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Bahtinov mask "Banana" spikes


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Hi all,

I'm looking for some advice on the 'curved banana' shaped spikes I have. Having used a DSLR connected to a 0.85 focal reducer & Skywatcher 80 refractor, the bahtinov diffraction spikes were perfecly straight including the diaganal lines. However, I am now using a ZWO mono 290 and a filter wheel with the adapters (55mm from the sensor to reducer) but now the lines are curved - see image. I know it's a noisy image but this was just a quick snapshot but my main concern is what is causing the curvature of the spikes.

Is this an issue of the camera not being the correct distance to the reducer? I have only had a quick imaging session of M27 the other day and too be honest, it seemed OK but I just wonder if this 'wobble' will be impacted on all the light frames I will be taking. Any advice please...

I Batonov.jpg.ce7572b861693c5be783775bbb463706.jpg

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10 minutes ago, mikeonnet said:

Hi all,

I'm looking for some advice on the 'curved banana' shaped spikes I have. Having used a DSLR connected to a 0.85 focal reducer & Skywatcher 80 refractor, the bahtinov diffraction spikes were perfecly straight including the diaganal lines. However, I am now using a ZWO mono 290 and a filter wheel with the adapters (55mm from the sensor to reducer) but now the lines are curved - see image. I know it's a noisy image but this was just a quick snapshot but my main concern is what is causing the curvature of the spikes.

Is this an issue of the camera not being the correct distance to the reducer? I have only had a quick imaging session of M27 the other day and too be honest, it seemed OK but I just wonder if this 'wobble' will be impacted on all the light frames I will be taking. Any advice please...

I Batonov.jpg.ce7572b861693c5be783775bbb463706.jpg

How long was the exposure as it looks like bad PA alignment....?

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This was just a snapshot from Firecapture, so roughly 1/8th of a second and yes, it wasn't properly polar aligned as I just took the scope out to try it again. It was last weekend that I tried the camera / filter wheel for the first time and properly PA the mount but had the same 'curved' spikes then (using APT software)., hence I thought I would quickly try it tonight but with the same effect.

I will try it without the focal reducer and just connect the camera + wheel onto the scope (providing I have enough back focus) and see what happens. I can only think of the spacing between sensor and focal reducer isn't quite right. Just seems strange that the lines are curved and not straight, yet my dslr attached to the reducer was perfect.

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When I attach my zwo camera to my field flattener, I add a 40mm extension to it to get the distance to the sensor roughly the same. Maybe you need to do the same?

There should be a line on your DSLR indicating where the sensor plane sits. Compare that with the distance the sensor on your ZWO will sit. It should be the same distance. 

Im no expert on this, but it seems like a likely cause to me. 

Edited by JimothyC
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I’m presuming you’re screwing the camera directly onto the t thread on the back of your flattener. You might find one of these helpful. 

https://www.firstlightoptics.com/adapters/baader-t2-extension-tube.html

sorry just reread your post and realise you’re already ensuring the correct spacing. Ignore me, I’ll go back in my box ?  

Edited by JimothyC
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15 hours ago, mikeonnet said:

Hi all,

I'm looking for some advice on the 'curved banana' shaped spikes I have. Having used a DSLR connected to a 0.85 focal reducer & Skywatcher 80 refractor, the bahtinov diffraction spikes were perfecly straight including the diaganal lines. However, I am now using a ZWO mono 290 and a filter wheel with the adapters (55mm from the sensor to reducer) but now the lines are curved - see image. I know it's a noisy image but this was just a quick snapshot but my main concern is what is causing the curvature of the spikes.

Is this an issue of the camera not being the correct distance to the reducer? I have only had a quick imaging session of M27 the other day and too be honest, it seemed OK but I just wonder if this 'wobble' will be impacted on all the light frames I will be taking. Any advice please...

I Batonov.jpg.ce7572b861693c5be783775bbb463706.jpg

I've seen this before and I think that it was a around the time that I was fiddling with back focus distance on my Esprit 880ED.

Have you allowed for the increased optical path length due to the filters? This is usually approximately 1/3 of the thickness of the filters. See here http://www.qsimaging.com/downloads/QSI-500-600-Series-Back-Focus.pdf

Also, are the bananas oriented the same all over the FOV, or do they gradually change curvature over the image? If they change, it might be another indication of incorrect spacing between the comma corrector and the imaging plane.

Can you post the whole image?

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Thanks for all the replies. It sounds as though it is a spacing issue then and I will give it another go when it gets dark & clear. I will try with the 1.25 nose piece adapter that I can then slide in / out from the filter wheel and test again because at the moment the camera is screwed on to the filter wheel with a M48 adapter, then the wheel has a 16.5mm extension tube that screws to the reducer so I'll try the nose piece connections for now and make subtle adjustments while in live view mode to see what happens.

Attached is the full image of the 'curved' bahtinov mask image. I will update when I get a chance to try with the nose piece fittings instead. Thanks again all.

Bahtinov.jpg

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18 minutes ago, mikeonnet said:

Thanks for all the replies. It sounds as though it is a spacing issue then and I will give it another go when it gets dark & clear. I will try with the 1.25 nose piece adapter that I can then slide in / out from the filter wheel and test again because at the moment the camera is screwed on to the filter wheel with a M48 adapter, then the wheel has a 16.5mm extension tube that screws to the reducer so I'll try the nose piece connections for now and make subtle adjustments while in live view mode to see what happens.

Attached is the full image of the 'curved' bahtinov mask image. I will update when I get a chance to try with the nose piece fittings instead. Thanks again all.

Bahtinov.jpg

The way it curves (presumably along the lengths of each little spectrum on the pattern) suggests its related to dispersion combined with the diffraction effect of the mask. Is the focal reducer at the right spacing? - Perhaps try the setup without it.

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18 hours ago, geordie85 said:

Could it not be caused by vibration due to the shutter of your camera?

AfaIk, this was with a zwo camera, which doesn't have a mechanical shutter. But the suggestion to also look for other causes is a valid one. Spacing offset should be visible as elongated stars towards the corners.

My first guess was actually seeing or turbulence. But then the curving would be different between frames.

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Yes, it's a ZWO camera and as others have suggested, it is probably a spacing issue. I will try and experiment when it's next clear. I have attached one of the Luminance subs on M27 (saved as a jpg to upload onto here). This was taken on the same night after the bahtinov mask picture above. Like I say, I will try different spacing from the camera to reducer and update if I have resolved the issue.

M27_L_1-Luminance_2018-07-14_23-33-52_Bin1x1_120s.jpg

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1 hour ago, wimvb said:

AfaIk, this was with a zwo camera, which doesn't have a mechanical shutter. But the suggestion to also look for other causes is a valid one. Spacing offset should be visible as elongated stars towards the corners.

My first guess was actually seeing or turbulence. But then the curving would be different between frames.

That is correct, my mistake. I got my wires crossed with the mention of a dslr in the original post.

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I had to reread the post as well, just to make sure.

From the full frame it seems that spacing is slightly off, but should that give the bent spikes on a star in the centre of a frame?

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If it is a spacing issue, wouldn't the banana bends all be going in the same direction? Can tilt be a possible cause?

I must admit I'm very interested in this thread as my middle bahtinov spike isn't straight either and I use a flatfield astrograph (WO star71). 

 

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Hey had to flick back and forth to find this post again.

Was setting up my QHY8L last night and for the 1st time ever saw the banana things like you posted then they disappeared.

Then came back and then disappeared.

OK...…… They came into my life... when slightly out of focus with very thin layers of high clouds with very muggy/warm temperatures.

Dissapeared quick too, but that's what I put it down to, just kinda strange it happened after youd seen them.

 

Hope you got sorted?

 

Stu

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Here's a theory.

When focused on infinity, a cloud layer will be slightly out of focus.

For a 1 meter scope, if the cloud is 1km high it will only be 1 micron out of focus. Refraction effects in the atmosphere might increase the focus difference, under the right conditions.

Thin, low, cloud preferentially scatters blue rather than red.

As a bahtinov mask is very sensitive, this very slight out of focus could be enough to create a superimposed blue image that is slightly displaced compared to the original one.

As the only the blue elements of the pattern would be affected, this would turn each spectrum into a short, curved segment.

This images of a BM pattern through a Ha filter shows how red light is 'grouped', imagine the dark parts being made up of blue light, then shifted as if out of focus

1300333805_HaBahtinov.jpg.fffb5112eaae704fabe0e128e0263f4a.jpg

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  • 1 year later...

I hate bumping an old post, but I am seeing the EXACT same problem as this thread is talking about. 

My Bahtinov mask is showing the exact same "bananas". I figured this was a problem with my mask that I 3D printed. It looked clean, but maybe it wasn't sort of thing. But no matter how much I zoomed in, looked at other stars in the field, and looked at varying brightness on different gains, and they all look the same, bananas. 

Then, while researching, I ran across this post and other information. One poster above said that "maybe you blue information from clouds is causing a second spike on a different focal plane", and then I thought about how Double EDs worked. They combine Red and Blue into the same focal plane. The whole point of the mask is to diverge the lines in the interference pattern according to focal plane. If a Doublet moves Red and Blue to one plane, and keeps the Green in another, then you would see a "U" shape across the rainbow, no?

This is my question, when using a mid-ranged focal length (F5-F6) and a super small pixel size (2.9um or smaller), can you see the "rainbow" in the form of a banana on a a B/W sensor? Being B/W, you wouldn't see the colors, just a white swipe of banana.

I am having poor skies for the next week or so, I can't wait to figure this out, it is bugging me. My thoughts are I can test with a narrow band filter. If I can bring that to focus (much like the image above), would that prove my point. Any information or anyone who could test/corroborate my theory, would be appreciated.

If you are wanting SOME proof of my idea, go check out the Bahtinov page on Wikipedia, the example image, if you zoom in, you will see the rainbows near the center of the image on the spikes, just like my theory postulates.

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On 03/03/2020 at 15:07, mos6522 said:

I hate bumping an old post, but I am seeing the EXACT same problem as this thread is talking about. 

My Bahtinov mask is showing the exact same "bananas". I figured this was a problem with my mask that I 3D printed. It looked clean, but maybe it wasn't sort of thing. But no matter how much I zoomed in, looked at other stars in the field, and looked at varying brightness on different gains, and they all look the same, bananas. 

 

Sorry, I've now had this problem myself and the answer is ridiculously simple: the scope needs collimating. It's a quick and reliable cure.

Edited by Stub Mandrel
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Quote

...the answer is ridiculously simple: the scope needs collimating.

I double checked my collimation, and sure enough, it needed some help. I am using a Skywatcher EVOSTAR 80ED that I thought was in good order. However, using my laser collimator from my reflector and my cheshire, I could tell it was not.

The telescope wasn't that far out, but to resolve it, I had to take the focus/back tube assembly off, elongate one of the holes that it mounted too (with a file) and then re-assembled using the slack to make up for the bad collimation. 

Finally, I noticed that the 2"->T-Ring was looser than I wanted from the focus rack to the camera train, so I replaced it.

I did do a star test, and the point in the out of focus is in the middle, but I still see slight banana'ing. This was all without a flattener, just Objective->Filter->Camera. The interesting thing, after I collimated, the bananas lined up better, like the ends were in perfect lines, prior to collimation, they were kind of all over the place.

The thing I am chasing is just what I feel are blurry pics. I will try and get a good sample distoration. 

What I am trying to fix is what I feel should be "more stars". The following is a Whale Galaxy shot done through a 7nm Ha filter using 5min exposures. It seems there are some "fuzzy" blobs that should be stars, but don't come out at all. Am I at the end of what I can do with this camera and scope?

image.png.fbad522d142c36a56b5f627a723c2e2e.png

 

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On 12/03/2020 at 13:51, mos6522 said:

I double checked my collimation, and sure enough, it needed some help. I am using a Skywatcher EVOSTAR 80ED that I thought was in good order. However, using my laser collimator from my reflector and my cheshire, I could tell it was not.

The telescope wasn't that far out, but to resolve it, I had to take the focus/back tube assembly off, elongate one of the holes that it mounted too (with a file) and then re-assembled using the slack to make up for the bad collimation. 

Finally, I noticed that the 2"->T-Ring was looser than I wanted from the focus rack to the camera train, so I replaced it.

I did do a star test, and the point in the out of focus is in the middle, but I still see slight banana'ing. This was all without a flattener, just Objective->Filter->Camera. The interesting thing, after I collimated, the bananas lined up better, like the ends were in perfect lines, prior to collimation, they were kind of all over the place.

The thing I am chasing is just what I feel are blurry pics. I will try and get a good sample distoration. 

What I am trying to fix is what I feel should be "more stars". The following is a Whale Galaxy shot done through a 7nm Ha filter using 5min exposures. It seems there are some "fuzzy" blobs that should be stars, but don't come out at all. Am I at the end of what I can do with this camera and scope?

image.png.fbad522d142c36a56b5f627a723c2e2e.png

 

That's not bad actually, that area isn't very starry the shapes in the galaxy are not points. Ha isn't ideal for galaxies, and for comparison here's a shot of mine and you can see my stars are nowhere near as nice as yours...

1349496761_miniwhaledrizzle.thumb.png.7a94c59a0703ddc387e3ad4649c610e1.png

 

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  • 2 weeks later...
On 18/07/2018 at 22:58, mikeonnet said:

Batonov.jpg.ce7572b861693c5be783775bbb463706.jpg

I was seeing exactly this on my SWED80.  I'd since moved to autofocusing, so the mask had been retired, but the issue had always niggled the back of my mind as unresolved

When I changed my focuser, I initially focused by hand before refitting the motor - no bananas! Doing this made me consider that it had been a collimation problem. Looking back, it happened when I swapped from a dslr to dedicated mono camera and spent some time finessing the slip out of the stock focuser to hold the weight more reliably

 

I might be biased, or more experienced but the first imaged after the focuser swap was the sharpest I've ever produced 🙂

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