Nikon D3200 and Celestron C90 Telescope

T_Tully

Hi Moose, I'm new to the group. Thank you for the D3200 cheat cards. I'm using the D3200 with a Celestron C90 spotting scope (90mm mirror/lens f/1250) for pictures of moon, planets, etc. The setup is completely manual and the camera is connected by T-ring to the prime focus of the spotting scope. I get some nice shots, but they just don't seem to be in absolute focus. I've experimented with a variety of ISO, white balance, exposure, and shutter settings. I'm using a very sturdy tripod and remote. Any ideas will be greatly appreciated. Thanks for your time.

bruto

I'm not Moose, obviously, but I'll throw in a couple of suggestions.

First of all, I'd check focus with live view. The focus system used there is actually entirely different from that used for the viewfinder, and uses the image sensor instead of a separate focus sensor. So issues of focus inaccuracy and back focusing that can plague some cameras should not appear there. You might have the additional help that the mirror is flipped up already, so there is no danger of vibration from that. Even a good tripod can shake a little with that big a rig on it.

With manual lenses, the focus confirmation dot in the viewfinder is not always accurate even if your AF is.

If you do need to use the viewfinder, the D3200's finder is marginal for manual focusing.

You can get a slide-on adapter, DK-22, which has a threaded hole. This allows you to fit the 2X eyepiece magnifier (DG-2). The DK-22 is only about 5 bucks, while the DG-2 much more, but sometimes it can be found used, as it has been around since the 1960's. This setup makes focusing much easier, though you don't see the whole frame. The magnifier flips up out of the way for regular viewing.

T_Tully

Hi Bruto, thanks for the reply. I use LV and use the focusing knob on the spotting scope to bring image into focus. It's really close.
Thanks again for your time.

bruto

I am not entirely sure how this all works, but as I understand it the camera must be in proper focus to the mirror in the lens. In a regular refractive lens, the lens focuses on the camera's focal plane. In a mirror lens, I believe the lens focuses internally, and the camera mounting tube must be precise enough to focus on the focused image in the lens. I wonder if there is some way you could adjust the length of the tube to sharpen the image.

I'm in alien territory here, but I do have (not at hand, loaned out unfortunately) a 500 mm mirror lens that worked fairly nicely when mounted to a camera,. With a scope-conversion eyepiece that works perfectly on every refractive lens I've tried, I cannot get good focus with the mirror lens. This is sort of the reverse of your problem, but it seems similar.

Edit to add: further investigation suggests I'm probably wrong about the above and how a mirror lens focuses, but it does not help one bit to understand why the mirror lens that is sharp on a camera is unsharp with an eyepiece. Something obviously is different here, but what it is seems hard to figure out.

T_Tully

I'll bet your right about adjusting the length. I have about 1" to work with and a 2" extender piece. I'll follow up the next time I set up. Off topic, the "Stars, Galaxy, and Northern Lights" cheat cards paid off big last night. That was the main reason I bought them.
Got some good shots of the Milky Way.
Thank you.

Moose

Hey @T_Tully - As always, @bruto offered up some great advice.

The one thing I'll add is shutter speed. If the shutter speed drops below say 15 or 30 seconds, you'll get subject blur no matter how sturdy your rig is. The reason for this is due to the natural orbit of the moon and stars. As you might imagine, they don't stay fixed at any single point for very long.

The closer you're zoomed into the moon, the shorter the shutter speed has to be. If you can't get shutter speeds shorter than 15 seconds at your current ISO, you'll need to increase the ISO a bit to get faster shutter speeds.

Give that a try and report back. All the best and thanks @bruto for your help!

okimar

Hey @T-Tully, did you find the solution you were looking for?

T_Tully

Hello, yes I was able to make a few adjustments and took some nice photos of the moon. Sorry so late in posting. Thanks to all.