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Old 03-04-2010, 08:52 PM
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Flexure breakthrough

Well I finally made some progress this weekend. I've been trying to eliminate the flexure in my system that has been limiting my exposure duration to a couple of minutes and I think I've finally isolated the source to the guide scope.

I recently purchased an old finder scope and have set this up for guiding after machining an adaptor to allow it to accept a webcam. I also fabricated a bar to allow it to mount firmly on the top of my Newt.

The clouds parted for an hour tonight and I managed to shoot 4x8 min subs of NGC1977. I ran the subs through deep sky stacker and after 32 mins I have 1.3 pixels of drift. Previously I was consistently getting 0.5 pixels per minute. I suspect the focuser on the guide scope is the culprit and will now be able to concentrate on this knowing the Newt is unlikely to be the source of the flexure.

Here's NGC1977 4x8mins with the DSI II - no darks but I'm pretty happy with the round stars!
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Old 03-04-2010, 10:25 PM
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Glad you had a breakthrough Peter. I've been sitting in the backyard playing with K3CCDTools for drift aligning - seem to have that licked. I was going to have a play with guiding and trying out some different focal reducers, but alas the clouds are rolling through... I've gone inside a few times already, and I think my next trip indoors might be my last tonight.

Bring on the end of the Wet Season!!!

Cheers
David T
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Old 04-04-2010, 09:05 AM
Dennis
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That’s great news Peter – congratulations on isolating the culprit and fixing the flexure.

Cheers

Dennis
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Old 04-04-2010, 09:30 AM
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Thanks David and Dennis. It's been a long slow process upgrading each of the suspects one at a time.

David - hopefully the weather improves soon!

Peter
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Old 04-04-2010, 02:58 PM
gbeal
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Good to see Peter, and to be honest why not just leave the "finder-guider" on instead. I have one, and am amazed at how it guides. In the near future I will be trying mine at an imaging focal length of about 1800mm and assuming it all goes as per plan it will become a fixture.
Mine is on a Losmandy style dovetail saddle that is part of the all original guide-scope ring setup. Heavy cumbersome and as you have found a great source for flexure. Next step for me is the fabrication of a saddle that firmly clamps the finder tube, and attaches the whole shooting box to the imaging scope.
Gary
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Old 04-04-2010, 03:10 PM
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Gary,

That thought has crossed my mind although I prefer to be able to remove the guide scope from the 10" when I move it around as it is at risk of being knocked off when I carry the gear inside at the end of the night. I'm also unsure whether the 40mm aperture will make it difficult to find a guide star in some shots. My tests so far have been with the Neximage and I've chosen bright objects. I've now mounted it on a Losmandy bar and will try it side by side tonight. If that works then I might look at getting a larger diameter finder - something like an 80mm Stellarvue.

Peter
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Old 04-04-2010, 05:53 PM
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Great shot Peter. You really have nailed it now and it's great to see that your flexture is gone - the stars in that shot are to die for.
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Old 04-04-2010, 07:27 PM
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Hmmmm, the weather is better tonight - but I have to be up at 5:30am to go to work tomorrow...
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Old 05-04-2010, 05:04 AM
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Quote:
Originally Posted by peter_4059 View Post
Gary,

That thought has crossed my mind although I prefer to be able to remove the guide scope from the 10" when I move it around as it is at risk of being knocked off when I carry the gear inside at the end of the night. I'm also unsure whether the 40mm aperture will make it difficult to find a guide star in some shots. My tests so far have been with the Neximage and I've chosen bright objects. I've now mounted it on a Losmandy bar and will try it side by side tonight. If that works then I might look at getting a larger diameter finder - something like an 80mm Stellarvue.

Peter
Peter, well in my case, portability of the guide-scope is something I considered as well, as I use more than one imaging scope. So it needed to be able to be shifted from one to the other without major surgery. The Losmandy saddle allowed this.
In my 50mm finder, and using either the DMK or the Lodestar, I am more than able to locate a guide star, and usually use a 1 second exposure. Try it before you plumb for an 80mm.
Gary
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Old 05-04-2010, 07:42 AM
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Gary - I must admit it's a tough decision - so many considerations.

I tried the SBS setup last night but obviously didn't have something tight and flexure was worse than ever. I swapped the guide scope back to on top of the newt and all was good again.

I'm finding it quite challenging to find a guide star in some fields with the camera I'm using and will probably look at getting something more sensitive. I had a look at the Lodestar and it looks like it has the same CCD as the DSI II pro (no wonder you have no trouble!) however I was surprised at how expensive it was.

I've had my eyes on a DBK for some time however I assume this won't be as sensitive as your DMK and definately not as sensitive as the Lodestar.

Do you have the 50mm finder mounted in rings and how do you locate a guide star - do you swap the camera out for an eyepiece?
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Old 05-04-2010, 10:11 AM
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Hi Peter,
well yes, there are some assumptions in what I said I suppose. The Lodestar IS sensitive, and yes it costs, but it is possibly the best. The DMK again it is not cheap, but has a dual role, lunar/planetary imaging.
Maybe try the DSI as the guide camera if you have another suitable for imaging.
I do have the 50mm in rings, but to be honest it is only until I find or fabricate a clamshell that will hold the lot onto the saddle.
My experience is echoed by many others, the DSBS isn't always the best system for getting away from flexure.
The picture attached may not show you as much as you need, and it is on the refurbished 10", but effectively is what I am using.
Sing out if you need more.
Gary
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Old 05-04-2010, 10:32 AM
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Thanks Gary - the CF tube looks the business.

Do you swap the guide camera for an eyepiece to find a guide star or is the camera so sensitive that a star is always visible in the field?

I also see you have the scope rotated so the imaging camera is lined up with the mount - I but that makes looking through the finderscope a challenge!

I've decided to try guiding with the DSI tonight if the weather permits. I'll shoot some images with the 400D for a change.

Peter
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Old 05-04-2010, 12:19 PM
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Peter,
the CF tube is another labour of love and as you can see from the reflections I ran out of love, and settled for "she'll be right".
Nope, the Lodestar is a fixture, screwed in place. I always find a guide-star.
If I am imaging (and by default therefore the guide-scope is also looking at) something like M42 or the Tarantula or Omega Cent (only really been at these ones since I got the Lodestar), I can normally see the nebulosity easily in a one second shot. Try the DSI and see.
The way I see it, the camera alignment means little to the image, so the closer all that weight is to the polar axis the less I need to have out the other side. I am tempted to dispense altogether with the finder, it is an RA job, which makes life slightly easier, but once I am aligned and synched I never use it again all night.
I have a copy-cat setup with the AT8" newt as well, simple swap the OTA only and go for it.
Gary
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Old 05-04-2010, 12:27 PM
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Thanks Gary - that all makes sense. I'll give the DSI a try. The Lodestar gets a good wrap in Craig Stark's testing.

http://www.stark-labs.com/craig/arti...oundup_API.pdf
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Old 05-04-2010, 01:12 PM
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Yeah, been happy with it so far.
Let us know how you fare with it tonight.
Gary
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Old 05-04-2010, 06:13 PM
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Had about 30 mins with the DSI guiding tonight - guide stars everywhere at 1 sec exposure using the 10x40 finder! Looks like I need to decide whether to get a new guide camera or use the DSI for guiding and get a new imaging camera

Here's last nights guiding trial: 5x8mins on the Horse using the DSI and Newt guided with the Opticstar.
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Old 05-04-2010, 06:40 PM
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If it where I, i would use the dsi for guiding and get a decent cooled CCD! you will never miss the target on that one! and the guider is fine for a 1200mm focal length, so 1800 shouldn't be much different! the SW finder is a f4.

gary, i love where you mounted your finderguider! might have to look into that too.

Currently i have mine set in the original skywatcher finder foot and have had no issues what so ever even up to 20 min guided images! i have still to do a bit more testing with the 10" newt but guiding in maximdl i have a guide error <.5 pixels! which is brilliant in my mind!
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Old 05-04-2010, 07:13 PM
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Have read all this with great interest. Glad to hear you've beat your flexure difficulties into submission Peter. It seems too easy a finder could do such a good job - keen to give it a try here too. Sounds like the number of successful finder/guider devotees is steadily increasing.

Another vote here for a new CCD main camera too......
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Old 05-04-2010, 07:25 PM
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Rob - it's a tough decision - what imaging camera to buy to replace the DSI if I use that for guiding.... Maybe a QHY??

The finder sure is a simple solution and I was surprised how easy a guide star could be found with the DSI.
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Old 05-04-2010, 08:25 PM
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you have

Qhy
FLI
SBIG
QSI

they have their advantages and disadvantages.
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