8" F4 Quattro and GSO newts on M33 first light review

[MartinB]

Steve (FLO) has very kindly loaned me a brand new 8" carbon fibre Skywatcher F4 Quattro for an imaging try out. This scope is designed primarily for imaging, having a fast parabolic mirror with an enlarged secondary moved down the tube to ensure plenty of back focus and a wide field of illumination.

About 3 years ago I bought an F4 GSO newtonian with similar design features which I have hardly used and was interested to compare the 2 scopes.

For these first images I was also giving my SX M26 it's first run. I wanted to try this camera rather than my usual mono camera to see how well the scopes illuminated the APS sized sensor. Fast Newtonians like this will inevitably be affected by coma. Fortunately there is the moderately priced Baader Multi Purpose Coma Corrector to address this problem. Spacing is critical. I needed to use some shims to get the 55mm spacing between chip and the back end of the focuser and it still wasn't quite right. I'm sure a bit more tweaking with the shims would have got things spot on.

The Quattro is certainly an impressive looking package with is black carbon tube and white tubes and fittings. Closer inspection suggests it is also well put together and impressively light meaning that the scope will be manageable for mounts such as the HEQ5. The tube is has substantial baffling and some people have suggested that this will cause excessive vignetting and increased air currents within the scope, I particularly wanted to check this out.

Collimation is particularly critical with a fast scope of this type. Collimation guru, TJ, identified a very small rotational error with the secondary which he corrected. After this tweak it was a 5 minute job to get a good collimation with my Hotech collimator. Rotating the laser around the draw tube saw the beam stay within the small centre circle on the primary mirror suggesting that the focuser was accurately mounted on the tube.

I was using imaging in my dome observatory so don't need to use dew shields but if working outside I would definitely recommend one since the oversized secondaries of this type of scope can be vulnerable. If using a dew shield I have been advised that a 10" shield with packing around the end of the tube would be less likely to induce vignetting.

At the end of a nights imaging the collimation was still spot on. However, I would recommend collimation prior to every session.

The Quattro also has the steel tube for £200 less than the carbon. The advantage of carbon is weight and thermal stability. Working under a dome scopes aren't subject to the same temperature variations as they are outside radiating to the sky.

My 8" F4 GSO had one or two outings when I first got it and things didn't run smoothly. The primary was a nightmare to collimate and didn't stay in position. Bob's Knobs came to the rescue with their much higher quality, more robust primary springs, along with collimation screws. With these fitted collimation was much smoother (at a cost of a little over £100). However I then had a couple of evenings of dewed up secondary (pre dome days) and I susequently got an FSQ 106!

Although not as light as the Quattro it is still no heavyweight. Fit and finish are fine although the aesthetics don't match the Quattro. Mine came with a Baader Steeltrack focuser which seems to be of good quality. The tube rings are flimsier than the Quattro and I experienced a little differential flexure using my side by side set up. This was almost certainly down to the rings since it never occurs with other scopes. An OAG would be a good option.

Following TJs advice I had a good look at the secondary position and was able to give it a little tweak. Adjusting the tilt wasn't quite as smooth as the Quattro but things came into line without too much trouble and the primary is very smooth with Bob's springs. Following the imaging session the primary had moved very fractionally, I'm pretty sure this was the result of me loosening off the primary springs excessively.

Both scopes had smooth 2 speed focusers although I preferred the feel of the Baader on the GSO. Focusing was very straightforward with a Bahtinov mask. I was imaging during a warm evening with very little temperature drop. I didn't need to adjust focus with either scope. A cold winters night outside would be a different matter.

The SX M26C is a OSC APS 10mega pixel camera with a pixel size of 6 microns. Ideal for small refractors. Unfortunately, being OSC and having small pixels it isn't the most sensitive of chips.

Below I have posted the combined data using both scopes and couldn't resist adding some old Ha taken with my QSI 532 and FSQ. This is 28 x 15 mins. The image is cropped because I made the mistake of putting the camera at a different orientation for the GSO run! Then there is 14x15 mins with the Quattro and the same with the GSO. On both nights the air was thick with moisture pulling up loads of light pollution which affected the images despite an IDAS LP filter. Finally I've shown the master flats captured with both scopes to show some idea of the vignetting. Both have received an identical stretch.

Captured calibrated and combined with Maxim and finished in PS.

Both scopes delivered tight round stars although the differential flexure with the GSO produced a little deterioration in the aspect ratio. The bottom left hand corner of the Quattro image shows elongated stars and this is more evident still with the GSO. I'm not sure to what extend these problems are due to tilt of the M26 chip and or sagging of the focuser. There are a couple of fairly tight doubles in the image which are resolved equally well by both scopes. At £399 the steel tubed Quattro is a stunningly good buy but imagers averse to repeated focusing may feel the extra £200 for a carbon tube is worth the extra.

[Black Knight]

Very interesting Martin. A good read

[NickK]

I assume you're using 2" filters?

[Macavity]

Good stuff. For obvious reasons, I'm glad the GSO wasn't totally "thrashed" by the Quattro? <G> Have invested in Bob's knobs for the secondary, now need to work a LOT on the (rather useless!) GSO primary adjustment. At least then, one starts with a more level playing field.

[MartinB]

I assume you're using 2" filters?

2" IDAS screwed into the nose piece

Good stuff. For obvious reasons, I'm glad the GSO wasn't totally "thrashed" by the Quattro? <G> Have invested in Bob's knobs for the secondary, now need to work a LOT on the (rather useless!) GSO primary adjustment. At least then, one starts with a more level playing field.

The GSO wasn't thrashed at all and optically was the equal of the Quattro. Bob's primary springs made a huge difference to my scope. They may have changed the springs they use now, I don't know. The weakest part of my GSO package is the tube rings which are a bit too bendy for my liking.

[Revs]

Interesting read, Martin, cheers

Out of interest, which version of the GSO was it? There are at least 3 with quite different specs.

[MartinB]

Interesting read, Martin, cheers

Out of interest, which version of the GSO was it? There are at least 3 with quite different specs.

I bought it when it was first introduced. It is the full imaging spec with larger secondary and Baader SteelTrack focuser.

[Revs]

I should point out that the later ones had a wider tube, reducing vignetting (though the other specs are still out there. TS sell both). I picked up a Aurora Flat-Field Panel recently so I'll be able to post a flat from mine to add to the comparison if ya like The forecast is good for tonight so I should be out with it.

[MartinB]

Modern Astronomy sell a TS scope and a GSO to confuse things. I thought it was the TS that came with the wider tube and monorail focuser (quite a bit more expensive. The only problem with posting a flat Paul is matching the stretch to the ones I posted. I would be interested to see the effect of the larger tube though

[Revs]

Very good point about the stretch, not thought of that. I'll post one anyhoo. As for Bern; he told me he was gonna stop doing the stock version, but it's still on his site I'll give him what-for when I see him at PSP