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GSO 6” f/4 Newton

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#1 Gianluca67

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Posted 01 October 2022 - 05:06 AM

The GSO 6” f/4 Newton can become a real astrograph capable to deliver beautiful images of the night sky and to enable the observer to do science in the field of photometry and spectroscopy but it definitely needs some modifications as the mechanics of this scope is totally inadequate to boost its potential. All the modifications I have made turned a mediocre instrument into an amazing scope.

Click here to view the article
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#2 Zombeaver

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Posted 01 October 2022 - 07:36 AM

Nice improvements.

#3 careysub

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Posted 01 October 2022 - 12:08 PM

I have the GSO 8" f/4 Newtonian and yes, the focal plane is way out on this one also. I will have to put a possible rebuild like this in my future projects queue.

 

I don't do astrophotography at all, just regular visual and nigh vision enhanced (the main reason for getting this scope). For now I just have time to use it, not rebuild it.



#4 iseegeorgesstar

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Posted 01 October 2022 - 03:30 PM

Thanks for sharing. Pics look great.



#5 Gianluca67

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Posted 02 October 2022 - 05:59 AM

Incidentally I have just finished a similar rebuild of the GSO 8" f/4 Newton that I have recently bought. The rebuild is sligthly different. The scope originally delivers about 2 mm of fully illuminated field so if you lengthen it by 80 mm you should get about 26 mm of fully illuminated field at the new focal plane. All eyepieces you can use with this scope are then fully illuminated (you probably have a front lens of about 25 mm in diameter for say 20 or 25 mm FL eyepieces) and you can effectively use the entire 200 mm of aperture over the whole FOV. The mechanics of the 8" is even poorer than that of the 6" given that the mirrors are bigger and heavier. Pay particular attention to the push and pull screws of the primary mirror cell. The cell has threaded holes and 3 of the 6 screws can hit the back of the mirror causing stress and even crack it if you push them too much (it is easy not to realise how easy it is). I am going to use this scope for a while before writing a review but if you need some tips send me a private message

 

 

I have the GSO 8" f/4 Newtonian and yes, the focal plane is way out on this one also. I will have to put a possible rebuild like this in my future projects queue.

 

I don't do astrophotography at all, just regular visual and nigh vision enhanced (the main reason for getting this scope). For now I just have time to use it, not rebuild it.



#6 John Miele

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Posted 02 October 2022 - 01:58 PM

Wow! Not only a very good review but what a slew of talents you put on display. Imaging spectroscopy, photometry, and ATM skills. You have inspired me to do more!

 

cs...John


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#7 rgk901

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Posted 02 October 2022 - 02:08 PM

I've noticed this as well... guessing to allow dslr back focus? and to have shorter/ lighter tubes one has to stick the optical train out a foot instead lol...

these things should come adjustable lenght with quick change secondaries to allow easier customization ;)

#8 e1fo

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Posted 03 October 2022 - 11:10 PM

I have the Orion version of the same scope and have been looking for solutions to fix the poor mirror cell and focal plane height off of the OTA. I was looking into getting a longer tube and moving everything over to that but I really like your solution of simply adding an extension tube to the end. I'd be really interested in hearing what tubing you ended up using as well as more details on the mirror cell modifications.

 

Cheers!



#9 Gianluca67

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Posted 04 October 2022 - 08:48 AM

As an extension tube you can effectively use either a piece of aluminium or steel. As the extension tube has to be secured into the original tube it needs calendering. The operation is not difficult for a local smith, or one that has a lathe.

For the primary  mirror cell the most important thing is to support the edge of the mirrors only with 3 contact points at the center of gravity of the mirror and use felt pads to support the mirror from the back (use GUI PLOP to find the best back support positions, but you have some mechanical contraints with the GSO cell, and Dobsonian mirror edge support calculator to find the position of the center of gravity of the mirror). Those measures are not critical however given that the mirror is small. The retaining clips must not touch the mirror and it is a good idea to replace them with either hard plastic or metal (again ask a local smith/lathe operator). The mirror is properly mounted in the cell when you can rotate it with your hands with no strain but it does not shift laterally.

Hope this helps

 

Cheers


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#10 StevenBellavia

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Posted 04 October 2022 - 06:32 PM

Looks like you did what I did:

 

https://www.flickr.c...157665075332593

 

I also glued the primary. 

 

(:

 

Steve



#11 calypsob

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Posted 04 October 2022 - 08:19 PM

Nice!  I replaced the cork in mine with computer thermal pads, it worked great and is very tacky


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#12 Gianluca67

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Posted 05 October 2022 - 12:00 PM

Have you checked star images after glueing the primary mirror? Some years ago I experienced bad results after glueing an 8" mirror under the effect of temperature drop during the night so I had to remove the silicone and modified the cell.

 

 

Looks like you did what I did:

 

https://www.flickr.c...157665075332593

 

I also glued the primary. 

 

(:

 

Steve



#13 Bearcub

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Posted 05 October 2022 - 01:04 PM

"Fig. 5 Image through a magnifier showing the correct position (black circles) and the orginal wrong placing of the collimating spot by GSO"

 

That is quite scary if it affects contrast and image sharpness. Its quite big difference in error there. But I wont even try measuring mine yet. Nice review though :)



#14 unimatrix0

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Posted 05 October 2022 - 07:08 PM

Wow! Excellent. I'm guessing all these branded Newtonians (Orion, GSO, Skywatcher) are coming from same factory with different branding. 

 

I have the 6" Orion and I struggled plenty to collimate it, but no wonder why.  I eventually figured it out. The center donut spot on the primary is not centered and the secondary was misaligned. I can imagine any rookie receiving the telescope as it is and tries to figure out what he/she is doing wrong. That's why I can't recommend these scopes to anyone new to astronomy viewing/photography, unless the person has enough experience and knowledge to notice the problems.    
I also removed the primary and I used a textile tape used by car mechanics/factory (you can see these if you ever removed your car radio) and taped inside the primary mirror holder clips. I also loosened the clip holder screws (they were full tight) to allow the mirror to be loose enough to rotate but not move sideways . 

 

I also replaced the focuser, which was substandard in many ways, even worse than the GSO focuser, with no counter roller inside so tightening the focus knob would produce a tilt. 
In a summary, a giant mess as they come out of the factory. 

 

I added an aperture ring also, essentially cutting down the diameter of the primary by a few millimeters around, to remove extra spikes used by the mirror holder notches.  I 3D printed an appropriate size and just stuck it on top of the notches. 

Another worthy upgrade is to replace the springs under the primary's collimation screws to longer ones, because it was a common problem that I "ran out" of spring load while collimating. 
I understand the budget nature of things, but it almost feels like that the designer of these scopes new what he was doing, but somewhere down the line to the factory worker, the translation was lost and the accuracy have gone out the window. 



#15 tomb1

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Posted 07 October 2022 - 01:15 PM

I wonder if these scopes have changed over the years or if there is substantial difference between models.  I have the Apertura model and I see a few differences.  First there is several more inches of tube length between the front end and the spider than I see in the above pictures.  Second, the secondary holder contains no plastic and is quite securely attached to the secondary via high bond tape.  The primary is secured with rubber hold-downs.

 

A little off-subject here but one curious thing I noticed while attempting to use a 3d printed aperture ring around the primary.  I ended up with triangular stars!  No tension on the primary from the hold-downs.  What would cause this?  Any ideas?   



#16 calypsob

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Posted 09 October 2022 - 03:21 PM

Have you checked star images after glueing the primary mirror? Some years ago I experienced bad results after glueing an 8" mirror under the effect of temperature drop during the night so I had to remove the silicone and modified the cell.

You have to shim the mirror if you glue it, the glass compresses the blobs of silicone but you are supposed to let it rest on popsicle sticks while it cures then remove the popsicle sticks. If you let it dry on glue with no shims it is going to create horrible astigmatism when it cures.


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#17 Gianluca67

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Posted 10 October 2022 - 06:02 AM

Yes, newer models have a longer tube above the spider vanes perhaps to prevent stray light from entering the focal plane but it is a nonsense as the original focal plane is still left so far away from the tube wall. The mounting of mirrors is poor, particularly the primary mirror that should only be slightly touched with edge supports at the center of gravity of the mirror to avoid pinching it. It is a very bad idea to use rubber hold-downs as the retaining clips push the mirror enough to spoil the images. I cannot answer for the ring aperture you have made but for sure the only way to get round stars is to use proper edge and back supports and avoid any other contact with the mirror.

 

 

I wonder if these scopes have changed over the years or if there is substantial difference between models.  I have the Apertura model and I see a few differences.  First there is several more inches of tube length between the front end and the spider than I see in the above pictures.  Second, the secondary holder contains no plastic and is quite securely attached to the secondary via high bond tape.  The primary is secured with rubber hold-downs.

 

A little off-subject here but one curious thing I noticed while attempting to use a 3d printed aperture ring around the primary.  I ended up with triangular stars!  No tension on the primary from the hold-downs.  What would cause this?  Any ideas?   


Edited by Gianluca67, 10 October 2022 - 03:13 PM.


#18 Chad7531

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Posted 10 October 2022 - 06:06 AM

Wow! Excellent. I'm guessing all these branded Newtonians (Orion, GSO, Skywatcher) are coming from same factory with different branding.

I have the 6" Orion and I struggled plenty to collimate it, but no wonder why. I eventually figured it out. The center donut spot on the primary is not centered and the secondary was misaligned. I can imagine any rookie receiving the telescope as it is and tries to figure out what he/she is doing wrong. That's why I can't recommend these scopes to anyone new to astronomy viewing/photography, unless the person has enough experience and knowledge to notice the problems.
I also removed the primary and I used a textile tape used by car mechanics/factory (you can see these if you ever removed your car radio) and taped inside the primary mirror holder clips. I also loosened the clip holder screws (they were full tight) to allow the mirror to be loose enough to rotate but not move sideways .

I also replaced the focuser, which was substandard in many ways, even worse than the GSO focuser, with no counter roller inside so tightening the focus knob would produce a tilt.
In a summary, a giant mess as they come out of the factory.

I added an aperture ring also, essentially cutting down the diameter of the primary by a few millimeters around, to remove extra spikes used by the mirror holder notches. I 3D printed an appropriate size and just stuck it on top of the notches.
Another worthy upgrade is to replace the springs under the primary's collimation screws to longer ones, because it was a common problem that I "ran out" of spring load while collimating.
I understand the budget nature of things, but it almost feels like that the designer of these scopes new what he was doing, but somewhere down the line to the factory worker, the translation was lost and the accuracy have gone out the window.

Two factories. GSO makes GSO, Apertura, Zhumell, and some of Orion’s scopes. Synta makes Skywatcher, Celestron, and some of Orion scopes.

Edited by Chad7531, 10 October 2022 - 06:07 AM.

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#19 mattyfatz

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Posted 10 October 2022 - 09:19 AM

Can you really determine an exoplanet transit with a 6”F4? Are you using the QHY268 for this task?



#20 Gianluca67

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Posted 10 October 2022 - 03:10 PM

Yes, you can do demanding photometry such as exoplanet transits with the modded GSO 6" f/4. I have used the QHY268 for the one illustrated in the review but in the past I was able to determine transits even with my Megrez 90 and an Atik 383L. Such measures require some practice in photometry (I have collaborated with the AAVSO for 20 years now) and a careful choice of comparison stars but are not so difficult after some practice. Of course a scope that delivers truly round stars helps and the modifications I have made helped me a lot in this task.

 

 

Can you really determine an exoplanet transit with a 6”F4? Are you using the QHY268 for this task?


Edited by Gianluca67, 10 October 2022 - 03:12 PM.


#21 StevenBellavia

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Posted 13 October 2022 - 06:56 PM

Have you checked star images after glueing the primary mirror? Some years ago I experienced bad results after glueing an 8" mirror under the effect of temperature drop during the night so I had to remove the silicone and modified the cell.

Yes.  It was much better after the gluing. And yes, popsickle sticks, or wood coffee stirrers, which are a little thinner.  But 6-inch is also much more forgiving than 8-inch. The 8-inch I built uses clips.  I didn't trust the glue for that weight.  The ideal thing is an R.F. Royce mirror with the conical shape and threads built into the center of the glass.  But sadly,  he retired.



#22 Gianluca67

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Posted 16 October 2022 - 08:50 AM

I fully agree with you about conical mirrors. I really don't understand why they have always been the choice for Schmidt-Cassegrains an RCs but not for Newtonians. I know that RF Royce has retired but I seem to recall that telescope service now offers conical 10" and 12" mirrors for Newtons. They are a much better choice over classic mirrors as they keep collimation, deliver no additional spike effects on stars, are lighter and reach thermal equilibrium much faster.

 

Yes.  It was much better after the gluing. And yes, popsickle sticks, or wood coffee stirrers, which are a little thinner.  But 6-inch is also much more forgiving than 8-inch. The 8-inch I built uses clips.  I didn't trust the glue for that weight.  The ideal thing is an R.F. Royce mirror with the conical shape and threads built into the center of the glass.  But sadly,  he retired.



#23 Old Speckled Hen

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Posted 27 October 2022 - 09:57 AM

Quote "As the extension tube has to be secured into the original tube it needs calendering."

 

?? I use a calendering machine to regulate the density of lithium battery cell foils.. could you perhaps explain the above slightly further please.

 

thank you.



#24 Gianluca67

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Posted 03 November 2022 - 03:06 PM

Quote "As the extension tube has to be secured into the original tube it needs calendering."

 

?? I use a calendering machine to regulate the density of lithium battery cell foils.. could you perhaps explain the above slightly further please.

 

thank you.

 

To get the extension tube you need to bend the aluminium or metal sheet so that it can enter the original tube from one end and get the same diameter of the original tube at the other end so that you can mount the mirror cell there. To accomplish this, the extension tube has not to be exactly a cylinder but it must have a slightly conical shape. You can either use a calendering machine or a lathe or have it done by someone that uses those machines as I have done.

Hope this clarifies further.


Edited by Gianluca67, 04 November 2022 - 09:38 AM.


#25 TareqPhoto

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Posted 04 November 2022 - 06:07 PM

I really wish to modify my GSO 6" F/4 like what you did to make it perfect, one day i will start to modify it one by one, mainly the secondary mirror because it failed in collimation, and i don't want to add extensions or Frankenstein the body itself, but the flocking inside is a great idea, if i know how to remove parts first and then find a good long lasting flocking material.




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