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Help with Wide Angle Astrophotography

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#1 Tony Pilato

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Posted 30 October 2013 - 08:12 AM

Good morning,

I am a total newbie to Wide Angle Astrophotography. I was trying to image nebula in Cygnus using a 105mm lens on an OM-1 35mm camera with a Red #29 filter and Hypered Tech Pan piggybacked on a C11 with a G11 Losmandy Mount.

I was off axis auto guiding through a Giant Easy Guider at F/11 for 2 hours. Needless to say mirror flop created star trails to varying degrees. Unacceptable. Tracking was good via ST-4 readouts –nothing higher than A2’s

So, what do I do? Can’t afford a guide scope to mount on my C11.

First, I am going to ditch the filter, exposure too long... Ideally, I am looking for about 30 to 90 minutes depending on F/Stop.

So my question is:

Knowing that at 135mm, the tolerance and drift time is:

Lens: 135 mm
Tolerance(arc sec):40"
Drift Time (seconds)2.5 sec.

And supposing that the G11 mount has an average period error of about 10” or less, can I get away with not guiding for about 1 hour? Less? Anyone with experience not guiding with this setup or something similar?

With this setup can I off axis auto guide with no trailing assuming some mirror flop for 1hour? Less? Anyone with experience off axis auto guiding with this setup or something similar?

Any other ideas or experience with 105mm lens, film, off axis auto guiding or not guiding with a SCT and an equatorial mount?

Thanks in advance.

Tony

#2 Michal1

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Posted 30 October 2013 - 08:40 AM

Are you sure that the camera doesn't move with respect to the telescope's tube? Some people use finder scopes to guide.

#3 TxStars

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Posted 30 October 2013 - 08:59 AM

I would add a brass shim to the baffle tube of the C11 to reduce mirror flop.
This will help all photography with it as well as guiding.
Also make sure the lens has no play in it, (some have flop) which will require a guidescope ring at the front to eliminate.

#4 Nebhunter

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Posted 30 October 2013 - 08:28 PM

Hi Tony. Lots going on with your set up. Having that monster tube on the G11 limits the setup. Any thoughts about having a switch out set up on a wider cross DUP with camera, small guide scope and the ST-4? Here is my set up and maybe it will spark an idea - or not. http://www.pbase.com/pentax67/inbox

I did replace the stock worm with the HP one piece Losmandy worm, and after doing some fine tuning and Pec training was able to do a 60 minute exposure with the Tec 140 with OK stars. No guiding - just Pec turned on. With the ST-4 (Pec off) with above average conditions I can go 7 seconds between iterations and still get strings of 0' with occasional 1's. Mind you, my drift alignment setups must do 60 minutes and be dead on.

Igor

#5 Nightfly

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Posted 31 October 2013 - 05:15 AM

Tony. Why are you using an off-axis guider for piggyback photography? I just put my illuminated reticle in a diagonal and guide. Mirror flop showing up on a 105mm lens piggybacked? That's a huge amount of flop. I've never encountered this with lenses up to 300mm using my old Meade 2080. Something else going on here?

Guiding a 135mm lens will require keeping the guide star within about two box diameters (at 2000mm FL) of a standard 12mm reticle eyepiece, roughly 40 arc seconds as you point out. In your C11 this will be about three box diameters.

Just make sure everything is tight and drop the OAG.

#6 Tony Pilato

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Posted 31 October 2013 - 09:06 AM

Thanks everybody,

I have alot of options to try.

If need be, I will start prepping my wife for a short refactor for guiding with Wide Angle for the next Milky Way season.

Thanks again for all of you input!

Tony






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