Help with Wide Angle Film Astrophotography

[apilato]

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

---------------------
Thanks,

Tony Pilato
api...@chatmosscable.com
www.darkstarimages.com
Brosville VA.
Latitude 36° 36' 31",
Longitude 79° 37' 02"

 

---

This email is free from viruses and malware because avast! Antivirus protection is active.

[Don Westergren]

Hi Tony,

 

Exposure time will definitely affect the quality of your tracking to get pin point stars.  So reduction in exposure time is obviously recommended.   However there are a couple other things that might be affecting your image tracking.

 

Wide angle FOV imaging is very susceptible to field rotation.  There are two kinds of FR when you are guiding (this is ignoring mirror flop).  FR can occur from polar misalignment (PA) and from Atmospheric Differential Refraction (ADR).  Both look alike in a photo.  They are a streaking of the stars in arcs around the guide star.  Since the guide star is by definition a point star (ignoring mirror flop),  all the stars farther away will be arcs around the guide star.

 

Take a close look at your photos to see if the stars are arcs or straight streaks (from mirror flop).

 

If you choose to image unguided, FR from polar misalignment will still be a problem, however unguided, the streaks will all be parallel.  So do the best polar alignment you can.  ADR isn't possible if you are unguided.

 

I can't tell you how well your unguided imaging will be because it depends on both polar alignment and the quality of your mount.  You will have to experiment to find how long you can image.

 

There are some things you can do to minimize FR and ADR if you choose to guide and things to be aware of.

 

1.  ADR and FR are affected by poor PA.  I can't stress this enough!

2.  Both ADR and FR are worse with greater angular distance from the guide star.  With a piggy-back camera, pointing at the guide star in the center of FOV is best.  Do not aim a piggy-back camera off axis.

3.  Longer exposure times cause more FR and ADR.   Duh!

4. ADR is a function of local altitude, so imaging above 30° above the horizon minimizes it.  Imaging very high is best. No ADR at the zenith.

 

Good luck and clear skies,

 

Don

To unsubscribe from this group and stop receiving emails from it, send an email to astro-photo...@seds.org.

[Butler, Ray]

Hi Tony,

 

I normally shoot wide-angle but I have never guided through an SCT - I have used 70mm and 120mm refractors for eyeball-on-crosshairs, old-school guiding, although I last did that in 2009.

 

Don makes some very good points, including this: "With a piggy-back camera, pointing at the guide star in the center of FOV is best.  Do not aim a piggy-back camera off axis." - Yup, I learned that lesson the hard way.

 

My main recommendation is to appreciate that although you have correctly noted that your EQ mount has a periodic error (PE) which is much less than the guiding tolerance for the short telephoto lens you are planning to use, that only means that you can take long unguided shots if your polar alignment is close to perfect. Imperfect polar alignment will cause Dec drift, additional RA drift on top of the PE, and field rotation [and I have the photos to prove it! :( ]. (Auto)guiding will take care of the first two, but field rotation can't be guided out; however it is usually the least of the three evils.

 

Also, you say you "Can’t afford a guide scope to mount on my C11"...but all you need to guide a wideangle shot is a cheap 50mm finderscope with your ST4 head in place of the finder's eyepiece.

 

Good luck!

Ray

This email is free from viruses and malware because avast! Antivirus protection is active.

[Jon Kolb]

Hi Tony,

 

Ray and Don make excellent points.  I would add making sure that your mount is balanced well, biased slightly heavy against the drive direction.  In any case, time your shot so the rig doesn’t pass through zenith during the exposure.  Also, make sure your camera mount is absolutely locked down tight.  It’s surprising how much something like a ball mount or really any tripod head can move even when you think it’s tight.  I eventually started using the camera bolted in a fixed position with a guidescope ring holding the lens, although a 105mm lens probably doesn’t need that.  I never guided off-axis so no help there, but I did many a shot manually guided and I think at the focal length you’re shooting, the suggestion to guide using a finder scope is worth trying out.  Heck, why not try guiding using the C11 itself without the off-axis gear, eliminating one potential source of movement in the setup?

 

Jon Kolb

 

From: astro...@seds.org [mailto:astro...@seds.org] On Behalf Of apilato
Sent: Wednesday, October 30, 2013 7:19 AM
To: astro...@seds.org
Subject: [APML] Help with Wide Angle Film Astrophotography

 

Good morning,

 

 

---

This email is free from viruses and malware because avast! Antivirus protection is active.

 

To unsubscribe from this group and stop receiving emails from it, send an email to astro-photo...@seds.org.

[apilato apilato]

Thanks everybody,

Lot's of good analysis of my problem. I willI 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

--

Tony Pilato
api...@chatmosscable.com
www.darkstarimages.com
Brosville VA. 
Latitude 36° 36' 31",
Longitude 79° 37' 02"

837 ft.