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Old 17-01-2018, 11:23 AM
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80 mm What is wrong?

I was in a rush to beat the clouds and thought I was polar aligned or at least reasonably so. ..but lookinh at my few images this morning the stars are elongated badly near the edges (which is probably why I did not notice last night).
It is an 80 mm espirt with a nikon 24 meg camera.
Any ideas what my problem is.
In the central area stars look reasonable but at edges they are terrible.
Alex
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Old 17-01-2018, 11:27 AM
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Here it is...
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Old 17-01-2018, 11:42 AM
N1 (Mirko)
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Field curvature?
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Old 17-01-2018, 12:25 PM
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Yes, it is field curvature. A field flattener will solve the problem.
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Old 17-01-2018, 12:28 PM
brisen (Brian)
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There should be a flattener in the case with the scope, I have recently upgraded to the Esprit 120 and a flattener was included. I am using a CCD with it, but the stars are round to the edges with the flattener in place.

Brian
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Old 17-01-2018, 01:05 PM
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Thats the problem the field flattener is on????
Must need adjustment...
Alex
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Old 17-01-2018, 01:09 PM
brisen (Brian)
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Have you got the spacing right - there is a DSLR adapter that comes with them - should be 55mm back focus from the flattener to the sensor.

Brian
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Old 17-01-2018, 01:49 PM
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The flattener is instaled see photo.
Alex
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Old 17-01-2018, 02:00 PM
brisen (Brian)
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The flattener is right and it looks like the DSLR adapter is there - there looks to be a bit extra on yours after the adapter though which might be taking the backfocus back too far. I have a canon t ring with mine and it looks thinner but not sure if the Nikon one is different. The ring screws straight onto the adapter with no extension tube. The other thing - is it the correct flattener for the 80 mm. I know some inadvertently got the wrong flattener. On the flattener mine has 120 - might be worth checking as this can cause issues.

Brian
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Old 17-01-2018, 03:32 PM
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Theres elongated stars even in this nice image near center ..not just edges ..tracking needs be a bit closer to pole perhaps..thought if spacing is not within even half a mm that can cause problems ..nice go Alex..gives us all an idea of what can happen.
PS:this unit has an extra spacer? is it needed?
bigjoe.

Last edited by bigjoe; 17-01-2018 at 03:35 PM. Reason: Add
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Old 17-01-2018, 03:53 PM
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The flattener is right and it looks like the DSLR adapter is there - there looks to be a bit extra on yours after the adapter though which might be taking the backfocus back too far. I have a canon t ring with mine and it looks thinner but not sure if the Nikon one is different. The ring screws straight onto the adapter with no extension tube. The other thing - is it the correct flattener for the 80 mm. I know some inadvertently got the wrong flattener. On the flattener mine has 120 - might be worth checking as this can cause issues.

Brian
I dont know the distance to the chip when Michael put it together I think he took the front of the flash plate as where the chip face was ..its supposed to be 61 mm but looking at it and with crud measuremeants to determine where the chip is at this stage it seems it could be too far out. But looking at the bits I have I cant see something I missed...I probably have.

Maybe it is the wrong flattener I will try and get a beeter alignment..also maybe the camera is not ligning up ..its just such a strange arrangement for the stars.

And the old (new) 8 inch out of the box worked...It must be me...

Thanks everyone I will have a play to night hopefully ... But other than my current problem I was happy with the image in so far as 30 seconds at 800 iso in the city is ok by me... And I think much longer captures will be possible...If nothing else I will see how long I can expose for and not worry about trailing for the moment ...Settings iso plus need to get a dedicated focuser mask ...so much to do.

alex
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Old 17-01-2018, 03:57 PM
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Quote:
Originally Posted by bigjoe View Post
Theres elongated stars even in this nice image near center ..not just edges ..tracking needs be a bit closer to pole perhaps..thought if spacing is not within even half a mm that can cause problems ..nice go Alex..gives us all an idea of what can happen.
PS:this unit has an extra spacer? is it needed?
bigjoe.
I will work on the alignment but I think its more than that..Its like what I expect as coma would be like in a reflector...

I think the camera needs to be closer ... but cant change that at the moment but maybe I will adjust before the flattner between flattener and objective... a bit see if that changes anything...

There is no spacer or any thing that allows me to put it together differently.



I will have a look on utube for someone setting up a rig like mine ..there must be one out there.

Alex
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Old 17-01-2018, 04:04 PM
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I must check mount level something may be different.
If it has gone out of level..how I dont know but thats what it looks look somehow...as well.

I love problem solving I love problem solving.1...2...3...4...

ANyways the way the clouds are rushing past I will have to go for 10 second captures...maybe the sky is the problem,...because I think even though it looks clear there must be a lot of water vapour so maybe clouds refracting the light???
Alex

Alex
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Old 17-01-2018, 04:45 PM
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From the ruff mesurements so far I think back focus is too short...maybe there is a ring I have misplaced...as it seem needs to move back.
I am not sure where the chip is and dont realise want to poke aroung with a probe of any kind ...but I will try and find the distance .
The back focus is 61mm and
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Old 17-01-2018, 04:51 PM
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Quote:
Originally Posted by xelasnave View Post
From the ruff mesurements so far I think back focus is too short...maybe there is a ring I have misplaced...as it seem needs to move back.
I am not sure where the chip is and dont realise want to poke aroung with a probe of any kind ...but I will try and find the distance .
The back focus is 61mm and
Hi ALex,

You don't need to poke anything inside the camera body (unless you really want to), since the Nikon F mount has a sensor to lens mounting flange distance of 46.50mm.

Best
JA
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Old 17-01-2018, 04:52 PM
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Continuing
But the chip seems past that so it needs to be closer MAYBE.
I think it could be the mount not level ..I cant look until dark as I dont want folk seeing me play telescopes...
The mount not being level could give this effect I would think...

Anyways start on it later.
Alex
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Old 17-01-2018, 04:55 PM
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Hi ALex,

You don't need to poke anything inside the camera body (unless you really want to), since the Nikon F mount has a sensor to lens mounting flange distance of 46.50mm.

Best
JA
Thanks I will get up and look with that in mind.
My legs are not happy that I am back into astronomy.
But what a good reason to get out of bed...its bed or astronomy maybe food but its secondary.
Alex
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Old 17-01-2018, 05:04 PM
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I got the scope and have it laying in the bed to love at some measurements and all I have is a plastic thirty year old dress makers tape..the decent stuff is in the garage but the stairs.. so far it seems off. There seems like 5 mm out ..but I will take a few more see if I can get a decent ruler...when I get the level from thr garage...baby steps.

Alex
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Old 17-01-2018, 05:17 PM
brisen (Brian)
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Hi Alex

You need to have 66 mm backfocus from the back of the flattener to the chip. As the chip is 46.5 mm you need 19.5 mm to meet the exact requirements for focus.

It can be challenging, I have spent a few weeks trying to get the exact distance on a 120 with a CCD and OAG and it is a tedious process but I think I have it right now.

Brian
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Old 17-01-2018, 05:29 PM
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Is it me...can you see my tape measure and improvised square ...but unless I missing something it looks as if the chip is way off being 61.5 mm away from the marker they give on the flttener.
Alex
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