First light GSO 8" RC

[TrevorW]

Collimation may be a bit out even though it was checked on the bench at the retailers, it didn't appear to be perfect when viewing a star out of focus.

Spent a good hour on DSLR Focus in and out trying to get perfect focus fiddled with the collimation although its years since I've collimated a reflector.

The scope is way back heavy I tried balancing it on a side by side saddle with my ED80 but I could not get it too balance on my home made saddle, I'll probably need a fully adjustable one for this arrangement to work. Took the saddle off and just had scope on mount and nearly got it to balance although not perfect.

Both images are less than 10 minute total exposure, unguided and uncropped

Had to use my 80mm extension tube in the focuser with the Canon 350d to attain focus

No filter used for these all taken under waxing gibbous moon 58%

1. NGC5139 30 frames x 20 seconds per frame no darks, flats applied stacked in DSS best 19 frames selected by DSS for stacking, Aligned RGB in DSS, adjusted for curves, levels and colour balance nn PS7, NC's light pollution action applied and RC Gradient Xterm.

2. M5 30 frames x 20 seconds best 22 frames stacked by DSS darks and flats applied adjusted for curves and levels in PS7

Once I have the balance and guiding issues sorted should be a good scope to image with and my feeble first attempt probably doesn't do it justice.

:thumbsup:

[jase]

Not sure what happened to Omega Trevor. I don't think this is an optical issue, but mount i.e. poor tracking/guiding. The on axis performance with M5 looks ok. I suspect keeping this scope well collimated is going to be key to extracting good images (no different to any other RC really). Looks like some fun times ahead...keen to see how you progress, not that I'm interested in buying one, more to validate the bang for buck arguement.

[gbeal]

Good start mate, looking good.
OK, the Omega shot is awful, I have to be honest, but given the circumstances it is a great start. While the moon is a nuisance get the balance and guiding sorted and then wave a big stick.
If there is a common "glitch" to overcome with these scopes it is the rearward balance issues, so maybe get a longer dovetail and rectify this, or a set of tube rings perhaps, rather than the screwed on dovetail?
It would have been nice to see a stack of 10 second shots from Omega perhaps.
Keep at it,
Gary

[TrevorW]

Tracking was atrocious probably more to do with the balance issue and my impatient setup.

I was impressed at the SNR collected though for such short frames

I've ordered another dovetail and some guide rings and looking at a counter weight system to hang off the front and may invest in a laser collimator (any suggestions)

:help:

[gbeal]

I agree, tracking issues are all that are really visible in the shots, which is unfortunate but fixable.
I have just revisited the shots of the scope on the mount, and even without anything in the focuser, the scope appears to be as aft on the dovetail as possible, so a set of OTA rings (as I suggested) probably won't be of any use.
If I were doing this I think the only fix is to have a dovetail rail extend out underneath the focuser, and in my case it is a Losmandy D sized rail. Your option of adding weight underneath and to the front is worth looking at, as much as I hate adding weight. It seems the only real quick fix. Make up a clip on weight that you can slide right to the front, something like a lead weight, or some speaker magnets and try it.
Gary

[dpastern]

mmm. Trevor - this is what I'm seeing with some of the shots showing up on CN. It could just be that you need the focal reducer/flattener to cure what might be field curvature. Given the shot you've posted of Omega Centauri, and what I've seen on CN, and what the far more knowledgeable people on there have come to conclusion wise, I suspect movement at the camera end and less than perfect collimation.

This is using the GSO focuser, whereas the shots on CN are from the Astro Tech unit, which I believe uses a featherlight focuser (obviously higher quality).

The 2nd shot looks better, but there is still weird star shaping away from the central section of the image. I'm hoping it just needs the flattener.

Dave

[gbeal]

I could be wrong, but understood even the AT versions from Astronomics were using the same stock GSO focuser.

[TrevorW]

Astrotech version uses same stock focuser just advertised as a feathertouch, just makes it sound better look at the photos on their site.

No way could you they sell this scope for $1300 in the US including an actual FT focuser.

[dpastern]

Well, you are right:

http://www.astronomics.com/main/product.asp/catalog_name/Astronomics/category_name/U5QNWB3RKWWL8N3EL99F9DX9A0/product_id/AT8RC

I *know* that they were originally selling with featherlight focusers originally though. I could find several past ads indicating this. Looks like they silently swapped it back out to a GSO/stock focuser.

As I said in a past post, a well known Telescope retailer in Australia tested these units and was (when reading through the lines of his EMail to me when I queried these products) unhappy with both the focuser and collimation. He would not sell them until these problems were resolved. He provided feedback to GSO, but I doubt that it has been taken. When I got this EMail from him, I suddenly became very wary of this product. At nearly 3 grand for the 8" unit, it is *not* cheap. Sure, it's cheap when compared to alternatives, which are all heavily over priced imho anyways, but 3 grand is not cheap. Anyone who thinks so is quite welcome to donate that money to me ;-)

I'm waiting to see Trevor investigate and sort out these issues and see what can ultimately be achieved with this unit. I do honestly think that a Meade 10" LX200 ACF unit will offer almost as good optical quality to be honest, if not better.

Dave

[dpastern]

If that is the case, then that would be false & misleading information. Feathertouch is a rather high quality, well known brand, and I'm sure that they'd be not too happy with a GSO focuser being touted as "feathertouch", don't you think? And rightly so imho.

Anyways, the jury is still out of me on these units. The images I've seen from them are all showing promise, but some worrying problems too. I tend to look before I leap. I might give into temptation for cheap things, but for something expensive like this, I want to make 100% sure that I'm spending my money on a 'sure thing'. So, please excuse my scepticism and wariness.

Dave

[multiweb]

Stars off axis look very consistent which is good. Once you have tracking sorted out you should get some real nice shots.

[AlexN]

Trevor, Good to see someone got their GSO RC out.. I am STILL yet to use mine do to the inability to balance it.. with the SBIG hanging out the rear of it there is just no way in hell im going to run the mount with such an imbalance.. I've ordered one of the vixen style counterweights from ADM to address this issue.. 3.5lb of weight hanging off the dovetail right at the front of the scope should be enough to see this issue sorted, then I'll be off! :)

The M5 image looks good, and stars do appear uniform across the image plane... Spend plenty of time on collimation... From my understanding talking to a few people in the US, they are a real pain to get from "close" to "perfect" collimation, however the time spent doing so is worth the hassle.

Cheers mate
Alex.

[Paul Haese]

Trevor,

A buddy and I spent a good 15 minutes last night getting collimation near perfect, note I say near perfect. It could always be better. I used a Tak flattener which made the images flatter but still had some defects. I found focusing is a bit of challenge and I will be getting a bahtinov mask made up to help with this.

I have also ordered a feather touch focusor and flange. It should arrive in the next week. Essentially I think these optics are pretty darn good, it is the collimation, focusor and flattener that are letting down the images.

Like I said else where balance can be handled by using a C9.25 dovetail D with C8 mounts rather than using the vixen dovetail. It is too short to be useful.

Work on the mentioned issues and you will be taking great shots before you know it. I am expect high results myself.

[TrevorW]

Resolved the balance issue see post under equipment.

Run CCD Inspector on a stack of 5 frames of M5 shows a curvature of 39.6% FWMH min 1.85 Max 4.37.

Correct me if wrong but if FWMH is between 2 and 4 would this not indicate reasonable collimation

The curvature % is better than from my ED80 when tracking was spot on however could this high figure be attributable to bad alignment as others posted images on CN show a figure of around 17% in CCD.

and true to form it's cloudy today but still no rain.

Cheers

[Peter Ward]

I seem to recall needing an asbestos suit when I suggested (albeit brazenly) :

1) accurate mount/guiding/self guiding/AO guiding.... first

2) optics..... a long second (corollary: you know you have a really good mount when you can see problems with the optics)

I suspect the RC is OK. But due the very large secondary for the aperture probably would have made a different choice at that aperture.

I also suspect you will will find imaging nirvana with decidedly better tracking.

Stick with it :thumbsup:

[Paul Haese]

Peter, absolutely correct. Good guiding is nearly everything as is often said.

[TrevorW]

Forgive my ignorance I just read on the CCD site that Clusters are not a good image to capture for testing optics using CCDI

Singular stars are better

[multiweb]

That's right. It's better to use a "uniform" star field. If you point on a globular cluster or bright nebula you'll get a bulge in your readings.