Bhatinov mask

[Rodd]

I am getting strange shapes in the B-mask images.  The spikes are slanted.  When focus is changed, the center spike moves left or right, but maintains its slant, so the bottom of the spike is at a different position relative to the side spikes than the top of the spike.  Also--the right hand spike is slanted to the right.  This makes it exceedingly difficult to use.  This is the first time this has happened.  I am imaging with the C11Edge.  last month, the shapes were just right--they way they are supposed to be.  The scope is permanently mounted (semi permanent) and has not been moved in months.  Polar alignment has remained the same, and the tripod is on poured concrete footings--so the mount has remained level.  

Does anyone know what this could be?  I have considered collimation, but the scope holds collimation extremely well and I have trouble understanding why it would lose collimation while on the mount.  I completed the RGB channels of NGC 5907 (earlier post page 2) not long ago  and the b-mask shapes were perfect. Also--seeing improved last night and I was able to achieve decent FWHM and eccentricity values.  Am I wrong to assume that FWHM values can be good with poor collimation?

You'll note that in the attached image, the bottom of the central spike is a bit to the right, but the top is to the left.  However, if you look at the furthest extensions of the spikes, they are textbook straight, and the central spike is centered.

Any ideas? (This is an iphone picture of my computer screen in the field)

Thanks,

Rodd

 

[Rodd]

Here is a 494 20 sec sub stack of luminance taken with the above Bhatinov mask issue--can it really be collimation?  It looks pretty good to me.  This is a screen stretch that has not been processed.  The median value is pretty high because my sky is not dark.  If I manually stretched this data it would look much better.  But for the purpose of this thread it is acceptable.  The first image is unbinned and the second image is bin 2 in software.   Sub frame selector is reporting sub 1 arcsec FWHM for the unbinned data, but there must be something wrong with that.  I used all subs with a FWHM value less than 3--most were between 2.3 and 2.8.  I imagine the FWHM is in the range of 2.5.  

Could there be something wrong with my Bhatinove mask?  Or could a slight collimation issue not severe enough to show up in the final image be causing the B-mask imperfections?

 

 

[newbie alert]

For me the stars look pretty good, if collimation is out it's not that drastic

How long were your subs on the bahtinov exposure,  you maybe just simply over exposing , although if I do I get a triple spike.. is it shop bought mask,  purpose for the c11 and f ratio?

[Rodd]

18 minutes ago, newbie alert said:

For me the stars look pretty good, if collimation is out it's not that drastic

How long were your subs on the bahtinov exposure,  you maybe just simply over exposing , although if I do I get a triple spike.. is it shop bought mask,  purpose for the c11 and f ratio?

It doesn't matter how long I expose--the B-mask shapes are like this (lately).  This is a purchased, plastic B-mask--never gave me issues before.  I was thinking maybe its bent, but B-masks are pretty toerant of deformity I think.  Anyway--it is pretty straight as I formed it so its pretty flat.  

I have never used the C11Edge native (without reducer) and figured I give it a try.   I don't want the extra field of view the reducer provides for the targets I am shooting (galaxies).  I wanted to shoot at 2800 and see what the fuss is all about.  So far, I find it relaxing.  I am not guiding, so allot of issues go away for that one reason.  I will add an OAG soon--I have it--its just I get 1-2 nights a month and don't want to lose them screwing around with the OAG (and, knowing myself, it will be a colossal frustration and I will be doubly frustrated because I will lose imaging time).

 

the stars look good, and the details in the galaxy look just like my Green stack that I shot when the Bmask was cooperating.  I guess the question is, will a very slight out of collimation cuase the B-mask to look distorted as above.  If so--I must conclude that if that is the case, then the B-mask must be considered the perfect tool to check collimation on a SCT.

[newbie alert]

It doesn't matter how long I expose--the B-mask shapes are like this (lately).  This is a purchased, plastic B-mask--never gave me issues before.  I was thinking maybe its bent, but B-masks are pretty toerant of deformity I think.  Anyway--it is pretty straight as I formed it so its pretty flat

if I showed you my bahtinov mask you wouldn't worry if you're was bent or mis-shapen..

I'm using a tri mask for collimation..I'm pretty impressed with it so far

[Rodd]

39 minutes ago, newbie alert said:

It doesn't matter how long I expose--the B-mask shapes are like this (lately).  This is a purchased, plastic B-mask--never gave me issues before.  I was thinking maybe its bent, but B-masks are pretty toerant of deformity I think.  Anyway--it is pretty straight as I formed it so its pretty flat

if I showed you my bahtinov mask you wouldn't worry if you're was bent or mis-shapen..

I'm using a tri mask for collimation..I'm pretty impressed with it so far

I recently saw instructions for making one of the collimation mask out of cardboard.  It looks pretty simple and effective.  I will probably try and make one.

[newbie alert]

1 hour ago, Rodd said:

I recently saw instructions for making one of the collimation mask out of cardboard.  It looks pretty simple and effective.  I will probably try and make one.

Plenty of people out there with a 3d printer, a face book friend made these for me.. better than shop bought

[newbie alert]

Here you go, I name him toothless Baht

Edited June 9, 2021 by newbie alert
Added info

[Rodd]

9 minutes ago, newbie alert said:

Here you go, I name him toothless Baht

Your not supposed to hit your B-mask...its not HIS fault the focus is bad!

[newbie alert]

On 09/06/2021 at 20:32, Rodd said:

Your not supposed to hit your B-mask...its not HIS fault the focus is bad!

It still works fine

[Rodd]

7 hours ago, newbie alert said:

It still works fine

That is what it is supposed to look like.  last night I had the same bent spikes.

[newbie alert]

3 hours ago, Rodd said:

That is what it is supposed to look like.  last night I had the same bent spikes.

What are you using for capture? Only thing I can think of is a screen stretch 

[carastro]

I presume you are referring to the spikes immediately after the blob in the middle.  They do look strange,  personally if the main long spike in the middle is central I would say it is focused, but I have no idea why those small stubby spikes are the shape they are. 

Carole 

 

[Rodd]

1 hour ago, carastro said:

I presume you are referring to the spikes immediately after the blob in the middle.  They do look strange,  personally if the main long spike in the middle is central I would say it is focused, but I have no idea why those small stubby spikes are the shape they are. 

Carole 

 

They do look strange.  Scope seems to be working.  Its a mystery