Somethings wrong - advise

[Eddie Jones]

Hi All,

recently moved from Canon DSLR to Altair 183 Pro Tec

Also moved from Backyard EOS to NINA

I seem to have what looks like diffraction spikes on my test images, (I'm using a 110mm refractor with a filter drawer and 0.8 reducer) thinking it may be lack of settling time between dithering.

This is a stack of 26 x 30 sec exposures on the  Pleiades
Would welcome more experienced opinions

 

Pleades-30sec-small.tif

[Eddie Jones]

Probably should add, was quite breezy and polar alignment was guesstimate as I setup before sunset to capture Jupiter Saturn conjunction.
 

[ollypenrice]

Strange! You weren't shooting with a clothes line or power cable in the line of sight? Or a spider's web in the dewsheild? We've seen both before now on here.

Rather than posting a link to a file,  just post a large JPEG here, not of a stack but of a single sub. That will contain all the info that's useful. The spikes are unrelated to wind or polar alignment.

Olly

[Eddie Jones]

Hi Olly,
No washing line, cant see any spider webs, single frame was FITs, I've converted it to tiff using DSS and moved it to  photoshop and converted to jpg (did a RGB levels edit to make it visible)  
Done a little more thinking , I was using a pier extension on my HEQ5, it's possible  didn't tighten it well enough to the tripod - maybe the wind was twisting the mount ? 

[Moonshed]

I’m sure the clever ones here will figure out what the problem is, all I can say is it looks as though your tracking or pa was a little adrift but that has nothing to do with those spikes, they remind me of the effect caused by a Bhatinov mask.  

[ollypenrice]

No, in my view the spikes cannot be explained by any problem associated with the mount. That imaging train would have produced those spikes on that night on any mount, however perfect. Nor are they connected with dithering. Some of the CMOS cameras are sensitive to internal reflections and I wonder if dew close to the chip or high haze might exaggerate this. However, the main suspect remains a physical obstruction in the light path. Could the filter drawer or filters, or some part of the fixations, be intruding into the light path? Take a good look down the OTA from the top with a clear view to the chip. This really is a likely explanation but they may be hard to spot.

Olly

[CraigT82]

👆What Olly said! 

These spikes look just like those produced by my heritage 130p (which has a single stalk secondary attachment).

Likely there is something with a straight edge intruding into the optical path, shouldn't be too hard to find if that is the case

 

[Moonshed]

38 minutes ago, ollypenrice said:

No, in my view the spikes cannot be explained by any problem associated with the mount. That imaging train would have produced those spikes on that night on any mount, however perfect. Nor are they connected with dithering. Some of the CMOS cameras are sensitive to internal reflections and I wonder if dew close to the chip or high haze might exaggerate this. However, the main suspect remains a physical obstruction in the light path. Could the filter drawer or filters, or some part of the fixations, be intruding into the light path? Take a good look down the OTA from the top with a clear view to the chip. This really is a likely explanation but they may be hard to spot.

Olly

I’m glad you explained about the fault most likely being something in the light path because it makes perfect sense. When I thought how it looked like the effect created by a Bhatinov mask it made me think it must be an obstruction of sorts in the light path, perhaps the filter drawer, but knowing nothing about this scope or filter drawers I decided to keep quiet and leave it to someone who actually knows what they are talking about, I’m still learning!

Edited December 21, 2020 by Moonshed

[david_taurus83]

1 hour ago, ollypenrice said:

Strange! You weren't shooting with a clothes line or power cable in the line of sight? Or a spider's web in the dewsheild? We've seen both before now on here.

I have had both as well! The spider web is an easy fix but the clothes line had a scissors took to it! The missus was not impressed lol

I replaced the line and its wrapped around the pole several times so I can lower it instead of cutting when needed.

[Eddie Jones]

also shot video of the moon (to get focus as I couldn't find my Bhatinov) and Jupiter Saturn conjunction (No setup changes) and saw no sign of the effect on short frames. 

[Moonshed]

34 minutes ago, david_taurus83 said:

I have had both as well! The spider web is an easy fix but the clothes line had a scissors took to it! The missus was not impressed lol

I replaced the line and its wrapped around the pole several times so I can lower it instead of cutting when needed.

You’re  lucky your missus didn’t hoist you up the pole!

[CraigT82]

4 minutes ago, Eddie Jones said:

also shot video of the moon (to get focus as I couldn't find my Bhatinov) and Jupiter Saturn conjunction (No setup changes) and saw no sign of the effect on short frames. 

You won't see any effect on short frames, even newtonians don't show any obstruction diffraction effects on shots like these. You'll only see them on longer exposures of stars (and planets if long enough)

 

[Eddie Jones]

Thanks all, I really can't see any obstructions, I'll take apart and clean everything later, maybe a dog hair or something has crept into the imaging path

 

[david_taurus83]

19 minutes ago, Moonshed said:

You’re  lucky your missus didn’t hoist you up the pole!

I was in the middle of a Horsehead run with 15 minute subs! There was no easier fix!

[Moonshed]

36 minutes ago, david_taurus83 said:

I was in the middle of a Horsehead run with 15 minute subs! There was no easier fix!

Ah, in that case totally understand.

[vlaiv]

I agree with above assessment that source of spikes is something in light path - some sort of edge.

I'd also like to point out that it is probably not straight edge and that it depends on light direction

One star has 3 spikes - this happens when the light passes close to multiple edges that are at an angle (bahtinov mask). Other stars seem to have single or max two spikes (second one being much less pronounced) and have varying angle between them - single edge can't produce this.

My vote goes to spider web.

[vlaiv]

Are all answers final? Can I change my answer?

My inner Sherlock Holmes went ballistic with this one. Here is my final answer and explanation worthy of Sherlock

It is hair that has been split near its end and is situated close to sensor I'll use diagram to explain it:

Light has already been "separated" into different converging beams depending on direction that it is coming from (stars in different parts of frame).

Single spikes are due to hair representing single edge where light cone hits it - slightly bending so different stars in different part of frame give slightly different angle. There is a place where hair splits into two at an angle - resembling Y arrangement on bahtinov mask:

and this happens only for one star - because converging beam from only one star is falling on the split.

And hereby I rest my case.

 

[CraigT82]

58 minutes ago, vlaiv said:

Are all answers final? Can I change my answer?

My inner Sherlock Holmes went ballistic with this one. Here is my final answer and explanation worthy of Sherlock

It is hair that has been split near its end and is situated close to sensor I'll use diagram to explain it:

Light has already been "separated" into different converging beams depending on direction that it is coming from (stars in different parts of frame).

Single spikes are due to hair representing single edge where light cone hits it - slightly bending so different stars in different part of frame give slightly different angle. There is a place where hair splits into two at an angle - resembling Y arrangement on bahtinov mask:

and this happens only for one star - because converging beam from only one star is falling on the split.

And hereby I rest my case.

 

Great sleuthing! 

[Moonshed]

Blimey!

[Eddie Jones]

Update

Think I've found it!   

Some kind of fibre on the nearest baffle to the focuser (had to take off the focuser to get to it)

Thanks all ! - waiting for clear sky to test!

 

Edited December 27, 2020 by Eddie Jones

[JamesF]

8 minutes ago, Eddie Jones said:

Update

Think I've found it!   

Some kind of fibre on the nearest baffle to the focuser (had to take off the focuser to get to it)

I've had something similar to that before, with a small piece of fibre or swarf close to the edge of the objective on my ED80.  It resulted in diffraction spikes similar to yours, but only in one part of the frame.  Took me some time to find it :)

James

[Eddie Jones]

Update, finally got some clear skies last night, Diffraction problem still there
Did another check this morning and found another fibre approx 8mm long (slightly thicker than a hair) on the very edge of the inside of the objective.
It was 'flexibly' attached at one end so with the scope upright it sat around the edge of the lens and wasn't visible, but when the scope was at an angle it dangles into the field of view.

I removed it by tapping a cotton bud to a violin bow and carefully running it into the scope from the eyepiece end.  Aimed it at the fibre It stuck to the cotton bud and I was able to remove it. (Not something I'd like to have to do ever again!)

Supposed to be clear again tonight - fingers crossed