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GSO 6” f/4 Newton
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GSO 6” f/4 Newton by Gianluca Rossi
I decided to buy a second hand Newton as I needed a scope bigger than my 90 mm Apo refractor. Most of the time I follow variable stars and record star spectra from my small balcony at home so a bigger telescope would enable me to detect and study fainter targets as well as to get a bigger SNR of the stars of my observing program at the same time. I came across an ad from an amateur that was selling his GSO 6” f/4 Newton at a reasonable price so I decided to get this scope which appeared lightweight and with a tube short enough to be put on my small balcony without the risk of hitting the railing.
At first sight the scope needed some cleaning but overall it was in pretty good conditions. The telescope has a short steel tube, the length is about 50 cm, and it is equipped with a power linear focuser from GSO. I soon realized that although pretty good for visual observing the focuser would not be able to hold the 2.5 Kgs of my equipment (Paracorr, filter wheel with OAG and camera) so I decided to use a feather touch focuser. I also realized that the mechanics of this scope is inadequate to get the most out of it so I decided to make some modifications.
First the focal plane of this scope is placed very far away from the tube. In my unit it originally laid at about 187 mm above the tube wall. If we use the Texerau’s formula to calculate the fully illuminated field:
d = ((D-b)a/F) + b
where d = size of secondary mirror
D = diameter of primary mirror
a = distance of focal plane to secondary
b = diameter of fully illuminated field
F = focal length
we can see that the 62.5 mm secondary mirror that comes with this scope is too small and in fact it delivers no fully illuminated field even on axis (i.e. you work with a smaller aperture than 6”). Besides at such a big distance from the tube flexure of the imaging train is likely to take place. In my opinion Newtonian reflectors should be equipped with low profile focusers to avoid using large diagonals and to get the imaging train as close as possible to the tube.
Other things to fix are the mounting of both mirrors. The primary mirror cell has clips that are very tight and push the mirror from the top causing astigmatism and even potential damage to the surface of the mirror and the back supports are placed in the wrong positions and are made of wrong material. The secondary mirror is put into a plastic holder that surrounds it and it is held in place with bi-adhesive tape that is likely to sag off over time (read further). There is also a metal retaining clip that is pushed very strongly against the mirror itself causing stress and potential damage. I have also found that the placement of the center spot of the primary mirror was off by about 1.5 mm which is well beyond the tolerance of an f/4 mirror and though the blackening of the interior of the tube can be acceptable under dark skies it is a good idea to flock the tube particularly if you plan to observe mainly from cities as I generally do.
To keep cost at an acceptable level and to simplify the project I have actually lengthened the scope with an extension tube of 85 mm using a piece of steel tube that I secured to the original one with tightened bolts. Though it may not be found aesthetically pleasing by some people this solution is cheap and works great with no flexure at all. The primary mirror is now placed at the right distance from the diagonal to deliver a fully illuminated field of 22 mm at the eyepiece. The aperture of the Paracorr Type 2 that I use is now fully illuminated.
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To fix the primary mirror cell I have first used 3 edge supports made of hard plastic that do not touch the mirror. The three clips have nylon grains that are the only points that touch very gently the mirror at its center of gravity to avoid any pinch. Though in theory 4 clips placed 90 degrees apart should be used I have found that 3 clips are adequate enough for a 150 mm mirror. As back supports I have used 3 felt pads placed at about 0.55x radius from the center of the mirror after removing the orginal cork supports that do not let the mirror rotate freely in its cell.
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Unfortunately I experienced an accident with the secondary mirror as it accidentally detached itself by its holder and fell onto the primary mirror as the bi-adhesive tape sagged off all of a sudden. I had to replace the diagonal while the primary got a mark measuring about 5x0.2 mm and with a depth that I estimated to be about 0.1 mm or so. Luckily that is not a chip but only a mark that did not alter the figuring of the mirror and did not affect the image quality at all. I have also double checked image quality visually at high power to watch close doubles and the planets.
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To avoid additional disasters and to avoid pinching the new secondary I removed three parts 120 degrees apart of the plastic holder surrounding the mirror and used 3 small dabs of silicone on the back of the secondary to secure it at its holder. This solution works very well.
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To correctly center the center mark of the primary mirror I have used an acetate template with the correct position to which I have attached the center mark. After removing the center spot from the primary I had to use some acetone to remove glue residues.
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Finally to flock the tube I have used some flocking material by Scope Stuff that is much blacker than the original painting and does not sag off over time
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Performance
Under this condition the GSO 6” f/4 becomes a real astrograph. I really get round stars across the frame of my QHY268 Mono camera that has small pixels and a diagonal of 28.4 mm. Images of deep sky objects are very pleasing and I can effectively use this scope for many science observations including exoplanet transits and spectra of many variable stars and peculiar objects such as novae.
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Fig. 10 Nebula IC 443 in Gemini
taken with the modded GSO 6” f/4 Newton through an H-alpha filter from the city
of Rome and moon above horizon.
Top left | Top right |
Bottom left | Bottom right |
Fig.11 Stellar field of variable star ZZ CAS taken through a Sloan I filter and 60 second exposure with the modded GSO 6” f/4 Newton and the QHY268 Mono. Shown are images of the four corners of the frames at full resolution with no elaboration nor calibration
Fig. 12 Image of the spectrum of
Nova Cassiopeiae 2021 at around 11th magnitude taken on July 22nd
2022 with the modded GSO 6” f/4 Newton and a star analyser 200 in grism
configuration showing many emission lines. The spectrum appears as a bright
line with many bright dots that are the actual emission lines of the nova
(mainly hydrogen and helium). The stars appear elongated as an effect of the
prism used
Fig. 13 Light curve of the secondary eclipse of variable star V2477 CYG at around 10th magnitude
Fig. 14 Light curve of exoplanet EPIC-211089792b in the constellation of Taurus. The parent star is a magnitude 12.5 star.
Conclusions
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. To summarize, the modifications to make are the following ones:
· use a longer tube or an extension as I have done to recover the fully illuminated field needed for both astrophotography and visual observations and to reduce the risk of flexure of the imaging train
· replace the original focuser with a good low profile focuser such as a feather touch or a moonlite focuser
· use different edge and back supports in the primary mirror cell to avoid pinching the mirror
· use a different support or modify the original holder of the secondary mirror to avoid pinching the mirror
· check the position of the center spot of the primary mirror and adjust it if needed
· flock the interior of the tube with black velvet or flocking paper to maximize contrast
I would also recommed using a good coma corrector such as the Televue Paracorr type 2 and good collimating aids such as the tools offered by Catseye collimation system to ensure good collimation.
- Bob Campbell, John Miele, dvb and 28 others like this
50 Comments
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.
Thanks for sharing. Pics look great.
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
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
these things should come adjustable lenght with quick change secondaries to allow easier customization
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!
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
Looks like you did what I did:
https://www.flickr.c...157665075332593
I also glued the primary.
(:
Steve
Nice! I replaced the cork in mine with computer thermal pads, it worked great and is very tacky
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.
"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
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.
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?
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.
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.
Can you really determine an exoplanet transit with a 6”F4? Are you using the QHY268 for this task?
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.
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.
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.
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.
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.
Very impressive. I was thinking of buying a 6" reflector. Now, I'm not sure it would be wise. I don't think I'm capable of these types of modifications. Does anyone know of any 6" reflectors on the current market that are made properly? This kind of dashed my hopes. I'm at a point of life where everything has to be lightweight. Just purchased an iOptron GEM28 which seems very accurate, Smooth and quiet as well as very well made. I was thinking a 6" reflector would be perfect. But, thanks for such a well documented approach to correcting the design. Your photos are amazing.