First light with new Serrurier truss: Hi-res NGC 6231 in Scorpius

[SkyViking (Rolf)]

Hi all, I finally had a chance to put the new Serrurier truss OTA to the test the other night.
(I have recently finished building a new OTA, details and photos of the beast are in this thread: http://www.iceinspace.com.au/forum/s...ad.php?t=74690)

I wanted to test the performance of the new scope on an open cluster. I thought it would be a good target for evaluating the (hopefully reduced) diffraction pattern and to see if there was any improvement in the resolution I could achieve.
Attached is an image of NGC 6231 in Scorpius taken with the new OTA. Also an inverted version which shows the faint diffraction spikes better. The last image is one I took last year of the Gem Cluster, I attached it for comparison. Notice how the halos and spikes are much more prominent and how details are lost near the brighter stars.

With the new scope I even had to stretch the NGC 6231 image considerably to bring out the diffraction spikes! I also like how even faint stars are easily resolved right next to the brightest cluster members. I could definitely not have gotten such a result before. This scope seems to deliver refractor-like images now, which quite surprises me.
It's not that I don't like spikes, they look pretty , but they also divert precious light away from the target and reduces the overall image contrast - which may make all the difference when trying to capture some faint quasar or other obscure target.
The most interesting aspect is that this new image was taken with the same optics as before, the only difference is that I have mounted the mirrors in a new OTA where I have removed all obstructions such as mirror clips, eliminated protruding focus barrel etc, and replaced my traditional 4 vane metal band spider with a wire spider using 0.23mm guitar strings.

All in all, I now have better focus and less difffraction. I think the mirror was slightly pinched in the old OTA too, something I took care to avoid when building the new mirror cell. I believe the biggest single improvement comes from the wire spider. I had massive diffraction spikes and halos before, and they are virtually invisible now, which translates to tighter stars and better resolution.

As an added bonus I seem to have gotten completely rid of the focus drift that plagued me before. The NGC6231 image was taken as a 5 part mosaic, each 100x15.5 seconds. So total imaging time was just over 2 hours. During this time I did not once have to readjust focus, which is something new to me! The last frame of the last panel was as sharp at the very first frame.
I also noticed that my finder now stays perfectly parallel with the optical axis. It used to shift slightly even when just changing direction of the scope. Now I can center a star on the crosshairs and it will be in the center of the eyepiece at 400x magnification. It means that I can center a target in the finder and it will be right on the camera chip the first time. Excellent .

Comments and critique is most welcome.

The OTA design itself is documented in detail in the other thread mentioned above.

[atalas]

Congrats on first light Rolf,a nice cluster with more to come I'm sure.

[multiweb (Marc)]

Significant improvement in stellar profiles. Gotta be happy with all that hard work paying off.

[Hagar (Doug)]

Nice cluster Rolf. Glad all your work has paid off. You have to be a happy Chappy.
Well done for both the image and the scope.

[richardo (Rich)]

Very nicely engineered Rolf.. and the proof is in the pudding, yes definitely better results although your present image tracked better than your previous.

Always wondered about the string mounted secondary when I was researching better designed secondary holders and spiders.. I think it was stellafane which brought my attention to the idea for visual scopes a long while back. I wondered how they would hold up for imaging.

Have you tested it to see how stable it is for movement on long exposures and when advancing to the meridian.. wind/ wind gusts??
How has it performed??

Thanks for sharing
Rich

[David Fitz-Henr]

Looking very good Rolf!
I note that your strategy appears to be lots of short exposures, is that correct? To avoid having to guide the camera? Will you be trying any long exposures with this setup?

[strongmanmike (Michael)]

Well, comparing these two images certainly there is a difference and looks to bode well for the future of your new musical instrument scope

I had a look at your scope ramblings and the new OTA looks very cool by the way, you are a cleaver dude

Mike

[SkyViking (Rolf)]

Originally Posted by atalas

Congrats on first light Rolf,a nice cluster with more to come I'm sure.

Thanks Louie And much more to come hopefully!

Originally Posted by multiweb

Significant improvement in stellar profiles. Gotta be happy with all that hard work paying off.

Thank you Marc, yes I think the improvement is huge. I'm definitely happy with the results so far

Originally Posted by Hagar

Nice cluster Rolf. Glad all your work has paid off. You have to be a happy Chappy.
Well done for both the image and the scope.

Thanks Doug, I'm very happy indeed

Originally Posted by richardo

Very nicely engineered Rolf.. and the proof is in the pudding, yes definitely better results although your present image tracked better than your previous.

Always wondered about the string mounted secondary when I was researching better designed secondary holders and spiders.. I think it was stellafane which brought my attention to the idea for visual scopes a long while back. I wondered how they would hold up for imaging.

Have you tested it to see how stable it is for movement on long exposures and when advancing to the meridian.. wind/ wind gusts??
How has it performed??

Thanks for sharing
Rich

Thank you Rich Yes the new image has slightly better tracking - still obviously a big improvement in the halos and diffraction though. I actually have the impression that the mount also tracks just a little bit better now, I wonder if that's because the new OTA is lighter - although the mount was in no way not overloaded before.

The wire spider seems to hold up very well for imaging. Collimation is rock solid in all orientations and as good as I left it after the first test now 2 weeks ago.
I also used to have to adjust focus slightly if I changed direction of the scope to another target. Everything is rock solid now and focus stays right where I put it, I just can't believe it

Originally Posted by David Fitz-Henr

Looking very good Rolf!
I note that your strategy appears to be lots of short exposures, is that correct? To avoid having to guide the camera? Will you be trying any long exposures with this setup?

Thanks David, yes I take short exposures but it's not really a strategy as such, I just image with a modified webcam and don't have any guiding solution available so I have to keep my exposures short to avoid tracking errors etc. Up to around 1 minute seems to be the maximum I can go
I'm planning to invest in a QSI583wsg camera with proper guiding later this year and then I'll expose for as long as I can!

Originally Posted by strongmanmike

Well, comparing these two images certainly there is a difference and looks to bode well for the future of your new musical instrument scope

I had a look at your scope ramblings and the new OTA looks very cool by the way, you are a cleaver dude

Mike

Thank you Mike, glad you liked it The results are promising so far. Now I just need the cam to go with it and then I'll be ready for some serious imaging.
Yours look very promising too! I saw your coloured Cat's Paw image and I think it was a nice image and a promising test, but didn't want to argue with the 'angry mob' in the other thread . Testing is what it's all about with new scopes right?

But now that we both have new scopes it'll probably stay cloudy for a while ...

[strongmanmike (Michael)]

Yours look very promising too! I saw your coloured Cat's Paw image and I think it was a nice image and a promising test, but didn't want to argue with the 'angry mob' in the other thread . Testing is what it's all about with new scopes right?

But now that we both have new scopes it'll probably stay cloudy for a while ...

Yeh thanks mate, I've certainly seen much worse they weren't angry just expecting better (or different?), regardless of the test nature and challenging combine and I understand that ...in time, in time...

Look forward to the new camera....you should get an FLI though

Mike

[SkyViking (Rolf)]

Originally Posted by strongmanmike

Look forward to the new camera....you should get an FLI though
Mike

Hehe yeah I have thought long about the FLI ML8300 but it'd be quite a bit more expensive all up because of the filter wheel and off-axis guider I'd need to buy separately, plus larger filters as well. Then all put together it'll have much longer backfocus too. The total backfocus of the QSI is around 5cm, that includes OAG and filterwheel .
With FLI I'd need a larger secondary mirror to avoid vignetting. With that comes larger central obstruction = less image resolution/contrast, which I've now realised the importance of while judging the test images from my new OTA... then it starts to look like a slippery slope

Of course with an FLI I'd be getting better cooling undoubtedly, and perhaps better electronics (although not sure about that) and faster download times - but this comes at a much steeper overall price tag plus the disadvantages mentioned above. So as long as we're talking about a KAF-8300 camera the QSI seems the way to go I must say.
For larger chip sizes though yeah I think I'd clearly favour FLI

[strongmanmike (Michael)]

Originally Posted by SkyViking

Hehe yeah I have thought long about the FLI ML8300 but it'd be quite a bit more expensive all up because of the filter wheel and off-axis guider I'd need to buy separately, plus larger filters as well. Then all put together it'll have much longer backfocus too. The total backfocus of the QSI is around 5cm, that includes OAG and filterwheel .
With FLI I'd need a larger secondary mirror to avoid vignetting. With that comes larger central obstruction = less image resolution/contrast, which I've now realised the importance of while judging the test images from my new OTA... then it starts to look like a slippery slope

Of course with an FLI I'd be getting better cooling undoubtedly, and perhaps better electronics (although not sure about that) and faster download times - but this comes at a much steeper overall price tag plus the disadvantages mentioned above. So as long as we're talking about a KAF-8300 camera the QSI seems the way to go I must say.
For larger chip sizes though yeah I think I'd clearly favour FLI

Ah well, don't say I didn't try and tell you ..

Nah just kidding, great stuff, exciting times ahead

Mike