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Refracting telescopes are expensive, especially those suited for photography. A great way to get started is to use an existing camera lens. There are pros and cons to doing this, but you can experiment and get started with narrow band imaging by using the right adapter.
This can save you some money upfront by allowing you to use a quality lens that you probably already own without buying a new telescope. You can always upgrade later after you get all the bugs worked out of your workflow.
Back Focus - Getting It Right
A Nikon F Mount lens has 46.5 mm of back focus. This means that you need to get everything installed from the lens so that it is 46.5 mm to the focal plane of your astrophotography camera.
The Nikon Adapter that I bought has a total of 34 mm of length. This was achieved by using the 5mm spacer that came with the adapter. My ZWO camera has 12.5 mm of back focus. Adding these two together achieved the correct 46.5mm of length. I had no problem at all getting my nebula in focus.
Now if you have a different camera, you can easily make it work by adjusting the spacer as shown here:
Watch The Tutorial
I’ll show you how I hooked up my ZWO 1178MM camera to my 80-400 mm Nikon Zoom.4
Using 1.25″ Filters
Out of the box, the adapter works with 2″ filters. You can easily use 1.25″ filter with an adapter.
Workflow Using Narrow Band Filters
For my first test using this adapter, I wanted to capture the Orion Nebula in narrow band. I first slewed to the subject and centered it using my AsiAir Plus.
Next I removed the adapter from the lens without disturbing the position of the lens. This has to be done carefully or it will throw the lens off target. I unscrewed the outer adapter and then installed my H-alpha filter. This was re-assembled and the the focus was adjusted with the narrow band filter installed. At this point imaging can begin.
I set the exposure for 60 Seconds. I had previously achieved less than 2 arc seconds total error with my polar alightment. Here is the very first shot I got in narrow band.
I was pleased with the first test especially since I did not use autoguiding due to the fact that I only have one dedicated astronomy camera.
As soon as the weather clears, i’ll shoot this again in the full Hubble Pallette and update this post.