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Old 15-05-2020, 09:53 AM
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pkinchington (Peter (Kanga))
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Nebula equipment question

I am a biological photographer and take photographs of a wide range of subjects both on land and underwater and through the microscope. I have plenty of lenses and my intention was to use my 300mm f/2.8 ED lens on a HEQ5 go to mount with a "feed back camera" to correct the position of the mount if any drift occurs. I have attached my first shot of the orion nebula using the 300mm f/2.8 lens on a full frame unmodified digital camera. I used a regular tripod and took about 3000 photos (1s @ 1600iso) for stacking (some dark and flat frames also). I recentered the orion nebula when it was about to move out of the frame. I was actually tossing up getting a modified dslr (to get the H-alpha band 656nm) or an apo telescope or if I find that I want to do some serious work then buying a monochrome camera with a filter wheel so that I can do narrow band work - I particularly like the results when the hubble colour palette is used. The reason that I was thinking about an apo telescope was that some stars looked mushy in my orion nebula shot and some had what appeared to be chromatic aberration.The William optics GT81 triplet apo scope sounds interesting and how much would a field flattener for this cost? I post processed the stacked image in photoshop where I stretched the histogram through several iterations. I would like to know the community's thoughts on the direction I should take.
Cheers Peter Kinchington - peterkinchington.com
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Old 15-05-2020, 11:15 AM
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iborg (Philip)
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Hi Peter


I think that's a pretty good astro shot.

Are you aware of star trackers? Take a look at https://astrobackyard.com/star-track...rophotography/ for ideas.

There is also one currently in the for sales. http://www.iceinspace.com.au/forum/s...d.php?t=182853

Have fun


Philip
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Old 15-05-2020, 02:52 PM
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multiweb (Marc)
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That shot is pretty good considering it's your first.

There is tilt in your focal plane that's why the stars are out of focus to the left of the fiels and sharper to the right. This is because a 300mm lens starts to weight a little and it doesn't keep square to the sensor plane when the camera point at different parts of the sky. That and that stars are point light sources so not forgiving on focus and field correction. You could remedy to that by bracing the lens in a centering ring to hold its weight.

If you want to get a refractor and a mono camera then you don't have to get an APO. Because you'll be focusing different colours independently. If you still want to use an OSC like this camera then get an APO which is better color corrected.

If you want to do narrowband that means long exposures, good tracking and auto-guiding so now it's bigger $ because you'll need a good equatorial mount.
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Old 16-05-2020, 07:34 AM
phomer (Paul)
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Quote:
Originally Posted by pkinchington View Post
I am a biological photographer and take photographs of a wide range of subjects both on land and underwater and through the microscope. I have plenty of lenses and my intention was to use my 300mm f/2.8 ED lens on a HEQ5 go to mount with a "feed back camera" to correct the position of the mount if any drift occurs. I have attached my first shot of the orion nebula using the 300mm f/2.8 lens on a full frame unmodified digital camera. I used a regular tripod and took about 3000 photos (1s @ 1600iso) for stacking (some dark and flat frames also). I recentered the orion nebula when it was about to move out of the frame. I was actually tossing up getting a modified dslr (to get the H-alpha band 656nm) or an apo telescope or if I find that I want to do some serious work then buying a monochrome camera with a filter wheel so that I can do narrow band work - I particularly like the results when the hubble colour palette is used. The reason that I was thinking about an apo telescope was that some stars looked mushy in my orion nebula shot and some had what appeared to be chromatic aberration.The William optics GT81 triplet apo scope sounds interesting and how much would a field flattener for this cost? I post processed the stacked image in photoshop where I stretched the histogram through several iterations. I would like to know the community's thoughts on the direction I should take.
Cheers Peter Kinchington - peterkinchington.com
Peter,
If this is your first attempt, then not a bad effort but there are a few things to be aware of :-
- Focus is much more critical for astrophotography than just about any other form of photography and your camera is not designed to focus in near darkness
- Most lenses are not optimal when wide open, try reducing the f stop

- There appears to be a difference in focus across the frame
- If you are pointing high in the sky then gravity may cause the focuser to shift
- 1 second exposures are really too short
- A 300mm focal length is starting to become very challenging, try shorter focal lengths until you have developed the required skills
Paul
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Old 07-06-2020, 08:21 PM
shoebox9 (Graham)
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As a fellow photographer I'm surprised at the suggestions the 300 prime is shifting vs the sensor. Is everyone aware these lenses come with their own mounting ring/foot and are never used 'hanging' off the camera. Either they are hand held (rarely) with one hand under the lens, or they are on a monopod. Occasionally someone may use a tripod, but always the camera is simply attached to the end of the mounted lens (supporting only their weight), not the other way around. I'm sure Peter uses the lens this way, like everyone I've ever met who owns one.

What can be an issue however, is the lens receiving a bump causing one of the elements to de-centre. This is a common problem. resulting in lenses with a tilted plane of focus. When it's only slight, it can be difficult to notice (especially with Canons when DPAF is being used- the part of the frame you focus on is always still in focus). Peter still may not have seen it before with his Nikon. A smaller aperture will cover it.

The truth is Astrophotography is a brutal sport.

In terms of advice, once you have a way if guiding your camera/lens, you can bracket your exposures so you capture both darker background stars, as well as exposures that don't blow out the very bright stuff. This will also give you more options with noise and allow you to dramatically improve the chunky blacks.

Others may disagree, but personally, if I were you I'd use the expensive glass you already own and spend money on stabalisation/guiding, before rushing out to buy a new scope.

Last edited by RB; 07-06-2020 at 09:03 PM.
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Old 16-06-2020, 06:22 AM
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pkinchington (Peter (Kanga))
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Thanks Graham - I'll try a smaller aperture and longer exposure with stabilisation and tracking.
Cheers P.
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