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Newtonian Focusing

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#1 JoeMom

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Posted 24 January 2020 - 10:09 PM

The focus knob can’t go far enough to reach focus with a DSLR. What if I just moved the primary mirror forward? I have an f/5, 5 inch newt and an AVX mount.

Edited by JoeMom, 24 January 2020 - 10:11 PM.


#2 J A VOLK

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Posted 24 January 2020 - 10:37 PM

that is commonly done...


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#3 ks__observer

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Posted 24 January 2020 - 10:50 PM

Interesting response re moving mirror forward.

You might lose a little light if the secondary no longer reflects entire optical cone -- so you might operate at say f6.



#4 JoeMom

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Posted 24 January 2020 - 11:06 PM

Videos or instructions online on how to do this? The ratio would actually be less than 5 because the length is decreasing as the mirror is moved forward in the tube. Right?



#5 J A VOLK

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Posted 24 January 2020 - 11:15 PM

The f# (and focal length) is set by the primary mirror shape and diameter.  If the secondary is undersized it causes vignetting, or darkening of the outer part of the image.  Assuming you only need to move the mirror 1/2" or so I doubt much if any additional vignetting would be noticeable, there could be none depending on what size secondary was used.  A more expensive, complex option is to install a lower profile focuser.   If you want to assess if there is any vignetting you can use this app and consider the size of the sensor.  Slight vignetting can be corrected by software.

 

http://www.bbastrode...om/diagonal.htm


Edited by J A VOLK, 24 January 2020 - 11:22 PM.


#6 TelescopeGreg

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Posted 24 January 2020 - 11:50 PM

The focus knob can’t go far enough to reach focus with a DSLR. What if I just moved the primary mirror forward? I have an f/5, 5 inch newt and an AVX mount.

Before you mess with the mirror, first take care of any coma that the Newt almost surely has.  I added a Paracorr-2 corrector to my 8" f/5, and its 15% Barlow effect took care of the back-focus problem, as well as the scope's coma.



#7 ks__observer

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Posted 25 January 2020 - 12:14 AM

The f# (and focal length) is set by the primary mirror shape and diameter.  If the secondary is undersized it causes vignetting, or darkening of the outer part of the image.  Assuming you only need to move the mirror 1/2" or so I doubt much if any additional vignetting would be noticeable, there could be none depending on what size secondary was used.  A more expensive, complex option is to install a lower profile focuser.   If you want to assess if there is any vignetting you can use this app and consider the size of the sensor.  Slight vignetting can be corrected by software.

 

http://www.bbastrode...om/diagonal.htm

If the secondary is undersized it wont reflect the entire aperture lihht cone -- reducing aperture.



#8 JoeMom

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Posted 25 January 2020 - 12:18 AM

Could I get extreme coma and vignetting if I moved the mirror forward? Reducing my aperture isn’t the biggest deal because I’m planning on doing some exposures.

#9 ks__observer

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Posted 25 January 2020 - 12:25 AM

Less coma if you cut the light path from outer optical cone.  Less off axis area.



#10 TOMDEY

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Posted 25 January 2020 - 12:42 AM

As-built, your scope is not properly configured for direct prime-focus imaging. Moving the PM up the tube and replacing the folding flat with a right-sized one would make sense. At F/5, a Coma Corrector is also needed. Some/many commercial Newts, for some reason, have focusers that are WAY too tall / too far back from the tube. Swapping the existing focuser for a shorter one might be the easiest remedy. The usual assumption that your mirrors are otherwise good wavefront. ~Solutions~ that involve clipping light reaching the camera are ... amateurish. But, come to think of it, that defines us!

 

Most direct remedy --- replace the focuser with a good shorter one.    Tom



#11 TelescopeGreg

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Posted 25 January 2020 - 12:56 AM

As-built, your scope is not properly configured for direct prime-focus imaging. Moving the PM up the tube and replacing the folding flat with a right-sized one would make sense. At F/5, a Coma Corrector is also needed. Some/many commercial Newts, for some reason, have focusers that are WAY too tall / too far back from the tube. Swapping the existing focuser for a shorter one might be the easiest remedy. The usual assumption that your mirrors are otherwise good wavefront. ~Solutions~ that involve clipping light reaching the camera are ... amateurish. But, come to think of it, that defines us!

 

Most direct remedy --- replace the focuser with a good shorter one.    Tom

I tried replacing the focuser.  Actually, the focuser was already replaced by a Moonlite unit, so I tried a thinner mounting shim.  That fixed the focus for the DSLR, but then I couldn't get the eyepieces to work (not enough outward reach), and I still had coma.  The Paracorr solved all of the issues, and I left the mirrors alone.  It's not cheap, but it worked very very well.


Edited by TelescopeGreg, 25 January 2020 - 12:57 AM.

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#12 cwm1091

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Posted 25 January 2020 - 12:58 AM

JoeMom,

 

Before you go mucking around with the insides - and possible screwing up the finely-tuned optics -  of your telescope, I suggest that you try reaching focus with via “Barlow Projection.” Based on an exchange of e-mails with Robert at TelescopeAdapters.com, I just bought a T-Threaded Barlow Lens (their item #BAR1), which attaches directly onto the T-Ring of my Canon camera. [If I didn’t already have a T-Ring, I would have had to buy the Canon EOS T-Ring (#TEOS) for my camera. You’ll need a T-Ring specific to your camera brand.] I just received the Barlow in the mail yesterday and admit that I haven’t tried it yet, but I assume that it’ll do what is required. (I could let you know in a few days if I succeed, or don’t). Before I spent the money on the Barlow, I looked around on YouTube and found a video of a guy talking about the problem you describe and demonstrating how to solve it with a Barlow lens. Here is the link: https://youtu.be/reFxoF3XoaU . You might also check out https://youtu.be/P47A97WlCSw . There is no voice-over for this video, but note the first configuration, which he calls prime focus, involves a Barlow lens. Hope this helps.

 

Chris



#13 TelescopeGreg

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Posted 25 January 2020 - 01:13 AM

JoeMom,

 

Before you go mucking around with the insides - and possible screwing up the finely-tuned optics -  of your telescope, I suggest that you try reaching focus with via “Barlow Projection.” Based on an exchange of e-mails with Robert at TelescopeAdapters.com, I just bought a T-Threaded Barlow Lens (their item #BAR1), which attaches directly onto the T-Ring of my Canon camera. [If I didn’t already have a T-Ring, I would have had to buy the Canon EOS T-Ring (#TEOS) for my camera. You’ll need a T-Ring specific to your camera brand.] I just received the Barlow in the mail yesterday and admit that I haven’t tried it yet, but I assume that it’ll do what is required. (I could let you know in a few days if I succeed, or don’t). Before I spent the money on the Barlow, I looked around on YouTube and found a video of a guy talking about the problem you describe and demonstrating how to solve it with a Barlow lens. Here is the link: https://youtu.be/reFxoF3XoaU . You might also check out https://youtu.be/P47A97WlCSw . There is no voice-over for this video, but note the first configuration, which he calls prime focus, involves a Barlow lens. Hope this helps.

 

Chris

A 2x Barlow will get you to focus -  I did what's shown in the first video too, as my first solution.  But that doubles your focal length, which messes up pretty much everything else unless you're going after planetary images. 

 

As a proof of concept, sure.  Give it a shot.  But you will quickly not like taking 2x longer for each image, having tacking / guiding issues, and the loss of field-of-view.


Edited by TelescopeGreg, 25 January 2020 - 01:14 AM.


#14 RJF-Astro

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Posted 25 January 2020 - 03:03 AM

There is also a GSO coma corrector with a slight barlow effect, I think 1.1. If the focus is close, this might be enough to get it into focus.


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#15 JoeMom

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Posted 25 January 2020 - 07:38 AM

Well, I already have a c6 so a Barlow won’t help because I’ll just end up at f/10 again. I am considering selling the scope for a focal reducer for the c6. Unless I really wanted the shorter focuser.

#16 kathyastro

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Posted 25 January 2020 - 12:16 PM

In order, try these solutions:

 

1. Get a coma corrector.  You need one anyway, and many of them will correct the back focus problem.

 

2. Replace the focuser with a lower profile focuser.  Measure how much extra back focus you need before you spend the money, and pick a model that will give you what you need.

 

3. Move the mirror.


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#17 Alex McConahay

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Posted 25 January 2020 - 12:29 PM

If the low profile focuser is too low for eyepieces, consider a simple extender on the focuser barrel, a set of extenders fitted to all your eyepieces, and parfocalizing rings (and not inserting the eyepieces all the way.

Alex
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#18 JoeMom

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Posted 25 January 2020 - 03:55 PM

Oof. I’m just gonna get a focal reducer for my sct. It’s not a perfect solution to help the scope’s speed, but it will work for now. This project seems more expensive than I thought it would. The coma correctors and low profile focusers are more money than I spent on the Newtonian it self. I could just buy a new refractor for that kind of money. Thanks for your help.

Edited by JoeMom, 25 January 2020 - 04:03 PM.


#19 TelescopeGreg

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Posted 26 January 2020 - 02:57 AM

Oof. I’m just gonna get a focal reducer for my sct. It’s not a perfect solution to help the scope’s speed, but it will work for now. This project seems more expensive than I thought it would. The coma correctors and low profile focusers are more money than I spent on the Newtonian it self. I could just buy a new refractor for that kind of money. Thanks for your help.

Well, yes.  But I got the new focuser because the original one was junk, not for the spacers.  And the coma corrector was needed because it's a Newtonian, and the fast ones just need it.  The focus fix kind of came for free (after spending all that money smile.gif ).  So, in the end I ended up spending about 3x in add-ons over what the OTA itself was worth.  But also in the end I had a pretty nice 8" f/5.6 telescope, with nice stars out to the edges of my APS-C DSLR's sensor, and I think I'd be hard pressed to get a large fast refractor for that kind of money.


Edited by TelescopeGreg, 26 January 2020 - 02:58 AM.



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