Prime Focus Astrophotography

[Likwid]

Hello,

I recently purchased an Orion ST80 along with a Skywatcher Star Adventurer to get my feet wet. I am a photographer and know my way around a DSLR. I also modified a Rebel XS and removed the UV/IR Cut filter myself a few weeks ago. When I got the telescope, I was able to barely make out the bands on Jupiter with a 10mm eyepiece and a 2x Barlow. After that, during the daytime I connected my modified DSLR to the ST80 with an Orion Variable Camera Adapter. I couldn't focus on anything at all. I took out the variable part of the adapter and the just connected the T-Ring to the small 1.25" tube to get the camera as close to the telescope as I could and I was able to focus normally. I thought that since my target was relatively close, it required the camera to be closer. Last night was the first clear night in over a week, so I was excited to check out the moon. Again, I had to get the camera as close to the telescope as possible to get anything in focus. I snapped a few shots and I attached the best one of the bunch. It is a single shot, not stacked. I want to try to add the barlow or an eyepiece to the mix to get closer to the moon. I read somewhere that I might need another extension tube to get focus, which makes sense to me. With the variable adapter extended all the way, the camera is probably 8" away from the telescope already and throwing off the balance as it is. Also, I tried attaching the Barlow directly to the T-Ring and I still could not focus. iWould it be safe to add an extension tube or 2 to the mix? Is that the way to go, or is that asking too much of the ST80?

[happy-kat]

Welcome

You'll need a 2 inch extension. To get a bigger Moon you'll have a lot more success with a webcam, the dslr has a large chip compared to a webcam.

My dslr with t ring and t mount fits securely into a focuser or extension. I don't know anything about your Orion variable camera adaptor that sounds more like for afocal imaging. Prime focus is where the camera body users the telescope as the lens.

[jambouk]

The sensor of your camera needs to be positioned to the light coming through the telescope archives focus on the sensor. Different set ups encounter different problems. Often with a reflecting telescope the user feels they cannot rack the focus in enough; with refractors people often need the focus tube to come out more (but there isn't enough length in the focus tube). There are ways around both of these situations. A Barlow commonly not only "magnifies" the image, they also usually bring the focal point out, away from the telescope a bit - this is useful if you are using a reflector and a DSLR. In your situation, it sounds like you need an extension tube to move the camera further away from the telescope. Potentially a focal reducer would also help as it will bring the focal point closer to the telescope and allow you to achieve focus. It might be worth searching on here for your scope and camera combo and see what others have done. Adding a Barlow to your current set up will make the focus issue worse. Do you have a star diagonal?

The other thing which I'd say if that you might want to do a custom white balance to get rid of the colour mis-alignment you are now getting that you have modified the camera. If you don't know how to do this, it will be mentioned in the camera's instruction manual.

Good luck.

James

 

[jambouk]

This page is quite useful:

https://starizona.com/acb/basics/equip_optics101_optical.aspx

James

 

[Likwid]

That makes a lot of sense now and that link has some good information! Thank you for all the replies! For the white balance, I tried compensating for that in Lightroom quickly but the color of the moon still looks off so I will play with that a little more. Its tough finding information when you start out because you don't know what you don't know.

[SonnyE]

Hi, and Welcome!

While I don't have your specific telescope, I do image through an ED80T CF Orion telescope.

When beginning, I found I needed some extension tubes to get the back-focus  I needed. Orion (and I am sure others) sell this focuser fitted tubes to allow that needed distance. I also have one of these fitted on the filter wheel housing.

Currently what I have is the 2" focuser tube fitted to my telescopes focuser, then I have a filter wheel (Optional), and a spool threaded to the back of the filter wheel where I can mount my Astronomy camera, or my DSLR with a T-Mount adapter.

My telescopes focuser has 600+ mm of travel. With the tube, my DSO camera likes about 400 mm of back travel.

But my DSLR likes about 100 mm or so of back travel.

DSLR's do need as big an opening in front of them as possible to avoid Vignetting. When I mount my DSLR to the back of my 1.25" filter wheel, it's awful. So I have to use the center most part of the frame for the subject.

I haven't tried it, but a friend advised using 48 mm adapters to get as much opening as possible in front of a DSLR's sensor. So food for thought.

These bits helped me find fixes for the back-focus issues I discovered.