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Newbie error. Is this coma? Collimation? Post? Something else?

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#1 dpaigen

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Posted 17 September 2019 - 10:01 AM

Thumbnail (not) attached, here is a larger version: https://astrob.in/6j1i20/0/

 

I have weird looking star shapes that seem to radiate outwards from the center.  My wife calls it "hyperspace."  I have three basic questions:

 

1. What am I seeing here?

2. Is this fixable and at what cost?

3. What can I still learn with this rig?

 

Shot details: 50 seconds times 43 lights with 37 darks, shot on an Orion Astroview with a Nikon d7200 through an Orion "shorty reflector" (114mm f/8.8 probably with an internal barlow).

 

Thanks,

-David

ps. sorry for the lack of thumbnail, I blame javascript and my browser.  Hopefully you can see the link



#2 Gipht

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Posted 17 September 2019 - 10:34 AM

In any deep space photograph with a reflector,  there is coma error, collimation error and tracking error.  If there is a barlow, then the barlow could be spaced for use with eyepieces, and may not be at the correct spacing for the camera sensor.

 

How are the views through an eyepiece?  Maybe a star test with an eyepiece and then the camera would help identify the problem.  If you search the web for star test reflector you can find some help with that.


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#3 Noobulosity

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Posted 17 September 2019 - 11:26 AM

Looks like pretty severe coma, to me.  Oval-shaped stars radiating out from the center of the image.  Do you have a coma corrector/field flattener?  Not sure if they make one for that scope, but they're pretty necessary for astrophotography to avoid exactly that type of issue.

 

Scopes typically have curved focal planes for viewing through eye pieces, as that's the shape the back of your eyeball has.  You get nice, clear views through your scope for visual use.  For astrophotography, a field flattener is used to flatten that focal plane so a camera sensor can capture it with nice, round stars.



#4 jdupton

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Posted 17 September 2019 - 11:56 AM

David,

 

   The aberration showing in your image is not normal Coma. Coma shows pointed arrow-like shapes pointing towards the center of the field of view not away from it.

 

   I think there are several aberrations at work here. To me, it sort of looks like Tangential Astigmatism combined with possibly Distortion and extreme Field Curvature.

 

   Are you using any auxiliary optics with the reflector? Something like a rmisconfigured coma corrector, or field flattener, focal reducer combined with a Barlow might show something like this if the order or spacings of the extra optical components are way off.

 

   To see what the various optical aberrations actually look like in an image, see the following Web page. Reflectors normally show Coma and Coma is often blamed for many ills in imaging but other aberrations are often present. Sometimes, by recognizing which are actually showing up most, you can figure out where they are coming from.

 

https://www.handprint.com/ASTRO/ae4.html

 

 

John


Edited by jdupton, 17 September 2019 - 11:57 AM.

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#5 Noobulosity

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Posted 17 September 2019 - 01:08 PM

I had just assumed normal coma, but looking at those examples, I think David is right.  It's "tangential astigmatism" / field curvature, from what I can make out.  Knowing more about the setup and any correctors/flatteners will help figure it out.



#6 dpaigen

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Posted 17 September 2019 - 02:46 PM

There was nothing extra in the imaging train.  The three optical elements (primary, secondary, and whatever is in the focus tube) of the telescope and the camera sensor.  



#7 Chuckwagon

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Posted 17 September 2019 - 02:59 PM

There was nothing extra in the imaging train.  The three optical elements (primary, secondary, and whatever is in the focus tube) of the telescope and the camera sensor.  

Which scope are you using?  I don't see an f/8.8 114mm reflector on Orion's site.  Are you using the 2x barlow?  Can you remove it?


Edited by Chuckwagon, 17 September 2019 - 03:00 PM.


#8 kathyastro

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Posted 17 September 2019 - 03:29 PM

It looks like reverse coma.  I have that in a homemade refractor made from a photocopier lens.

 

In your Bird-Jones reflector, check that the corrector lens (it is not a barlow, though it has a barlow-ish effect) is inserted the correct way.  You have to remove it for collimation, so it is possible that it got reinserted backwards.  Try it in the opposite orientation from the way it is now.  If the images improve, that was the problem.



#9 Chuckwagon

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Posted 17 September 2019 - 03:51 PM

In your Bird-Jones reflector....

Are the Orion 114mm reflectors a Bird-Jones (Jones-Bird) design.  Orion lists them as 500mm f/4.4 scopes.  They do not indicate the Bird-Jones design in their specs.  Both the Starblast and Starseeker models have a package that includes a 2x "Shorty" barlow, but they do not indicate any integrated sub-aperture elements such as the Bird-Jones has.  Also, they only refer to the barlow as "Shorty" not the scopes.

 

If the OP is using an Orion 114mm, and it has the 2x "Shorty" barlow they sell with it, I'd suspect that's the culprit, and would remove it.  Then verify collimation and try the imaging again.


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#10 dpaigen

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Posted 18 September 2019 - 03:55 AM

The 'scope I am using is not currently for sale at Orion.  I bought it in the mid 90s.

 

I'm not sure how to remove the corrector lens; I haven't noticed it before, only inferred it's existence.  Do you think I will need to remove the focuser to get at it?  I collimated via star test, not with instruments (well, other than my eyes).

 

The other thread about Bird-Jones has convinced me that this OTA is a piece of crap barely a half step above a department store model.  I have another reflector, but at 200mm it is twice the weight of this one and with a much larger wind profile to boot.  I just got a new mount which could handle it but I am leery of making a double jump at this point in my AP learning curve (

 

So let me reiterate question number three.  What can I still learn with this rig?  Or is it so impaired that switching to a larger 'scope will be the lesser of problems?



#11 Chuckwagon

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Posted 18 September 2019 - 11:18 AM

Ah, that would explain why there are no references to the Bird-Jones design in the current docs, yours is a discontinued design.  And sadly, it's a much maligned design as well.  I'm afraid you are likely to suffer a great deal of frustration getting decent results.  As Kathy pointed out, the recommended practice is that you'd need to remove the "corrector" in order to collimate properly.  And that's likely going to require the disassembly of the focuser.  Too much hassle in my book.  I'd scrap it.

 

As to what you might still be able to learn, I'd say that sometimes, for your own sanity, it's best to bite the bullet, accept defeat, and move on.  :)

 

Sure, you can probably fight with it for a while and make it work.  But the odds are, you are going to be disappointed with your results, and that might discourage you from continuing.  Getting started in imaging can be daunting enough without using equipment that is difficult to work with.

 

As a "newbie," you'd probably have better success using the Nikon 70-200 2.8 in your signature.  Fewer potential issues to deal with.  The shorter focal length is more forgiving of poor tracking, so if you are not ready to guide (which you are going to need to do for decent results at longer FLs) you can still get fairly acceptable results.  It's also a faster lens, even if you stop it down a bit to improve sharpness.  And with a Nikon camera, the attachment should be solid, you'd have no issues with spacing, fit, or other connecting issues since it's designed for that camera.  There are still a lot of targets you can shoot at the shorter focal length, such as M31 @200mm FL at this time of year.  And you'd be spending more time learning the processing side of the equation and not fighting with the equipment.  Then, as you feel you have the image acquisition and processing better in hand, using your 200mm scope on your new mount wouldn't be such a leap forward.

 

That would be my chosen path at least.  :)


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#12 bobzeq25

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Posted 18 September 2019 - 11:50 AM

Ah, that would explain why there are no references to the Bird-Jones design in the current docs, yours is a discontinued design.  And sadly, it's a much maligned design as well.  I'm afraid you are likely to suffer a great deal of frustration getting decent results.  As Kathy pointed out, the recommended practice is that you'd need to remove the "corrector" in order to collimate properly.  And that's likely going to require the disassembly of the focuser.  Too much hassle in my book.  I'd scrap it.

 

As to what you might still be able to learn, I'd say that sometimes, for your own sanity, it's best to bite the bullet, accept defeat, and move on.  smile.gif

 

Sure, you can probably fight with it for a while and make it work.  But the odds are, you are going to be disappointed with your results, and that might discourage you from continuing.  Getting started in imaging can be daunting enough without using equipment that is difficult to work with.

 

As a "newbie," you'd probably have better success using the Nikon 70-200 2.8 in your signature.  Fewer potential issues to deal with.  The shorter focal length is more forgiving of poor tracking, so if you are not ready to guide (which you are going to need to do for decent results at longer FLs) you can still get fairly acceptable results.  It's also a faster lens, even if you stop it down a bit to improve sharpness.  And with a Nikon camera, the attachment should be solid, you'd have no issues with spacing, fit, or other connecting issues since it's designed for that camera.  There are still a lot of targets you can shoot at the shorter focal length, such as M31 @200mm FL at this time of year.  And you'd be spending more time learning the processing side of the equation and not fighting with the equipment.  Then, as you feel you have the image acquisition and processing better in hand, using your 200mm scope on your new mount wouldn't be such a leap forward.

 

That would be my chosen path at least.  smile.gif

+1.  It's a better path to becoming a good imager.


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#13 TelescopeGreg

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Posted 18 September 2019 - 01:08 PM

The 'scope I am using is not currently for sale at Orion.  I bought it in the mid 90s.

 

I'm not sure how to remove the corrector lens; I haven't noticed it before, only inferred it's existence.  Do you think I will need to remove the focuser to get at it?  I collimated via star test, not with instruments (well, other than my eyes).

 

The other thread about Bird-Jones has convinced me that this OTA is a piece of crap barely a half step above a department store model.  I have another reflector, but at 200mm it is twice the weight of this one and with a much larger wind profile to boot.  I just got a new mount which could handle it but I am leery of making a double jump at this point in my AP learning curve (

 

So let me reiterate question number three.  What can I still learn with this rig?  Or is it so impaired that switching to a larger 'scope will be the lesser of problems?

I guess the answer to "what can I learn" question depends a good bit on what you want to learn.  DSO AP is a hobby of details and precision.  If you persevere with this telescope and get it properly aligned and adjusted you will have learned a lot about not only the particulars of that telescope (which, honestly, may be of limited value), but also a hands-on appreciation for the care and precision demanded by the hobby.  That later skill is universally applicable, and you'll probably have some good images to show for your struggles, which will be all that more satisfying given what you did to create them.


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