#1  
Old 29-12-2018, 12:02 PM
Startrek (Martin)
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Diffraction Spikes

Iíve noticed over the last week that my AP images of larger stars in the FOV have dual diffraction spikes only on one axis ( east / west ) where the spike starts off single from the star core and then slightly splits into 2 on a very tight angle away from the core ( itís just noticeable)
Could it be that one axis of my spider vanes are not at 90 degrees to the adjacent
I donít think itís collimation as I use a Cheshire and laser collimator and visually get it spot on,totally concentric in secondary and primary with a bulls eye in the middle
Scope is a Bintel GSO 6Ē f6 newt
Any advice or comments appreciated
Cheers
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  #2  
Old 29-12-2018, 12:46 PM
astro744
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Yes a 3 vane spider causes 6 spikes and a 4 vane spider causes 8 spikes which are seen as 4 brighter ones if the vanes are directly opposite and in line. You have either a twist on one vane or slight misalignment or even a slight bend half way along.
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  #3  
Old 29-12-2018, 03:19 PM
Startrek (Martin)
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Thanks,yes I thought it might be the spider vanes
They are not bent or twisted but you can see a slight off set from east to west
To pull everything apart and fix it might not be worth the trouble as it could end up worse than leaving it
Photo attached of a view down the sight tube of my Cheshire. Looks ok to me ?

Cheers
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Old 29-12-2018, 03:41 PM
astro744
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It's difficult to say based on the photo. There is a slight flare toward one side but it could just be the way the photo was taken. You need to check final collimation using a star test.

Unclean diffraction spikes are typically misaligned spider vanes and this is visible visually. If collimation is an issue then yes stars will exhibit coma along the axis of the in the misalignment.

I'm visual and perhaps those doing imaging can chime in on how better to narrow it down, perhaps taking a closer look at some images.
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Old 29-12-2018, 03:59 PM
Startrek (Martin)
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You will always get a very slight secondary off set in f6 to F3 newts , itís an optical illusion due to the reflection in faster optical systems
I imaged last night and used Rigel for my frame and focus and the Star shape was nice and crisp with uniform shape except for the diffraction spike east west as mentioned in my post
Today I pulled out the primary gave it a wash and clean , reinstalled it , cleaned the secondary in situ and collimate again using my Cheshire and 2 different lasers
Tonightís cloudy so will have to wait until early next week , maybe Tuesday before I can image again
Thanks for your reply
Newts can be a bit of a problem child sometimes but I enjoy the aperture especially my 12Ē goto visual scope

Cheers
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Old 29-12-2018, 04:41 PM
Wavytone (Nick)
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OK Martin, nice pic which tells all. Two problems evident - and its not your spider vanes:

1. You have 3 mirror clips protruding over the edge of the primary. These cause 6 weak diffraction spikes.

Lose the clips and find another way to restrain the mirror without impinging on its circular perimeter. Google is your friend.

2. Large lump at the bottom protruding into the light path, this will create a big pair of diffraction spikes. At a guess this is the bottom of the drawtube of your focusser. From the photo you can estimate how far it sticks in, moving the mirror up by the same amount closer to the secondary mirror will move the focal plane out and ideally enough to ensure the draw tube does not stick into the optical path.

FWIW the intensity of diffraction spikes is related to the area of the optical path obstructed by each object that obstructs the path - the bigger the obstruction the more intense the spikes.

There are three ways to eliminate diffraction spikes:

a) find the cause and remove it, out of the optical path (schiefspieglers, maksutov-newtonians);
b) ensure an obstruction has a curved shape of the same radius as the perimeter of the primary mirror (curved spider vanes);
c) ensure an obstruction is precisely circular and concentric with the primary mirror (secondary mirror cells).
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Old 29-12-2018, 05:37 PM
Startrek (Martin)
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Hi Nick
I love diffraction spikes in my images, I do t want to eliminate them
I merely pointed out in my original post that one axis had a single spike that morphed into a slight dual spike about half way out ( I had a hunch it was to do with the spider vanes not being geometrical at 90 degrees to each other ie a true cross hair or bulls eye target a perfect right angles to each other
I have moved the primary up the tube already ( about 5mm ) but that was to give me more travel in my focus on the DSLR which is working really well when I frame and focus my objects
The primary clamps on these GSO 6Ē newts are quite small , only a 3 to 4 mm overhang on the mirror edge
Cheers
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Old 29-12-2018, 05:53 PM
Wavytone (Nick)
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Quote:
Originally Posted by Startrek View Post
The primary clamps on these GSO 6” newts are quite small , only a 3 to 4 mm overhang on the mirror edge
Cheers
Long ago I made a 6" f/8, 8" f/7, 4" f/2.8, 6" f/5 and then converted the 6" f/5 to f/19.

I HATE diffraction spikes and to me they are the sign of a very lazy telescope maker who doesn't know any better.

if you like them... well... thats up to you. If you want really huge SPIKES put a square or hexagonal mask in front of your scope.

FWIW I know someone who used to put a hexagonal mask on a Celestron deliberately to ADD diffraction spikes - don't ask me why as I have no idea after 30 years.

Last edited by Wavytone; 29-12-2018 at 07:55 PM.
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  #9  
Old 30-12-2018, 08:55 AM
Kunama
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Quote:
Originally Posted by Startrek View Post
I’ve noticed over the last week that my AP images of larger stars in the FOV have dual diffraction spikes only on one axis ( east / west ) where the spike starts off single from the star core and then slightly splits into 2 on a very tight angle away from the core ( it’s just noticeable)
Could it be that one axis of my spider vanes are not at 90 degrees to the adjacent
I don’t think it’s collimation as I use a Cheshire and laser collimator and visually get it spot on,totally concentric in secondary and primary with a bulls eye in the middle
Scope is a Bintel GSO 6” f6 newt
Any advice or comments appreciated
Cheers
Judging by the picture you need to loosen the screw on the bottom spike a little and tighten the top one to bring the east/west vanes into a straight line. The cause of spikes that split more as you go further from the hub is the two opposing vanes not making a straight line, which then means that their secondary reflections are also showing that deviation but in the opposite direction..... (a 3-vanes spider is the ultimate extension of this as its vanes reflect in the same manner to show 6 diffraction spike)
(I am not discussing the offset look of the secondary shadow etc, just the alignment of the vanes)
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Old 30-12-2018, 09:03 AM
Kunama
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Quote:
Originally Posted by astro744 View Post
Yes a 3 vane spider causes 6 spikes and a 4 vane spider causes 8 spikes which are seen as 4 brighter ones if the vanes are directly opposite and in line. You have either a twist on one vane or slight misalignment or even a slight bend half way along.
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Old 30-12-2018, 12:42 PM
Startrek (Martin)
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Hi Kunama
I adjusted the east west spider vane axis by a few mm so itís reasonably straight now ( see photo ) used a piece of white string across the face of the OTA to check alignment
Popped the Cheshire in and still had a very slight deviation but only slight
The mechanics of the spider vane / secondary mirror design set up only allow you to adjust in or out from the centre ie: straightening out a bow from the centre into a straight line
These GSO newts are obviously built for a cost.I find the adjustment arrangements are a bit crude and take a fair bit of skill to accomplish, definitely not Swiss watch precision
Iíd be happy to pay extra money for an advanced , quality engineered adjustment arrangement on these newts
Cloudy nights until Tuesday so I will check out the scope then
Thanks
Cheers
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Old 30-12-2018, 01:06 PM
Kunama
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Martin, check the up/down line pictured above as well, looks a little bowed to the right....
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Old 30-12-2018, 03:59 PM
Startrek (Martin)
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Matt

Yeh just a little out so Iíll tweak it
Lots of testing when I get a clear night , hopefully Tuesday or Wednesday night
Thanks for your input
Hows to 18Ē must be a great view !!
I have a 12Ē goto and itís fantastic at my dark site near Ulladulla
Cheers
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