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DSO imaging with old C8 and celestar fork drive.

Astrophotography
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#1 Tallpedro

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Posted 03 December 2021 - 09:48 PM

The gear:  1980's C8 mounted on newer Celestar fork mount with basic motor drive  good wedge and tripod. Camera Sony A6000 (24 mp ASPC format).  Visual  off access guider with illuminated cross hair  lens.  I have a F6.3 corrector lens for the C8. Scope is all clean and shiny inside and out. I have hand controllers for motions and focus. I'm   heading to Joshua Tree National park in February for new moon, Lots of targets are available. First trip out with this gear for DSO imaging (attempt).  I have a large 12 vt battery to run both the camera and scope drive. I have a bahtinov mask for the telescope for focus. I have astro dlsr on a Mac laptop but the interface with Sony is very week so may just use android app to remote control the camera.  I also have dew shields. but not heaters.   If the gear is not up to snuff, I 'll work in the Hershel 400 catalog, (and some fine whiskey).

 

 The joke:

I know It's antiquated but  it's all I've got on a budget.   Yeah, Yeah, yeah,  laugh but it's what we all did 30 years ago.  I'd like to get a newer GEM mount but not in the cards now.  And a better camera. etc etc etc.

 

The questions:  I need hints and help. I feel confident that I can get polar alignment with drift method. With this set up how long of exposures would be recommended. From all the reviews and research,   I know that the Sony camera likes to run at 1600 ISO for deep images. Scope is going to be  f6.3 with the corrector lens.  I guess it depends on how  the old tracker gears will behave and how much I need to guide.  If I take numerous short images will I be able to get any rich detail or color.?Any suggested best targets?   I will also have the ability to mount  the camera piggy back with a 70- 200mm  f4 lens  and 19mm f2.8 wide angle. Setting suggestions?    Is the 200mm lens ( 35mm equivalent to 350 MM) worth bothering?

 

Thanks all.

Cloudy nights in Portland Or.

 

 

 



#2 Borodog

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Posted 03 December 2021 - 10:16 PM

Wrote a big post and lost it. Moral, you'd probably be able to shoot up to say 8s, maybe a bit longer, if you are well polar aligned, before you start running into trailing due to either polar alignment error or more importantly the periodic error in the drive. I don't know anything about visual guiding. You will also have a smallish FOV, 1x0.7 degrees, that will not allow you to fit large objects like Andromeda, Orion nebula, Rosette, etc. Calibration frames (darks, flats, and dark flats) will be absolutely critical.

 

If I were you I would piggy back the camera and shoot wider angle stuff. You will be able to shoot much longer exposures and fit large targets. The camera has terrible H alpha sensitivity. Pick targets that don't require H alpha to look good, Andromeda, the Witch's Head, etc. The 19mm lens will take great Milky Way shots. Don't think you can get away without piggybacking & tracking there because field rotation through the lens distortion field will get you.

 

You can use astronomy.tools' FOV calculator to check your framing of targets.

 

https://astronomy.to.../field_of_view/


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#3 MJB

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Posted 03 December 2021 - 10:27 PM

Borodog has offered some sound advice. I concur with the piggybacking suggestion in particular - you should get some nice results that way. Of course, since you've got the C8 at hand, experiment with that setup as well - but if you are unguided I think the camera and associated lenses are your best bet. Sounds like a fun trip to dark skies, enjoy.



#4 Aaron Small

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Posted 03 December 2021 - 10:30 PM

Although I deforked my Celestar 8 many years ago, as I recall, there was a ST4 port for guiding which made AP possible with the clock drive.  I didn't have the DX (I think that is what it was called) version, but I was able to take up to about 5 minute guided exposures occasionally.  Most of my frames were on the order of 60-120s.  Polar alignment was key and drifting is your best bet.  I did eventually add a JMI declination motor so my alignment didn't have to be as precise.  Worth the $100 or so.   I also added a DIY digital setting circles (google Dave Ek) so I could connect to the computer.

 

I still use the OTA but on a EQ mount.  Excellent optics.


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#5 Tallpedro

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Posted 04 December 2021 - 01:17 PM

Thanks for the imput. Just found this article

 

http://uncle-rods.bl...es-with-c8.html

 

What is the FOV when camera is mounted? No lens.


Edited by Tallpedro, 04 December 2021 - 01:23 PM.


#6 Aaron Small

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Posted 15 December 2021 - 10:05 PM

TallPedro,

 

The C8 is your lens.  Without a reducer (read Rod's article), you have a f/10 lens that has a fixed focal length of 2032mm. As Rod points out, this is both good and bad.

 

I suggest you download a planetarium program such as Stellarium or Cartes Du Ciel.  Both are free.  In each you can enter the parameters of the telescope and your camera (3.92 micron pixels & 6000x4000 resolution) to see how your target FOV fits.

 

For a C8 and your Sony A6000, I computed a FOV of 39.74"x26.49" or 0.66x0.44 degrees.  That is about 0.40 arcsec/pixel which means your camera is almost too fine and you need decent seeing. I prefer to have a scale of 0.5 arcsec/pixel or higher which is also the Dawes Limit of the C8.  The Dawes Limit is essentially the smallest that the optics can practically resolve.  Therefore, the C8 is the limiting factor and you can't take advantage of all those pixels in your camera.  Doesn't mean you can't use it but I suggest you bin the images as part of your preprocessing to help control the noise.  It has the added advantage of improving processing speed too.



#7 Tallpedro

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Posted 16 December 2021 - 01:22 PM

Aaron,
I do have a .63 focal reducer. I'm getting the feeling this gear is not going to make any useful DSO photos especially with the uncorrected drive system and even hand guiding. I been told don't expect exposures over 8 sec. and the narrow FOV will also limit me. I do have a friend bringing a Sony 7 Mark IV (64 mp) with 400 mm lens we can mount on the C8 and hand guide the scope. Any suggestions


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