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How to Move Prime Focus without cutting or drilling the OTA

astrophotography imaging Orion reflector 3d printing
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11 replies to this topic

#1 kevinrfrancis

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Posted 13 November 2019 - 06:06 AM

In the Orion Astroview 6 reflector telescope, there are 3 rubber hold downs that keep the mirror attached.  I replaced them with longer 3D printer hold downs which effectively moved the prime focus.  This allows me to use a DSLR camera with this telescope.  I've used for more than 1 year, so this recommendation comes with test results.

 

See this link.

 

See one of my photo results.

 

M42_Orion_1600_2_26_19_ST_v4_web.jpg

 

Would love your feedback.


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#2 happylimpet

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Posted 13 November 2019 - 06:32 AM

Nice work! I managed to buy some extension blocks that sit behind the mirror of my 300p to move the mirror forwards 40mm when I had the same problem.

 

This sort of thing is a common problem so its great to see such a neat solution. Well done!



#3 Jon Isaacs

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Posted 13 November 2019 - 06:43 AM

Kevin:

 

Your photo came out nicely.  :goodjob:

 

My concern with this method is that the mirror is supported at the edges rather at the original support points.  For photography this may not matter but for visual where one is using high magnifications, it will probably affect the view.

 

Jon



#4 dmcnally

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Posted 13 November 2019 - 08:23 AM

Very nice job!!

 



My concern with this method is that the mirror is supported at the edges rather at the original support points.  For photography this may not matter but for visual where one is using high magnifications, it will probably affect the view.

 

 

It holds the mirror far enough in the OTA to achieve prive focus with my DSLR camera. I can also return the telescope to original condition for resale or a night of viewing. The base of it matches the original rubber clamps and the screws hold them tight to the mirror frame. The best part is that this is non-destructive to your telescope!

 

This was cut/pasted from the link he posted above:  he can take out the extensions for observing.

 

Clear skies,

Dave

 

 



#5 Jon Isaacs

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Posted 13 November 2019 - 08:56 AM

This was cut/pasted from the link he posted above:  he can take out the extensions for observing.

 

 In that sense it's like the old Caves and Meade RGs that had two sets of mirror holes in the tube.

 

And hopefully it hasn't affected the shape the mirror sufficiently for astrophotography but it is something to be aware of. 

 

BTW. I couldn't get the popup to disappear so I couldn't read much of the text.

 

Jon



#6 dandabson

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Posted 13 November 2019 - 09:31 AM

I have been toying with the idea of modifying a 4.5 StarBlast imaging OTA for DSLR use.  This might be a better route.  Pick up a used 6" and use the modified parts. 



#7 Eddgie

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Posted 13 November 2019 - 12:50 PM

The only negative to this is that the aperture is probably being reduced.

 

These small reflectors usually have woefully small secondary mirrors that are often incapable of illuminating more than a very small field, maybe on the order of 2 or 3mm.

 

For every multiple of the focal length that you move the primary closer to the secondary, you decrease the size of that illuminated field by 1mm.  

 

For example, if the fully illuminted circle is 3mm, and the focal ratio is f/5, them moving the mirror more than 15mm closer to the secondary will mean that you are at the start of aperture reduction.

   

Now none of this matters at all if you are content with the result, but if you have not ray traced it or measured the aperture in any way, then you really don't know what focal ratio you are working at. 

 

Again, that means absolutely nothing if you like the result, but if you want to know if there is a negative consequence to your action, then this would be it. Is it negative enough for you to care?  It seems that you have answered that question for yourself already.  


Edited by Eddgie, 13 November 2019 - 12:58 PM.

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#8 kevinrfrancis

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Posted 13 November 2019 - 08:02 PM

Kevin:

 

Your photo came out nicely.  goodjob.gif

 

My concern with this method is that the mirror is supported at the edges rather at the original support points.  For photography this may not matter but for visual where one is using high magnifications, it will probably affect the view.

 

Jon

I absolutely agre with your observation.  Because the mirror was moved up 25 mm, visual observations are impossible.  The original supports need to be reinstalled for visual use.  This is the only reason I kept them. Ok and to sell the OTA in the future as originallay purchased.



#9 kevinrfrancis

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Posted 13 November 2019 - 08:09 PM

The only negative to this is that the aperture is probably being reduced.

 

These small reflectors usually have woefully small secondary mirrors that are often incapable of illuminating more than a very small field, maybe on the order of 2 or 3mm.

 

For every multiple of the focal length that you move the primary closer to the secondary, you decrease the size of that illuminated field by 1mm.  

 

For example, if the fully illuminted circle is 3mm, and the focal ratio is f/5, them moving the mirror more than 15mm closer to the secondary will mean that you are at the start of aperture reduction.

   

Now none of this matters at all if you are content with the result, but if you have not ray traced it or measured the aperture in any way, then you really don't know what focal ratio you are working at. 

 

Again, that means absolutely nothing if you like the result, but if you want to know if there is a negative consequence to your action, then this would be it. Is it negative enough for you to care?  It seems that you have answered that question for yourself already.  

Eddgie,

 

This is something I will begin to consider and play with different length hold downs.  My original intent was simply to get the DSLR to work without having to purshase a different OTA.  Now with your feedback I can adjust to see if field of view or focal ratio are negatively impacted in my photos.  Thanks for the feedback.



#10 Eddgie

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Posted 14 November 2019 - 10:39 AM

Eddgie,

 

This is something I will begin to consider and play with different length hold downs.  My original intent was simply to get the DSLR to work without having to purshase a different OTA.  Now with your feedback I can adjust to see if field of view or focal ratio are negatively impacted in my photos.  Thanks for the feedback.

Yeah, what you did was fine in that it got you a result that you wanted and there is no other way to describe that than as a win.  

 

Your results speak for themselves. It worked and worked well and you got a great result.   My post was simply intended to say that when you change the geometry of a telescope, you are changing some other characteristic of the instrument as well, and in many cases that change, while undesirable, may not prevent you from getting satisfactory result.  In this case, rather than working at 150mm and f/5, you may have been working at 145mm and f 5.2 but that is not so different than 150mm and f/5 as to be easy to see and your sensor and target size meant that vignetting was not an issue.

 

It is just something to be aware of, that's all.  As the sensors get bigger and scopes get faster, these things can have more impact.


Edited by Eddgie, 14 November 2019 - 10:40 AM.

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#11 sg6

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Posted 14 November 2019 - 01:02 PM

Here a similar operation was reasonably common.

Skywatcher said you could attach a DSLR, so people bought the scope and attached one. The "missing" bit was that you couldn't get the DSLR and image to coincide.

 

Solution was to move the mirror "upwards" and people did. Usually 20-25mm. Then they attached the DSLR and took images.

 

Skywatcher eventually caught on to this idea and produced the 130, 150 and 200 PDS newtonians. Mirror was a bit higher up the tube and they added a slightly bigger secondary. Oh yes, and increased the price.

 

Sounds like you have peeformed a similar action , maybe not exactly identical but similar it would seem.


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#12 kevinrfrancis

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Posted 16 November 2019 - 06:50 AM

Here a similar operation was reasonably common.

Skywatcher said you could attach a DSLR, so people bought the scope and attached one. The "missing" bit was that you couldn't get the DSLR and image to coincide.

 

Solution was to move the mirror "upwards" and people did. Usually 20-25mm. Then they attached the DSLR and took images.

 

Skywatcher eventually caught on to this idea and produced the 130, 150 and 200 PDS newtonians. Mirror was a bit higher up the tube and they added a slightly bigger secondary. Oh yes, and increased the price.

 

Sounds like you have peeformed a similar action , maybe not exactly identical but similar it would seem.

Yeah, Orion ignored my original request for assistance on how to put a DSLR camera on the scope successfully.  That led to this idea and result.  So now I share my results to help others and to keep the cost of this hobby reasonable.


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