Old 14-09-2017, 04:57 PM
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Tinderboxsky (Steve)
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Lowest aperture to resolve double star HJ5188

I spent some time last night observing HJ5188, a double star in Sagittarius to find the lowest aperture with which I could resolve the faint companion.

The companion is mag 10.13, the primary mag 6.63 and the separation is 3.3. So, the challenge is resolving a close faint (relative to the primary) companion .

The seeing was reasonable and transparency good. Seeing limited the best useful magnification to 100X in my 140mm refractor. H5188 was at 72 degree altitude, so nice and high for observing. I had a series of simple cardboard aperture stops that could be slipped into a holder on the dew shield.

140mm aperture - straight forward with faint secondary clearly visible
120mm aperture - difficult observation with faint secondary still visible
100mm aperture - very difficult observation. Very faint secondary only visible in moments of better seeing
90mm aperture - failed to observe the faint secondary.

I have a feeling it might be possible to improve on these observations slightly on a night of good seeing.

Why this double star? Well, it is one of the test pairs in the Sissy Haas Double Star project. Sissy has put out a call for more observations for this pair.

Scope: Vixen NA140SS with LVW8 giving 100X.
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Old 14-09-2017, 05:12 PM
Tropo-Bob (Bob)
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Thanks for your report Steve. I notice its been cold and wet in your area.

I will give it a go once the local clouds and bushfire smoke clears.
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Old 18-09-2017, 06:58 AM
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The Mekon (John Briggs)
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Thanks Steve for bringing this challenge to my attention.

I spent almost an hour finding and observing this star under quite good conditions last night.

I had my 18" set up which I can use with a 7" off axis mask, + my 106mm Sharpstar for which I have an 80mm mask for courtesy of 3D printing by Meru.

Since there are no prominent stars or even any NGC objects nearby, it was dial up by RA& Dec to find the star. I did this in the 18" first up and located HJ5188 and the 10th mag companion was of course bright in this aperture, but as usual the separation not so good.
masking down to 7" saw lovely airy disc and easy separation - the companion was clear, well separated and distinct.

Now with the 106mm. Found the star - it is in fact in a nice field with a 7th mag star well to the north, another 8th mag about 2' to the north and a nice little 8th or 9th mag double about 3' or 4' ENE.

Knowing just exactly where to look made it not too hard in the 106mm - with a bit of patience I would have detected it regardless. It was not easy by any means. I was using 140x and I could also see & split the other double ENE at 106mm.

Now down to 80mm - using a mask means you are assured of correct focus which is a help. At 80mm and 140x the companion was only observed fleetingly and only because I knew where it was. The double ENE was also not easy, insufficient light gathering to make the split distinct. I would doubt any observation that showed this straight away in a non Apo 80mm.

All up a great test for scopes, thanks again Steve, If I were you I would try again at a slightly higher magnification than 100X.

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Old 18-09-2017, 08:19 AM
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Tinderboxsky (Steve)
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I enjoyed your report John.
Yes, I will certainly give this target another try on a night of good seeing. I tried higher magnification the other night but the seeing just made a mess of the image. 100X was the best I could achieve on the night.
I live on the coast south of Hobart and unfortunately nights of good seeing are few and far between. Just got to be patient.
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