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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#551 cytan299

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Posted 19 July 2022 - 12:17 PM

how would you orient the mask to do this? 

I orientate the collimation adjustment screws (red circles) to line up with each spine of the mask. My original design had both the rectangle slots and the white screws to help me with setting the mask to the correct orientation.

 

track.jpeg

 

cytan


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#552 calypsob

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Posted 19 July 2022 - 01:24 PM

I orientate the collimation adjustment screws (red circles) to line up with each spine of the mask. My original design had both the rectangle slots and the white screws to help me with setting the mask to the correct orientation.

 

attachicon.giftrack.jpeg

 

cytan

Ok but how can you do this with a normal bahtinov?



#553 astroserge

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Posted 19 July 2022 - 01:49 PM

Ok but how can you do this with a normal bahtinov?

Earlier in this thread it was discussed that a normal bahtinov with rotations cannot replace a tribahtinov mask. So my suggestion is to get a tribahtinov first.



#554 calypsob

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Posted 19 July 2022 - 02:30 PM

Earlier in this thread it was discussed that a normal bahtinov with rotations cannot replace a tribahtinov mask. So my suggestion is to get a tribahtinov first.

Did you read my question on the last page?



#555 cytan299

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Posted 19 July 2022 - 06:19 PM

Ok but how can you do this with a normal bahtinov?

My  answer was a suggestion to Steve to use a normal Bahtinov to check Kirk's program and there are reports that the normal Bahtinov cannot be used for collimation despite what I posted earlier.

 

So, I'm not sure how I can answer your question because I don't use the Bahtinov for collimation at all.

 

cytan


Edited by cytan299, 19 July 2022 - 06:21 PM.

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#556 Broscheanu

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Posted 19 July 2022 - 09:24 PM

I've tried to collimate my C9.25 SCT last night using a Canon M200 in prime focus, with focal reducer, using the Farpoint tri-bahtinov.

 

tri-bahtinov.jpg

Do the stripes seem aligned well enough?

The 9/3 o'clock stripes are affected by focus, and if they seem off center I can center them better using the focus knob.

 

Defocused.jpg

The de-focused star seems to be slightly bloated toward the bottom.

 

The star (Altair) is defocused by turning the knob counter clockwise.  When I tighten the knobs/screws on the secondary the corresponding part on the defocused star gets wider, and when I loosen them it gets narrower.  So I should loosen the bottom knob. Does that seem right?

 

Thanks for any feedback.



#557 cytan299

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Posted 20 July 2022 - 07:24 PM

I've tried to collimate my C9.25 SCT last night using a Canon M200 in prime focus, with focal reducer, using the Farpoint tri-bahtinov.

 

attachicon.giftri-bahtinov.jpg

Do the stripes seem aligned well enough?

The 9/3 o'clock stripes are affected by focus, and if they seem off center I can center them better using the focus knob.

 

attachicon.gifDefocused.jpg

The de-focused star seems to be slightly bloated toward the bottom.

 

The star (Altair) is defocused by turning the knob counter clockwise.  When I tighten the knobs/screws on the secondary the corresponding part on the defocused star gets wider, and when I loosen them it gets narrower.  So I should loosen the bottom knob. Does that seem right?

 

Thanks for any feedback.

I have no idea what I'm seeing with your de-focused star because the central circle is not a shadow. In general, to check collimation, the scope should be just defocused.

 

cytan



#558 Broscheanu

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Posted 20 July 2022 - 07:35 PM

I defocused the star a little more so I'd get a bigger donut with more detail in it.. I don't know what that bright dot is in the center either. Could it be because my 0.63 reducer was on, while the mask is designed for the full focal length?



#559 cytan299

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Posted 21 July 2022 - 07:45 AM

I defocused the star a little more so I'd get a bigger donut with more detail in it.. I don't know what that bright dot is in the center either. Could it be because my 0.63 reducer was on, while the mask is designed for the full focal length?

There's something messed up in your image train, like a light leak or reflection or maybe your image train is OK, but you've really cranked up the gain to obtain this image. I had a 0.63 Celestron Focal Reducer as well when I used my mask and I've never seen this.

 

Furthermore, you're too far out of focus to know whether you are collimated or not. The central obstruction image, i.e. image with a doughnut, only tells you that you are way off. You need to go more into focus to check. For example, here's my video of what you should see as you go in and out of focus:

 

https://www.youtube....h?v=Ie9j0Z9MNC8

 

There are literally circles around a bright dot as you move in and out of focus. If the circles on average are symmetric around the bright spot, you are collimated. As you can see it's a real PITA to actually judge this, that's one of the the reasons why I created the TriBahtinov.

 

cytan


Edited by cytan299, 21 July 2022 - 07:46 AM.


#560 Psittacula

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Posted 21 July 2022 - 06:36 PM

Satoru, would it be possible to do this with a newtonian that uses 4 secondary screws instead of 3 ? 

My Newtonian OTA's secondary mirror has a 3-axis adjustable type with a pull screw in the center and three push screws around it (that makes a total of four screws). The primary mirror is also a 3-axis adjustable type.
If so, the tri-bahtinov mask pattern would be convenient to use.

 

On the other hand, if it is an orthogonal two-axis adjustable type (like the Cassegrain Secondary Holders from Astro Systems), it may be more appropriate to construct a new mask pattern by combining two orthogonal bartinov mask patterns.

Regards,

Satoru



#561 Psittacula

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Posted 30 July 2022 - 03:08 AM

My  answer was a suggestion to Steve to use a normal Bahtinov to check Kirk's program and there are reports that the normal Bahtinov cannot be used for collimation despite what I posted earlier.

I have the same results from similar examination. It seems that it is difficult to check the collimation simply by rotating the normal bahtinov mask.

Both patterns of tri-bahtinov masks are off axis, which I consider to be the key to their function.
tbms.png

Regards,

 

Satoru


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#562 ET_PhoneHome

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Posted 21 November 2022 - 03:16 PM

So, is it best to focus so one set of spikes are centered, and then adjust the screws to center the other two?



#563 Whichwayisnorth

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Posted 22 November 2022 - 05:11 PM

So, is it best to focus so one set of spikes are centered, and then adjust the screws to center the other two?

This is what I have come to understand.  And after making an adjustment you must remember which of the two were related to focus and refocus them after each adjustment.  Also keep the star centered.



#564 cytan299

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Posted 25 November 2022 - 08:04 PM

This is what I have come to understand.  And after making an adjustment you must remember which of the two were related to focus and refocus them after each adjustment.  Also keep the star centered.

Yes, use 1 triplet of spikes to focus and the other triplet of spikes to collimate. Always recenter the star and then refocus with the preselected triplet of spikes after each adjustment.

 

cytan



#565 jetty

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Posted 04 December 2022 - 01:11 AM

Got a new EdgeHD 9.25” and literally saw UPS drop it off the back of the truck.

 

Out of Collimination, fixed it with a Tri-Bahtinov and Cover, took only 20 mins in less

than perfect seeing.  Tri-Bahtinovs rock!

gallery_397257_21709_1543418.jpeg

gallery_397257_21709_1198311.jpeg


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#566 ET_PhoneHome

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Posted 05 December 2022 - 08:20 AM

I do like my tri-mask, but think this needs to be done with a camera, right?  I tried doing it visually, but he spikes were just too small or faint to see clearly.



#567 Zebul

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Posted 05 December 2022 - 03:56 PM

Quit easy to realize this job with a camera naturaly, but I have done it with a meddium or hight magification occular. It take a little bit more time but possible even with a single bahtinov mask turning 120° time to time...

Now I use my DSLR or mobile phone to take picture for much better analyse. It works too.

You need to be 'patient' because it takes a little bit more time. waytogo.gif


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#568 john rozakis

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Posted 11 December 2022 - 05:31 AM

Hi to all

I have  the Takahashi Epsilon 160ed and want to make a tri-bahtinov-mask for collimation . Is this going to work ?

 

John



#569 cytan299

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Posted 11 December 2022 - 06:36 PM

Hi to all

I have  the Takahashi Epsilon 160ed and want to make a tri-bahtinov-mask for collimation . Is this going to work ?

 

John

In principle it should work because the TriBahtinov can be used on a Newtonian. Well, assuming that the Epsilon is a Newtonian.

 

cytan


Edited by cytan299, 11 December 2022 - 06:36 PM.

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#570 AstroCub

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Posted 11 January 2023 - 12:56 AM

Got a new EdgeHD 9.25” and literally saw UPS drop it off the back of the truck.

 

Out of Collimination, fixed it with a Tri-Bahtinov and Cover, took only 20 mins in less

than perfect seeing.  Tri-Bahtinovs rock!

gallery_397257_21709_1543418.jpeg

gallery_397257_21709_1198311.jpeg

Sorry to hear about your experience with UPS but glad you got your OTA back in collimation. 

Where did you get the mask & cover from? I note that the cover design is a lot different from the earlier covers that simply just covered off 240 degrees of the mask itself. I wonder if that cover design as shown in your photo above is a new & improved version??

Thanks,

Siouxsie   


Edited by AstroCub, 11 January 2023 - 12:59 AM.

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#571 TareqPhoto

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Posted 11 January 2023 - 03:25 PM

I wish if there are more of Bahtinov mask but in transparent one same as William Optics but much cheaper, WO smaller one is still expensive, anything with WO brand/trademark is more expensive than alternative, i don't need everything to be high quality name or brand, some had issues even with that expensive WO transparent one so what is the point.



#572 jetty

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Posted 13 January 2023 - 01:32 PM

Sorry to hear about your experience with UPS but glad you got your OTA back in collimation. 

Where did you get the mask & cover from? I note that the cover design is a lot different from the earlier covers that simply just covered off 240 degrees of the mask itself. I wonder if that cover design as shown in your photo above is a new & improved version??

Thanks,

Siouxsie   

I made the design myself and printed it with my 3D printer.  For the mask pattern, I used the resources linked in this thread.  The notch and cutout on the edge are to clear the screw that's in the tube edge (notch) and

the celestron dew heater ring wires (cutout).  There rest are just modifications to fit over the secondary holder.  The more solid cover is also linked on this thread.

 

I've put white marks on, to line up the mask with the collimation screws, and line up the cover to the mask, so I just rotate white to white.  Rotate the cover so white marks line up, it blocks out the

other 2 sets of spikes and I adjust that screw (NE one on the cover).

 

If you have an EdgeHD 9.25" with the same setup, I'm happy to send over the design (or possibly print one for costs).


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#573 Whichwayisnorth

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Posted 15 January 2023 - 01:22 PM

Hi all,

I have created a web based mask generator, that can directly emit STL files (for 3D printing).

The tool is available at this link.

Here's a post announcing it on CN.

This is cool.  I wish there was a database of existing SCT's that would have the data included because typing in the focal length kills the page.


Edited by Whichwayisnorth, 15 January 2023 - 01:24 PM.



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