Imaging quality with ED80mm vs 200mm camera lens

[alex_wells88]

Hi!

I am very new to astrophotography, and I have 'upgraded' from a 200mm camera lens I was using to a Explore Scientific ED 80mm apo. 

I don't have clear skies yet so can't test it astro-objects, but with my daytime testing, I'm a little confused about the optical clarity I'm getting with the DSLR mounted on the telescope vs with a camera lens vs with my phone down the eyepiece; so I'm hoping someone can help!

 

Attached is my comparison images below:

1) Nikon D7200 coupled directly to Explore Scientific ED 80

2) Nikon D7200 couples to its 200mm telephoto lens (and zoomed in to match the FOV)

3) Samsung phone photo down the 25mm EP + Diagonal mounted on the ED80

 

As you can see, the camera lens seems to provide a much sharper image, as does the phone down the eyepiece.... Have I coupled something incorrectly? Would a field flattener help with the sharpness in the middle of the FOV? Why does the eyepiece yield a better image than camera direct to scope?

 

Many many thanks for any replies!

 

 

[ollypenrice]

The phone image proves that the scope is optically sharp so I suspect that you were not as well focused for the prime focus scope image. Did you still have some focus travel left when using the scope at prime focus? Often you'll find you need an extension tube between drawtube and camera, particularly on closer daytime targets.

Olly

[alex_wells88]

Hi Olly,

Thanks for the reply!

The drawtube (I presume this is the tube that moves out of the scope as I focus and has a ruler etched into it) extends 10.5cm and seemed to have plenty of travel either side of focus. I have a 1" extension tube on the camera mount, but it seems that if push the extension tube all the way in, I just focus out the same distance, and if I pull it out, I have to travel the drawtube the equivalent distance closer to the scope. I'm probably missing something obvious, but the extension seems to just act as a longer draw tube and I have plenty of extra play in that.

I'm wondering now if the camera was mounted at a slight angle to the scope, as it is only affixed by two screws.

Alex

[Anthonyexmouth]

looks like theres movement in the ed80 image too. 

[Thalestris24]

Hi Alex

Yes, focus is important and is perhaps more critical with a telescope. With astro, you are (normally manually) focussing at infinity on extremely distant objects and there is no depth of field between different objects at infinity. Also, unlike a camera lens, the image produced by most telescopes isn't flat and needs a corrector to compensate. With a refractor that will be a field flattener, or more likely, a reducer which also flattens the field (but needs precisely the right spacing!) and reduces the focal ratio, i.e. effectively shortens the focal length. There is nothing stopping you doing wider field astrophotography with your 200mm or other lens, though some lenses are better than others for this purpose. The most import thing is the mount you are using .

Louise 

[Han Solo]

To me it looks like your focus is out on the ed80 / DSLR combo. An phone held up to an eye piece can correct for slightly out of focus telescope. Also the longer focal length of the telescope will accentuate and shake which is present, I.e. DSLR mirror slap if your mount isn’t rock solid.

also shooting through a window doesn’t help focus

[alex_wells88]

Thank you to all for your comments! It turns out the camera was just not perfectly flat to the mounting, and light was hitting the sensor at a slight angle. Once lined up properly I had a nice sharp image

And excellent point about through window shooting, Han Solo