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Buzzard75

Camera Selection for EAA with a Dob

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I've had a 12" dob for almost two years now and have loved the visual observations I've done with it. I've also dabbled in astrophotography using an iOptron Sky Guider Pro and a DSLR camera with fair results. I've wanted to step up my astrophotography game, but I just don't have the funds at the moment for a mount, the scope, a CMOS camera, a guider and all the other accessories that go along with it. However, I've done a bit of reading and it sounds like I may be able to do some EAA with my 12" dob, which would be amazing. It's an absolute monster of a light bucket. My thinking is, if I got something like a QHY camera with a 1.25" body, I shouldn't have any problem with achieving focus because I can use the 1.25" eyepiece adapter and push the camera into the focuser body as much as necessary to achieve focus. Is my thinking correct on that?

If so, that just leaves me with a couple of questions. Can you use focal reducers, say a .5 or .33, with the QHY cameras? Would that require spacers? Would a focal reducer present any issues using it with an eyepiece adapter on a dob that has limited in focus?

I know that with the dob being Alt-Az and such a long focal length, I'm going to be limited on exposure length. I figure with such a fast focal ratio though, I shouldn't need very long exposures.

As a "bonus", when I start getting a dedicated AP setup I can repurpose that camera as a guider. I was looking at the QHY5III178 color and a .5 or .33 focal reducer.

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If you have motorized dob, then yes, it should work. Not sure how well it will work, you are right - that scope is monster at 1500mm FL (I'm guessing here) and you will have very high "magnification". Reducer is a must.

I've used very small sensor of QHY5IILc with 1.25" x0.5 reducer from GSO (probably Revelation brand in UK) and it worked well on my 8" F/6. For such small sensor (same as ASI120) I did not even need coma corrector.

Best way to go about it is to get that cheap x0.5 reducer and vary the spacing between it and the sensor. More distance will provide higher reduction, while less distance - lower reduction. You can "dial" in reduction that you see as acceptable, because the more reduction you use - worse outer field will be - both from scope coma and from reducer it self.

I remember I needed to mod my 1.25" x0.5 reducer, but it was fairly simple task - it required reversing lens in the cell - easy to do, just unscrew retaining ring, reverse lens and tighten back retaining ring. For some reason it gave better performance in "reversed" orientation. It might be that it was accidentally reversed at factory, or maybe it just works better that way with sensors - it was meant as EP reducer originally.

Using reducer pushes focal plane "inward", but you should not have trouble reaching focus with camera and your scope.

There is another approach that you can try - using EP projection. You'll need suitable adapter for this, and regular plossl - like 32mm one. Here too distance from EP to sensor plays a part in reduction, and can even act as barlow if you place sensor too far away. In this case, placing sensor closer to EP results in higher reduction, but it also moves focal point further away from tube - you might need extension between focuser and EP if you take this route.

In general, go with camera with the least read noise and highest QE that you can afford. ASI290 and ASI385 are very good candidates. ASI224 is also, but it lacks "pixels" it is only 1280x1024 or something like that.

You'll probably need to keep exposures fairly short - like couple of seconds - up to ten or so. I think that motorized dob mount lacks precision to go any longer than this. It might even not be suitable and you might find it too jumpy - something that will not cause problems for visual but can be problematic for EAA and imaging. At this point - its really thing of trying it out.

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Thanks, vlaiv! Yeah, it's 1500mm and a motorized GoTo. The tracking on it is actually pretty good. If I do my alignment well enough I can put something, say Jupiter for example, centered in my TV Delos 10mm eyepiece and come back over an hour later and it's still in the same place or at least extremely close. Was figuring at least a 0.5x reducer would be needed, and possibly even 0.33x, with that much focal length. A 0.5x would get me roughly the same field of view as my Delos 10mm. I know lowest read noise and highest QE are best, but my concern with something like an ASI290, ASI385 or anything that style would be being able to get focus. I almost feel like I have to have something that goes down inside the focuser like an eyepiece would. The stock focuser is pretty tall so I don't have a lot of inward focus. I know I need to use my Barlow 2x with my DSLR, otherwise I can't get focus, but the sensor is 55mm deep. Next time I get it out, I'll try and figure out where the end of the eyepiece sits in the focuser tube. If I understand correctly, that should be approximately where the camera sensor needs to sit in order to achieve focus. Obviously everyone's eyes are different and eyes looking through an eyepiece are also slightly different than a digital sensor in their place, but it should put me in the ballpark.

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I've got the same GoTo DOB as you ... XX12GT. I drilled a second hole in the strut secondary are as per the photo's ... brings my Canon DSLR APS/C camera to focus nicely. If I used EP's init to do alignment or visual I simply stick a small 2" extension tube in and EP's will also then come to focus. EAA with the APS/C sized sensor worked a treat with EOS Utilities controlling camera ISO and exposure times. Setup EOS Utilities to save images to a folder monitored by Astrotoaster on a bootcamped Macbook. Astrotoaster uses DSS in the background to do the stacking (if necessary) and Toaster to do the AP-like stretches so it made the EAA image happen. None of my EAA shots from that DOB available unfortunately, as when Photobucket went to a cash-to-use model I declined. And those buggers simply wiped the account photos and all.

However, my example inspired a friend of mine in New Zealand to do the same. Link to his shots below using DSLR on Skywatcher DOB for years with all sorts of scopes piggyback on it. Being adjustable struts he did not have to drill any holes but simply adjusted the length via the adjustable set-point knobs. He rigged a counterweight system on a adjustable rail so the Alt motor could see the load as balanced ... and so would still work in Alt. He carried ED80 + a second DSLR for widefield, and whole bunch of piggybacked scopes as per the photos of his rig down in the link below.

The APS/C sized sensor gives a good 1.2 degree FOV on the 1500mm DOB. A great view of most Messiers. The stacking capability of toaster allows much more SNR in the final stacked image which allows some very good shots from the combo.

BTW. You can also get bigger primary mirror bolts to "push" the primary mirror into the OTA more thus pushing the focal point further out the tube (instead of modding the XX12GT struts).

My photos showing how I moved the focal point further outside the tube thus allowing the DSLR to come to focus. 

https://flic.kr/s/aHskZvmNDE

Very long video when I first tested that the shortening of my struts had worked nicely. It had reached focus and was amazingly sharp ... live HD desktop capture of Moon surface at Canon EOS Utilities 5x sensor zoom. Dont watch it all as its 15 minutes!

https://youtu.be/LQJ9WyE2nmA

Friend in NZ showing his amazing balanced DOB using moveable counterweight system ...

https://flic.kr/s/aHsksGCqYM

His amazing DSLR images using that setup ...

https://flic.kr/s/aHskGspbb6

Edited by Howie_Oz
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2 hours ago, Howie_Oz said:

I've got the same GoTo DOB as you ... XX12GT. I drilled a second hole in the strut secondary are as per the photo's ... brings my Canon DSLR APS/C camera to focus nicely. If I used EP's init to do alignment or visual I simply stick a small 2" extension tube in and EP's will also then come to focus. EAA with the APS/C sized sensor worked a treat with EOS Utilities controlling camera ISO and exposure times. Setup EOS Utilities to save images to a folder monitored by Astrotoaster on a bootcamped Macbook. Astrotoaster uses DSS in the background to do the stacking (if necessary) and Toaster to do the AP-like stretches so it made the EAA image happen. None of my EAA shots from that DOB available unfortunately, as when Photobucket went to a cash-to-use model I declined. And those buggers simply wiped the account photos and all.

However, my example inspired a friend of mine in New Zealand to do the same. Link to his shots below using DSLR on Skywatcher DOB for years with all sorts of scopes piggyback on it. Being adjustable struts he did not have to drill any holes but simply adjusted the length via the adjustable set-point knobs. He rigged a counterweight system on a adjustable rail so the Alt motor could see the load as balanced ... and so would still work in Alt. He carried ED80 + a second DSLR for widefield, and whole bunch of piggybacked scopes as per the photos of his rig down in the link below.

The APS/C sized sensor gives a good 1.2 degree FOV on the 1500mm DOB. A great view of most Messiers. The stacking capability of toaster allows much more SNR in the final stacked image which allows some very good shots from the combo.

BTW. You can also get bigger primary mirror bolts to "push" the primary mirror into the OTA more thus pushing the focal point further out the tube (instead of modding the XX12GT struts).

My photos showing how I moved the focal point further outside the tube thus allowing the DSLR to come to focus. 

https://flic.kr/s/aHskZvmNDE

Very long video when I first tested that the shortening of my struts had worked nicely. It had reached focus and was amazingly sharp ... live HD desktop capture of Moon surface at Canon EOS Utilities 5x sensor zoom. Dont watch it all as its 15 minutes!

https://youtu.be/LQJ9WyE2nmA

Friend in NZ showing his amazing balanced DOB using moveable counterweight system ...

https://flic.kr/s/aHsksGCqYM

His amazing DSLR images using that setup ...

https://flic.kr/s/aHskGspbb6

I wondered about either some different struts or modifying mine somehow. Wonderful video of the moon you were able to capture. I haven't been able to get the full disk because I've had to use a Barlow. Was there a specific measurement you took for the new hole or did you just move it down as far as you were comfortable?

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There's probably some smart way to calculate it all. But being not smart enough for that, I made up a jig. Imagine a plank of wood on top of two saw-horses down by the bay near where I live. The whole xx12 tube lay on top horizonally and pointing across the bay towards the port facility on the other side of the bay about 5 km away. The camera was inserted into the focuser (along with its 2" Baader MPCC Canon fitment) and (importantly) with the focuser wound out about a two centimeters to allow for any possible adjustment around the focus point later on. But ... the Orion struts were not holding the primary and secondary tube assemblies apart on that "jig"! The primary and secondary tube assemblies were resting in a 'bay' created by two lengths of straight timber screwed onto the plank. Thus gravity held the primary and secondary tube assemblies hard against the horizontal plank/timber guides, which being straight still held the optical alignment between primary and secondary mirror cells. I simply moved the secondary mirror assembly containing the focuser/camera slowly towards the primary cell until I got focus. Then I measured the distance between primary and secondary assemblies. Back home I reassembled the xx12gt using the proper struts and checked that measurement to find I needed to drill those second holes exactly 40mm towards the primary mirror to work for my Canon 700D. So the four attachment plates on the struts where the struts attach to the secondary mirror cell, needed a second hole drilled 40mm closer to the end pointing to the primary mirror. I took the struts to a local machine shop where they had proper drill presses, to guarantee each hole was the same distance from, but still aligned with the centre of the original holes. They were so intrigued with it all, and in disbelief that an amateur telescope could be THAT big, that they didn't even charge me for it! LOL. Note that it was not required to perfectly optically align the primary and secondary cells in the "bay" on the plank. As long as the camera showed focus, and the focuser knobs allowed me to wind up to then past the point where the camera got the best focus on that 'rig', that's all that was needed. For once the struts were drilled mm perfect at the machine shop, and I then did a collimation, the focuser would look after that last absolute perfect slight adjustment to bring it properly to focus out there in the dark. Leastwise, it worked out well for me. Cheers

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