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Star spike question


MalcolmM
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horsehead.thumb.jpg.11735044ea22591fd6576a5c3f4d4dc7.jpgHi,

Would someone be able to tell me what is causing the spikes in the bright stars in the attached pic? This was taken with a Nikon attached to a Tak FS60CB directly. No flattener. It's 10 x 10 sec exposures at ISO 1600 on an alt/az mount stacked in Sequator. Though I have seen something similar just using the camera plus a 200mm zoom lens which might imply it is the camera itself?

Many Thanks,

Malcolm

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Did you check the scope from the front to see if there is anything protruding into the scope? The type of spike indicates that there is a single foreign object in the light path. This could be a screw that is too long. This type of spike will also occur when imaging along an edge like the slit of a dome.

Nicolàs

 

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There is a straight line obstruction perpendicular to the diffraction spike. Look into the ”business end” of your scope/lens and see if there is a straight edge somewhere in the light path.

there is also a shadow in your diffraction pattern. Lens clips maybe? A small obstruction at the very edge of the optical elements can be the cause.

Edited by wimvb
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I am not sure about the spikes (possibly something protruding into the light path as suggested) but the dark lines going inwards are well known for TAKs if I remember Olly @ollypenrice correctly and I think they are called the light house effect (or something like that).

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11 minutes ago, gorann said:

I am not sure about the spikes (possibly something protruding into the light path as suggested) but the dark lines going inwards are well known for TAKs if I remember Olly @ollypenrice correctly and I think they are called the light house effect (or something like that).

That is indeed the case. I have been considering buying a FSQ85, but found that quite a few images on Astrobin taken with that scope suffer from those dark lines. Those dark lines, however, rotate around the centre of the image, see this one for example: https://www.astrobin.com/full/pmh3a6/0/

In contrast the dark lines shown in Malcolm's image seem to have a perpendicular orientation to the spikes and thus a fixed orientation (provided that this is true all over the image, which is hard to assess) and thus have another cause.

Nicolàs

PS: I ended up buying a SW Esprit ED80

Edited by inFINNity Deck
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9 hours ago, gorann said:

I am not sure about the spikes (possibly something protruding into the light path as suggested) but the dark lines going inwards are well known for TAKs if I remember Olly @ollypenrice correctly and I think they are called the light house effect (or something like that).

The bright spike running roughly up-down is not normal and will probably be due to something in the light path. The dark bars at right angles to the that are classic Takahashi. I originally called them 'inverse lighthouse beams' because they look light dark versions of a lighthouse's beams. I've had them on all the FSQs I've used and can't say they bother me.  According to Greg Parker they're caused by pinching. I always think of pinching as causing triangular stars but I'm not a retired Professor of photonics and Greg is!

Olly

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Hi,

Just as an update I cannot see any obvious obstructions in either the camera or scope (pics attached - though very difficult to see!).

Also I have attached a picture that was taken with the camera and a 200mm zoom lens. No telescope. The same artifact appears there too.

Malcolm

IMG_20211202_181339372.thumb.jpg.561b537ecbdb90eacce6822c99eb671f.jpgIMG_20211202_181352758.thumb.jpg.f67b9a82fa88c8cf5fdf112ea188643a.jpgIMG_20211202_181513890.thumb.jpg.8cccfc90f40a85ed1193f3922a19a381.jpgIMG_20211202_181528029.thumb.jpg.d097c0c4a5ecb915edcd06a372aaa70e.jpgIMG_20211202_181533772.thumb.jpg.bfc9382f2dc594a2a1158c9fd36c562a.jpg

1737324773_DSC_0006(2).thumb.jpg.7c2d4880e5f69e2c34fea3d422a0e4c9.jpg

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Hi Malcolm,

at least that shows that the bright spikes are not caused by the scope. Could you produce another image with the scope, but with the camera at a different orientation (or even several images with various orientations)? If the dark spikes still are perpendicular to the bright ones, then we know for sure that all artefacts are caused by the camera.

I have no experience with DSLRs in astro-imaging, so the next could be totally wrong/rubbish: I presume the camera has a flip-mirror? Could it be that it does not fully fold away and that we see reflection of it?

How come the orientation of the bright spikes when used with the 200mm lens is mirrored to those when using a scope?

Nicolàs

 

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1 hour ago, inFINNity Deck said:

Hi Malcolm,

at least that shows that the bright spikes are not caused by the scope. Could you produce another image with the scope, but with the camera at a different orientation (or even several images with various orientations)? If the dark spikes still are perpendicular to the bright ones, then we know for sure that all artefacts are caused by the camera.

I have no experience with DSLRs in astro-imaging, so the next could be totally wrong/rubbish: I presume the camera has a flip-mirror? Could it be that it does not fully fold away and that we see reflection of it?

How come the orientation of the bright spikes when used with the 200mm lens is mirrored to those when using a scope?

Nicolàs

 

That's a great idea and I'll definitely try that as soon as I get another clear night. Last night was the first time I'd clear skies in weeks! 

The inverse lighthouse beams do not bother me and I'm a visual observer just dabbling in AP for fun but I'm intrigued as to what's causing the spikes! @inFINNity Deck thanks again for taking the time to help me 

Malcolm

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Quite a while back (a few years I believe), there was another thread about unexpected diffraction spikes. At that time the culprit turned out to be a power cable or something. Clearly the definition of light path is a loose one.

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8 hours ago, MalcolmM said:

That's a great idea and I'll definitely try that as soon as I get another clear night. Last night was the first time I'd clear skies in weeks!

Please keep us posted on the results, I am always intrigued by these matters, so would like to learn about this one.

Nicolàs

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The star images taken with the 200 mm lens are elongated by camera shake.

However, the spikes in the Tak image show definite chromatic abberation which is very much an optical effect.

I think the two issues are completely unrelated.

Did you have a completley unobstructed view of the target? A washing line or telephone cable in the line of sight could cause this effecl.

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I have have just gone outside to check for obstructions as a few of you have suggested and it's possible that a washing line may have been in the way! I'm hoping to try that target again sometime if the rain and clouds ever go! I'll wait till it's higher in the sky and be careful of washing lines and telegraph lines!

Many thanks for all your help and advice,

Malcolm

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Don't start me! I have a constellation of them over my back garden. And it's such an effort to wind up the phone! If only they would invent something like those mobile phone thingys that the original Star Trek crew used :)

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  • 4 weeks later...
On 04/12/2021 at 11:06, MalcolmM said:

I have have just gone outside to check for obstructions as a few of you have suggested and it's possible that a washing line may have been in the way! I'm hoping to try that target again sometime if the rain and clouds ever go! I'll wait till it's higher in the sky and be careful of washing lines and telegraph lines!

Many thanks for all your help and advice,

Malcolm

Hi Malcolm,

any luck with the weather lately (and with chasing the root cause of the artefacts)?

Nicolàs

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Hi @inFINNity Deck,

I actually have not had one clear sky since! Well maybe one but I was cooking at the time and when I was done the clouds had rolled in!

I'm working on the assumption that it was the washing line; I had managed some images before that did not show the spikes. See attached. I'm really a visual observer but like to occasionally try and capture some images; generally no more than 20 subs of 10 seconds or so is my limit. But I'm getting an eq mount (hopefully arriving in the next couple of weeks) so I might get more adventurous :)

Thanks for your interest and I hope you have been having better luck with the weather than me!

Malcolm

M45.thumb.jpg.3d7374aab1d97ba58ad1cc6ae9a8303c.jpg

 

 

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