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Old 27-12-2011, 11:58 AM
Nortilus (Josh)
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Getting focus with Prime focus?

I seem not to be able to find any definative information about getting focus using certain cameras in prime focus on Newtonian scopes.

In my own limited experience so far i know that you need to have infocus using DSLRs and even my SPC900 webcam. I have thus cheated a little bit so far and have had an adapter made up (see images). I do however have a GSO coma corrector what i havent been able to play with yet imaging and as far as i know this will add outfocus.

Im planning very soon to add the QHY9C to my expanding equipment list and I am curious to know will this camera be in the same boat as the others I have tried so far. Will I need more infocus? If so does that mean either getting a low profile focuser or moving the mirror up the tube? Not really keen on cutting into my scope however but I will if i have to. I read that usually about 25 to 30mm is suffieciant enough and that there are ways of checking (have looked at the DIY here on IIS in regards to this).

Any added info on this will be magical, and not only to myself but others in the same boat.
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Old 27-12-2011, 12:34 PM
gbeal
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Josh.
depends, depends on a lot of things, but you can establish the focus point for yourself quite easily.
Line the scope up on a distant terrestrial object if you have the room, or something like the moon at night if you don't. Take a piece of lunch paper or similar and hold it over the end of the focuser draw tube, and focus. When the image is as close to focus as you can get it, this is the point. I'd suggest you make a measurement from the focuser base, or the outside edge of the tube or something, and record that.
The rest is just mechanical requirements. The QHY9 for example will have a distance of about 15 - 20mm from the chip to the front of the metal section,so your focus point needs to accommodate at least this 20mm.
A DSLR is worse, it needs "about" 45 - 50mm.
Another way is to hold the DSLR body (no lens) at the end of the focuser draw-tube and look through it. If you can focus it, then you are in luck, but I suspect you won't be able to.
Remedies are as you suggest: a lower profile focuser, or shifting the primary mirror towards the focuser. Neither is too daunting, but if you are shortening the tube, remember to measure twice and cut once.
Try shifting the mirror on the collimation bolts/springs first, the little amount you can move it might be all you need. I tried longer collimation bolts for one scope, it worked.
Gary
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Old 27-12-2011, 01:33 PM
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rainwatcher (Peter)
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Common problem with newtonians, mine is the same. I have to move my mirror for my Canon 100d at prime. Its only a couple of mm. I have to be carefull because my 6.3 mm eyepiece projection setup is in focus almost at the end of the travel, although this is easier to fix with an adapter.
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Old 27-12-2011, 02:14 PM
adman (Adam)
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Josh with that setup you will need to do one of two things - you will need to bring the focal plane further out of the focuser, (by either cutting the tube shorter, or getting longer collimation bolts to bring the mirror further up the tube) OR you can get a lower profile focuser to bring the camera closer in.

Finding the rough location of the focal plane during the day time with a piece of translucent paper as per Gary's suggestion is a good idea. The location of the imaging sensor in various cameras should be available by a simple google search - all canon eos camera have the same distance from the metal flange at the front that the lenses attach to and the sensor which from memory is 44mm (maybe 46??) so then you can figure out roughly where the camera needs to be to get focus.

There are quite a few threads addressing this issue on IIS, so have a search around and see some of the solutions that have been used before. But when it comes down to it, it's just simple measurements, and getting the focal plane and the sensor in the same place.

Adam
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Old 27-12-2011, 02:31 PM
gbeal
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In addition to the measurement that Adam suggested, 44mm or so, the T ring at about 10mm needs to be considered, this giving the magical 55-56mm of distance that is used with the MPCC and other flatteners.
Gary
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Old 27-12-2011, 04:02 PM
Nortilus (Josh)
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Im starting to feel that the best solution to this problem will be to cut the tube...after some of the research i have been doing...
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Old 27-12-2011, 08:10 PM
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RobF (Rob)
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Yep, lots of threads on this if you dig around on IIS. Definitely worth trying to move the mirror up a bit first before you get too carried away.

Another option aspiring Newt imagers have to consider now is the Baader Rowe coma corrector. Peter Ridgeway showed me one recently, but I haven't played with it at all. In theory, looks nice - gives 91.5mm backfocus if you can get it into position properly in the focuser. I think the more common Baader MPCC helps a little with backfocus too.
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Old 27-12-2011, 10:34 PM
Nortilus (Josh)
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i have a GSO coma corrector...not sure really how it performs but hey...gotta give it a go...
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