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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.
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.
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.
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.
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.
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
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.
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.
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.
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