GSO 8" Astrograph, first light & review

[Tim]

A little while ago I purchased a GSO F4 8" Astrograph from Modern Astronomy. Whilst collecting the OTA I talked myself into picking up a QHY8, and have spent most of the time since then getting to grips with that, but a recent holiday has given me some time to devote to the GSO, and give it a proper first light testing. I bought it after using a smaller F4 astrograph newt and being very impressed with the images, and the speed of f4 imaging.

In essence the GSO is a Newtonian. I went for the 'astrograph' version, which has a larger secondary mirror than the standard, and I also opted for the Baader Steeltrak focusser. These options set you back a few extra quid. There is a 12v fan on the back of the primary mirror for quicker cooling, but I have not found the need to use this yet as I keep my scopes outside and cool all the time.

To help with the coma problems associated with a Newt, I also picked up a Baader Coma Corrector. Unfortuneatly my trial of CCD Inspector ended just before I ran the tests, so I cant give a real idea of how flat the images appear, but on the large chip QHY8, the enlarged secondary neatly illuminates the whole chip, as it would for a DSLR. With flats, any remaining vignetting is easily taken care of.

On unpacking the assembly, I found the Baader focusser had not been attached as securely as it should have been, and that caused the knobs to rub on the housing. A quick tweak with an Allen key sorted that, and the focusser proved very smooth indeed, and holds well even with a heavy setup behind it. Once focus was achieved, I found that tightening the lock thumbscrew just edged the focus out. Maybe because of the weight of the camera. I got around this by figuring out which way it was pushing the focus, and then compensating that way, but it isnt ideal.

The OTA proved fairly easy to collimate, I followed the online guide, although after a car journey it needed adjusting again. Some aspects I found a little tricky, but experience will sort those I think. I used a laser collimator and the job was done in about one minute. One improvement I might make would be to add Bobs knobs to the secondary for collimation. Using a screwdriver is tricky and fraught with the dangers of dropping it onto the primary!

The rings that come with the GSO have flats on the top, and three mounting holes per ring. I used these to mount my WO72 via scope rings. I plan to use an off axis guider with this OTA, which will make balancing the assembly easier, but even with the WO72 on top, DSI on the back of that, and QHY8 on the other end, the EQ6 handled the load very well. The GSO itself has some weight to it, but it is fairly easy to handle, not too cumbersome. I havent weighed it yet. I dont have and didnt appear to need any kind of dew prevention for this OTA, which was a bit of a novelty tbh. I did wonder if wrapping a dew heater tape around the outside of the OTA/primary would help, but with its metal construction I think most of the heat would be conducted away from the mirror before it did any good.

Once the camera was live, I played with the focusser. Even tiny movements make a big difference, so i was glad I opted for the smoother focusser. The fine controls work like a dream, but I am told that the standard focusser is also of reasonable quality. All that remained was to start imaging! I planned a whole night session on that most worthy, and yet easy to use as a benchmark of a target, M42. I was hoping to capture a whole series of different length subs, but the freezing fog put a stop to that, so the result below is around 2.5 hours, with 360sec subs.

Even through the poor visibility and fog, the OTA managed to suck in a lot more light and detail than I expected, and I was genuinely surprised to see how much extra faint detail had come through the murk. I'm quite excited about getting this OTA to a really dark site and seeing what it can do there.

So here we are, the old warhorse, M42. Two and a half hours with QHY8, Captured in Nebulosity, guided PHD, Processed in PS with Noels tools and a DBE from Pixinsight LE. Stacked in DSS of course with flats and bias, no darks. Image here at 20% of the original size. I have cropped it square, but the top to bottom is the size and FOV straight off the camera.

All in all I find the GSO F4 8" Astrograph to be a relatively cheap, fast, and effective tool in the arsenal. I havent tried it visually, but photographically it seems to be able to deliver. It was recommended by Bernard at Modern Astronomy, and i'm glad I took his advice.

Thanks for looking.

TJ

(click to see it slightly bigger)

[blinky]

Very nice review (And nice image too!), would love to get my hands on one of them astrographs.

[Psychobilly]

Nice Review and Image TJ....

Aperture and speed certainly help in grabbign the faint stuff....

Billy....

[Doc]

Wow sounds like the ideal tool to me.

Very imformitive report.

So could you make it faster still by putting a focal reducer in line to bring it down to F2.8 or something.

[MikeP]

Nicely written review Tim, both interesting and informative.

I'm not the best judge of images but to me that is one of the crispest M42's I've seen.

Mike

[Tim]

Here you go Steve. I always crop out all unwanted parts of the image at the start of processing, so that the histogram is only showing data you actually want to adjust, so this is just a single 6 minute exposure with flat applied and one application of levels and curves. The wierd bit in the corner is ice on the corner of the flats, I ran out of time to take them and couldn't wait for the ice to disperse fully!

The stars show a bit of deformation in the top left corner. I am not sure if this was the way I had mounted the camera. Also, I dont know enough about reflections etc to know whether the bright reflection around the blue star Ori should be centralised if the ota was perfectly collimated. I struggled with this, away from the online guide and in the middle of a camp field at minus 3. As the distortion seems only to be on one side, I am inclined to think the camera was mounted a bit cockeyed. IIRC only the coma corrector was used in the 2" tube, ideally I would have had a longer fitting in the focusser to help centralise it properly.

Even so, seems pretty good for a newt, to me at least?

I just wish the 3 hours of other subs I have for this were useable, but the mist just wrecked them

TJ

(click to enlarge)

[Tim]

So could you make it faster still by putting a focal reducer in line to bring it down to F2.8 or something.

I shouldn't think so Doc. As far as I've seen, the only way to go faster is via hyperstars on scts, but thats another story I'm not good at maths or figures, but the way I see it, the bigger the opening and faster the scope, the easier it is, and this tube seems to offer a decent compromise for both.

[centroid]

Nice report Tim, and they say that "the proof of the pudding, is in the eating".

Looking at that result, I'd say that 'pudding' tasted pretty good.

Looks like it will prove itself to be a very capable 'tool', to have around.

Dave

[RichieJarvis]

Great review TJ - and a great M42!

I've had the 6" GSO for a while, and can heartily recommend them. I've never needed any sort of dew system to keep the mirrors clear as yet. I believe you can get dew on the secondary on really bad nights though - I've seen secondary heaters on the web to cure that problem though.

I am picking my (non-deluxe) version tomorrow, so can't wait to give F/4 imaging a shot with the SXV-H9.

[barkis]

Bern's recommendation is always sound TJ. and it certainly 100% with this speed merchant. At f4, Coma is bound to occur, but the CC does it's job very well. This is a very sublime M42, and when you have learned your way around the QHY8, then you have a very potent outfit.

Ron. :salute:

[PNJ]

Did you have any problems reaching focus with your ccd camera, ie did you need any spacers or anything to get the right distance to the chip.

cheers

Paul

fantastiv m42 by the way

[Tim]

Paul,

The recommended spacing from Baader MPCC is 56mm. I fit this assembly into a 2" extension tube, and then focus comes easily.

You can get focus through the MPCC without having the spacing right as well, but this manifests itself as a coma in various ways. In an ideal world you could just slap the camera into the focusser and not need an MPCC or such, as with the Skywatcher Mak-Newt, but then you sacrifice a lot of speed. F4 is a lot quicker than f5.3.

I'm very pleased with the image, you should see the full size especially for just 2.5 hours on an average to poor seeing night. Cant wait to test it from a really dark site on a good clear night!

TJ

[roundycat]

I think you will find that any bright star that is off centre will show eccentric reflections. It has nothing to do with the alignment of anything per se but more to do with the fast f ratio and the star being off centre. You can prove this by taking a quick exposure of a bright star having centred it as perfectly as you can manage. Then move it to near the edge and do it again. That should give you the answer.

Dennis