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Make focuser or telescope longer?

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#1 bokemon

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Posted 23 February 2021 - 05:59 PM

Hello folks,

I have a seemingly poorly-engineered GSO truss Newtonian.

They designed it such that the focal plane is so far above the focuser that I have to extend the focuser all the way, and then pull my imaging train out another 0.2".

(Camera + filter wheel + oag + 2" coma corrector)  Right now, the coma corrector is held by two thumb screws.

One option is that I can just make some 1" tall aluminum blocks and insert them in where the truss tubes connect to the upper section. (Or the lower section, where it is a little wider)

The other option is to make some kind of adapter that will raise / extend the base of the focuser.  (haven't figured out how yet)

What are the pros and cons of each approach?

BTW, what would be the symptoms of "focuser droop" on images?



#2 Robindonne

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Posted 23 February 2021 - 06:19 PM

Cant you use an extender on the camera side of the focuser? Or play some with the main mirror?

Your telescope is such a widespread piece of metal/carbon among users and multiple brands.  Its probably user error.


Edited by Robindonne, 23 February 2021 - 06:22 PM.


#3 Gazpacho

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Posted 24 February 2021 - 01:06 AM

Can you replace the trusses?



#4 bokemon

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Posted 24 February 2021 - 03:30 AM

Cant you use an extender on the camera side of the focuser?

 

How would I do that when the coma corrector is supposed to slip inside the draw tube?
 

 

Your telescope is such a widespread piece of metal/carbon among users and multiple brands.  Its probably user error.

Sounds like victim blaming.



#5 bokemon

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Posted 24 February 2021 - 03:30 AM

Can you replace the trusses?

no



#6 Jon Isaacs

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Posted 24 February 2021 - 03:48 AM

Imaging Newtonians are normally built with a considerable back focus to accommodate a variety of cameras and equipment.

 

What coma corrector are you using? Do you have it spaced correctly?

 

Is it not possible just to use an extension tube?

 

Jon


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#7 bokemon

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Posted 24 February 2021 - 05:35 AM

I think imaging newtonians are designed to have the coma corrector sit inside the draw tube and almost, but not quite, protrude into the main body.

The coma corrector is a 0.95x Maxfield from TS.

100% it's spaced correctly.  I pulled the whole imaging train out of my other newtonian that was making  pinpoint stars out to the edge.  This telescope also makes (almost) pinpoint stars to the edge, limited by seeing.

Please explain where and how I stick on an extension tube with a non-threaded draw tube.



#8 TxStars

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Posted 24 February 2021 - 05:58 AM

I would go with solid aluminum riser blocks for the truss mounts.

Locate them under the top pole connector brackets on top of the central plate, this would make for a good solid connection.

Check exactly where the focal plane is, you may want (1 1/2") or 2" to give you the option of replacing the stock focuser.

Keeping the corrector out of the light path if you can will be best.


Edited by TxStars, 24 February 2021 - 06:00 AM.

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#9 bokemon

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Posted 24 February 2021 - 07:06 AM

Should have shown pictures at the beginning.

 

IMG_20210224_032512137.jpg

 

This is what it takes to achieve focus on this setup.  Note that the coma corrector is clamped by the two thumbscrews, and I had to purposely pull it out by about a quarter inch.

I feel like those refractor people with their focusers sticking way out, and then extra spacers added.

 

IMG_20210224_032418985.jpg

 

Where should I stick the aluminum blocks?  I was thinking I could just get some aluminum 1" square channel (hollow but thick wall), drill two holes, and get longer screws. 



#10 Robindonne

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Posted 24 February 2021 - 08:04 AM

Is it possible to adjust the mainmirror first? I mean so that the distance to the secondary and camera increases
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#11 Pinbout

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Posted 24 February 2021 - 08:56 AM

Should have shown pictures at the beginning.

 

attachicon.gifIMG_20210224_032512137.jpg

 

This is what it takes to achieve focus on this setup.  Note that the coma corrector is clamped by the two thumbscrews, and I had to purposely pull it out by about a quarter inch.

I feel like those refractor people with their focusers sticking way out, and then extra spacers added.

 

attachicon.gifIMG_20210224_032418985.jpg

 

Where should I stick the aluminum blocks?  I was thinking I could just get some aluminum 1" square channel (hollow but thick wall), drill two holes, and get longer screws. 

the easiest spacer, mechanically, is  a spacer under the focuser.  but you'll have to see if the 2ndry is big enough... but your out that far already... but I'd still check.

 

but if you install spacers under the mirror box truss, you'll still need to look at how that will affect the 2ndry size also. but not a bad idea, keeps the weight away from the front of the scope.

 

so 1" will put your focusing plane somewhere close to 1/2 the focuser travel length?


Edited by Pinbout, 24 February 2021 - 08:57 AM.


#12 Mirzam

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Posted 24 February 2021 - 10:11 AM

Not wishing to blame the victim but this setup looks very wrong.  Usually a focuser must be racked inward to achieve focus with a long imaging train.  Is there any chance the coma corrector is reversed? If not, then if this scope is new I would return it.  The mirror focal length is incorrect.

 

JimC


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#13 rhetfield

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Posted 24 February 2021 - 11:40 AM

My best guess for what the problem might be is that if the entire secondary assembly was somehow flipped over and reattached to the truss, everything would be an inch or two further out from the primary.  You can see in the pictures that the focuser is closer to the truss side than the end.

 

Otherwise the primary mirror is too long of a focal length or the primary mirror is too far forward in the mirror cell.  Sounds like a call to the manufacturer is needed.

 

I am not sure it matters much which end the spacer blocks go to.  I can think of pros and cons for each location.  Probably be pretty much a wash.  My inclination would be to make them beefier and tighter tolerance than you would think are needed though.



#14 Pinbout

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Posted 24 February 2021 - 11:50 AM

or he left it out in the rain, it got wet and shrunk...lol.gif


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#15 TxStars

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Posted 24 February 2021 - 04:09 PM

Ummm

Just to double check..

What were you focused on?

Did you focus on a star ?

 

If this was on a star then yea you will want to lengthen the tube.

 

As I said I would place a solid riser at the lower location near the center of the tube.



#16 Robindonne

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Posted 24 February 2021 - 05:46 PM

The focuser looks to be on the correct position

Attached Thumbnails

  • 4CBFF358-CB73-448A-BCAD-55193406DDF8.jpeg

Edited by Robindonne, 24 February 2021 - 05:48 PM.


#17 bokemon

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Posted 24 February 2021 - 06:46 PM

Is it possible to adjust the mainmirror first? I mean so that the distance to the secondary and camera increases

no, the mirror cell is bolted to the bottom plate.

 

the easiest spacer, mechanically, is  a spacer under the focuser.  but you'll have to see if the 2ndry is big enough... but your out that far already... but I'd still check.

 

but if you install spacers under the mirror box truss, you'll still need to look at how that will affect the 2ndry size also. but not a bad idea, keeps the weight away from the front of the scope.

 

so 1" will put your focusing plane somewhere close to 1/2 the focuser travel length?

That sounds about right.

Well, isn't it the case that the closer the focal plane is to the secondary, the smaller that it needs to be?  And also I am a bit worried that the coma corrector is being vignetted by being so deeply recessed in a drawtube / focuser body.  (haven't measured to see if that would be a problem)

 

Not wishing to blame the victim but this setup looks very wrong.  Usually a focuser must be racked inward to achieve focus with a long imaging train.  Is there any chance the coma corrector is reversed? If not, then if this scope is new I would return it.  The mirror focal length is incorrect.

No, the coma corrector is not reversed.

 

 

My best guess for what the problem might be is that if the entire secondary assembly was somehow flipped over and reattached to the truss, everything would be an inch or two further out from the primary.  You can see in the pictures that the focuser is closer to the truss side than the end.

 

Otherwise the primary mirror is too long of a focal length or the primary mirror is too far forward in the mirror cell.  Sounds like a call to the manufacturer is needed.

 

I am not sure it matters much which end the spacer blocks go to.  I can think of pros and cons for each location.  Probably be pretty much a wash.  My inclination would be to make them beefier and tighter tolerance than you would think are needed though.

No, it's not flipped over.  The secondary is "centered under the focuser" according to a sight tube.
I think other people with this scope have the same issue.  See here:

https://www.cloudyni.../#entry10473841

Now that I think more about it, a better location to put the spacer blocks is at the joint near the secondary.  The reason is that this configuration actually has the center of mass lower.  If I put the spacers near the primary, that shifts all the truss tubes, plus their mounting blocks, upwards.



#18 Mirzam

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Posted 24 February 2021 - 07:02 PM

Can you see the entire mirror when you place your eye at the focal plane?  If not, send this piece of junk back where it came from.

 

JimC


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#19 TxStars

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Posted 24 February 2021 - 07:22 PM

Wow ..(I understand wanting to have back focus for filter wheel and OAG, but this is crazy) https://www.cloudyni.../#entry10473841

 

I would  measure the secondary size to double check field illumination.. http://www2.arnes.si...10/diagonal.htm

 

It will not matter if you put the risers at the top or bottom of the upper truss blocks .

The closer you can get the camera/corrector to the secondary the better  (reducing possible vignetting)

 

I would also rotate the upper portion of the scope to the bottom plate placing the focuser inline with the dec axis. (makes it easier to balance)


Edited by TxStars, 24 February 2021 - 08:06 PM.


#20 MitchAlsup

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Posted 24 February 2021 - 07:43 PM

no, the mirror cell is bolted to the bottom plate.

Is the mirror cell bolted above or below the bottom plate ??

And how thick is the bottom plate ? and the mirror cell bracket ?



#21 bokemon

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Posted 24 February 2021 - 07:47 PM

But it's Sky and Telescope's HOT PRODUCT for 2016!!!!1!11!


Edited by bokemon, 24 February 2021 - 07:47 PM.


#22 xiando

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Posted 24 February 2021 - 07:49 PM

If someone hasn't already said it, if you own a newt astrograph (ie a newtonian intended for use as an imagine platform) whether or not you use a coma corrector, if you wan want to *view* rather than photograph through the scope, you'll need an extension tube added to the focuser to recover the missing path length.

 

For instance, my 610mm fl 6" aperture F4 astrograph requires the addition of a 2" extension tube if I want to use eyepieces instead of a camera. That is, if I want to achieve focus...

 

(same btw, for my little 80mm celestron refractor, where I use a  diagonal to achieve the necessary path length.


Edited by xiando, 24 February 2021 - 07:54 PM.


#23 bokemon

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Posted 24 February 2021 - 07:57 PM

Is the mirror cell bolted above or below the bottom plate ??

And how thick is the bottom plate ? and the mirror cell bracket ?

I hope you're not about to ask if I can bolt the mirror plate to the outside...



#24 TxStars

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Posted 24 February 2021 - 08:02 PM

The focal plane is the focal plane does not matter if it is a camera or eyepiece.

For what ever reason they decided to make the scope this way.

Now the customers have to rebuild them to work as an imaging scope.

 

btw Is this the 250mm?   correct?


Edited by TxStars, 24 February 2021 - 08:05 PM.


#25 Gazpacho

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Posted 24 February 2021 - 08:14 PM

My instinct would be to replace the visual back on the focuser tube with a Baader Clicklock.




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