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Lunt 50 1st light (imaging)


Pete Presland

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Well finally I got to follow the 1st "visual" light with 1st "imaging" light, but only a couple of AVI's though :sad:  As I unable to achieve focus using the eyepiece tube and my DMk21 i switched to the ASI1200mm screwed directly into the B600 diagonal. Pulling the diagonal out of the focuser approx 10mm enables focus to be achieved comfortably.

Using Firecapture in max size 1280x960 the full disc is in frame :smiley: I didn't realise this when I captured the video hence the bright area on the western limb :rolleyes: Very much a work in progress.

Initial thoughts were that the contrast seems a lot less than the PST, but its obvious by the pretty average image posted here that the illumination is much more even.

Again it is disappointing that the scope will not reach focus with a standard camera in the eyepiece holder especially when the B600 filter is supplied for imaging purposes at extra cost. I think I will hang on to the PST  for a little while yet, especially for close ups of any big proms that occur (got a fleeting glimpse of todays)

As always with a 1st light comes a few questions.

If as seems focus is an issue that can only be resolved by screwing the ASI directly into the diagonal how do I increase the image scale? can I put a barlow between the diagonal and the scope to increase the focal length?  Also any tips on increasing contrast without double stacking?

Any comments as always very gratefully received :grin:

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They're certainly good first efforts, but I'm a bit confused by your comments.  Perhaps I've misunderstood.

You say you can only get the ASI120 to focus if it is screwed onto the B600 and then the B600 is moved out of the focuser tube by about 10mm.  I'm assuming you're using the T2 thread on the ASI120 for that.  Obviously that moves the camera further forward on the optical path compared with using the nosepiece, but I'm struggling to see how that would be more than 10mm compared with using the nosepiece (looking at my own 120, I'd say the entire depth of the flange with the internal T2 thread, plus the thickness of the nosepiece shoulder, can't be more than 9mm), in which case I'd have thought using the nosepiece and having the B600 fully in the focuser tube would work as well.

Would I be correct in assuming that you seem to be running out of inwards focuser travel?

James

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Yes it is inward travel that sems to be the problem. The eyepiece holder is 30 mm in length so when that is removed i read that the diagonal needs to be withdrawn from the focuser slightly so that fouser is not at the end of its travel when in focus. The focuser only has around 30 of travel itself.  The distance from the chip to the end of the nose pice is just over 30 mm on my 120mm.

I have not tried a barlow with the ASI, but i did with the DMK because i remembered similar issues with the PST and a lack inward travel which i resolved by cutting down the nose piece or adding a barlow.

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What barlow are you using?  If you've not tried already, perhaps it might help to use one with a fairly short barrel such as the Orion shorty or the Celestron Ultima (assuming you can find one), or even the Revelation 2.5x.

James

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There's a thread here that might help, though I'm really not sure: http://www.cloudynights.com/topic/496517-lunt-50-b600-back-focus-observing-imaging-tips/

James

Interesting discussion thank you for the link. I think I am going to have to do some tests with various barlows.

Sent from my GT-N7100 using Tapatalk

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I think I am going to fit the ASI120mm direct into the diagonal. Then leave the diagonal fully in the focuser, then measure how far out the focuser is when focus is achieved.

Hopefully this will give me some idea of where the focus point is.

Sent from my GT-N7100 using Tapatalk

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Great start Pete.  When I saw a Lunt 50DS at Astrofest earlier this year I thought, yes, one would be nice for a portable set up, especially as I was carefully eyeing up a Lunt 80DS at the same time.  I have seen some good images posted elsewhere from the little 50 and on paper the specs look good, so I think you will improve.

A barlow usually gives you out focus, but by how much depends on the barlow and the distance between the barlow and ccd.  Perhaps your barlow is either not giving very much out focus or is just too powerful, making focus very tricky.  I use a screw in barlow and then adjust the barlow lens to camera distance by using different nosepieces or spacers.

I think it is just a case of trial and error to see what the optimum set up is.

Robin

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