pics to post
concerning collimation to see if i am missing anything, but i got real close with the Catseye
system. The images on the autocollimator were slightly skewed, but they were very close.
In addition, I put the older paracorr in the loop and that seemed to resolve the edge focus
issue. Not sure if it was Paracorr or just a little better collimation as it did look better
even when i pulled the paracorr lens but there is no doubt the paracorr seemed to improve the
sharpness of the edge stars. I attempted a star check, but the mirror had not cooled down enough
so instead of concentric rings, i got the swirling/moving images when i slightly defocused.
it was pretty cool tonite so that big mirror takes a little to cool down when days are warm and nights
cold. I did get the chance to peek a a few clusters high in the sky and they looked fantastic! Got
to start planning my observation pad and possible enclosure down my hill!

#19 CHASLX200

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Posted 13 May 2021 - 06:20 AM

An off topic note.

Be careful when wheeling your scope around that the ota doesn't slide out of the rocker.

If you have the handles attached in the other direction then the rocker back board will act as a stop.

My Obsession wants to take a dump out of the rocker box when i push it out my door as it hits a bump.  It is a pain getting it in and out as the door is only 32" wide so i pull my scope out and in.  

I am a lazy viewer and roll out the whole scope set up and even leave the handles on. Since i only view the planets i have no problem with the handles left on, but sometimes the wheels rub in the grass. So i am gonna make a pad that will bring the scope up a little so that should stop the rubbing.  I would like to rig up a new system for the handles once i dream up a way to fab something up.


Edited by CHASLX200, 13 May 2021 - 06:21 AM.

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