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Lovely double star session with simple equipment


Ags
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My main telescope is currently set up for imaging, so last night I ended up outside with an ST80 attached to a travel head on a photo tripod. The cheap diagonal was somehow occupied by a Revelation achro 2x barlow and 20mm plossl.

I had no plans to do observing last night, but a draft of my book "Discovering Double Stars" had arrived and I wanted to test it under some real stars.  There weren't many stars though, with midnight twilight and city lights I could only see the brightest star in Cepheus, so fourth magnitude and fainter stars were whited out.

In brief the book worked well, and was easily read by red light torch. The star patterns easily guided me to doubles I had never seen before, so of course I was really pleased.

But what will stick in my mind are the doubles, all viewed at 40x. In the refractor they were such perfect pinpoints compared to the less precise splots in my SCT. Zeta and Beta Cygni were delightful, and the colors in Delta Cephei were gorgeous! I was surprised I could split Xi Cephei at such a low magnification.

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1 hour ago, Ags said:

...  all viewed at 40x. In the refractor they were such perfect pinpoints compared to the less precise splots in my SCT. Zeta and Beta Cygni were delightful, and the colors in Delta Cephei were gorgeous! I was surprised I could split Xi Cephei at such a low magnification.

Your Schmidt-Cassegrain may need collimation. I do agree that the views through smaller apertures can be surprising and delightfult.

 

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If I go hunting for doubles I much prefer to use one of my ‘fracs as the star views are much more pleasing. If I’m hunting down faint galaxies then my 8” SCT hits the spot there.

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Posted (edited)
55 minutes ago, mikemarotta said:

Your Schmidt-Cassegrain may need collimation. I do agree that the views through smaller apertures can be surprising and delightfult.

 

I'll double check next time I have it out, but even perfect collimation seems to leave a little residual splodginess. The SCT never quite matches the cleanness of the views in my Skymax 102.

Edited by Ags
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4 minutes ago, Ags said:

I'll double check next time I have it out, but even perfect collimation seems to leave a little residual splodginess. The SCT never quite matches the cleanness of the views in my Skymax 102.

That sounds about right for an SCT, always seems to be a bit of fuzziness around stars in my experience, rarely nice neat airy disks. Cooling and seeing are normally to blame I guess.

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3 minutes ago, Ags said:

I was amazed by the beauty of the views given it was just a humble ST80 and Plossl.

Small scopes have lower resolution and present larger airy disks. This means that stars actually have a lovely, clean appearance in these small scopes as the airy disk is much easier to see, and double stars can be easier to split (at least down to the achievable separation for a particular aperture)

I love the views in my little Telementor and TAL Alkor, stars like Izar look absolutely beautiful in it, lovely round primary and the secondary looks like a gemstone on the first diffraction ring.

A larger scope will split tighter doubles, but the views are often nowhere near as beautiful as the airy disk is much smaller, harder to see and often gets lost in the seeing disturbances.

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On a good night my C9.25 split 0.7" and had very tight, tiny airy discs. Mostly though a bit fuzzy. I do believe it is the seeing conditions - large scopes in general are more vulnerable.

My 80ED, an ageing Celestron, gives lovely star views. Not much matches it for colour rendition. Everything seems more vivid.

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It's a balance  isn't it? Fracs give a lovely view, but with dinner plate sized Airy disks that preclude close doubles, whereas compound scopes give a messy view, but split closer doubles and equally important, show fainter companions. I compromise and get the best of both worlds by having an ED80 alongside my 180 Mak.

Chris

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Posted (edited)

Every once in a while my ST80 does this and I start shopping for ED doublets...

Edited by Ags
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1 hour ago, chiltonstar said:

dinner plate sized Airy disks

🤣🤣 a little harsh Chris but I know what you are saying.

 

2 hours ago, Mr Spock said:

I do believe it is the seeing conditions - large scopes in general are more vulnerable.

Totally right, if seeing were no issue, the larger scope wins every time. Maybe we need to move to the Kalahari desert…

I’m sure compound scopes suffer more too; long focal length, multiple passes up and down a tube which may not be in thermal equilibrium, larger central obstruction. When everything is right though they can obviously give fine images.

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All roads lead to spending more money... I am thinking of saving my pocket money for one of these:

https://www.teleskop-express.de/shop/product_info.php/info/p4964_TS-Optics-ED-APO-102-mm-f-7-Refraktor-mit-2-5--R-P-Okularauszug.html

https://www.teleskop-express.de/shop/product_info.php/info/p1151_TS-Optics-70-mm-f-6-ED-APO---Reiserefraktor-fuer-Beobachtung-und-Fotografie.html

I am thinking the 70 mm might give more charming views of doubles and would be great for travel and photography, but the 102 mm would be able to replace my Skymax 102 and would let me get up to around 100x on planets (I am restricted to exit pupils of at least 1 mm). Probably I need both, but couldn't justify that many telescopes!

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2 hours ago, Ags said:

All roads lead to spending more money... I am thinking of saving my pocket money for one of these:

https://www.teleskop-express.de/shop/product_info.php/info/p4964_TS-Optics-ED-APO-102-mm-f-7-Refraktor-mit-2-5--R-P-Okularauszug.html

https://www.teleskop-express.de/shop/product_info.php/info/p1151_TS-Optics-70-mm-f-6-ED-APO---Reiserefraktor-fuer-Beobachtung-und-Fotografie.html

I am thinking the 70 mm might give more charming views of doubles and would be great for travel and photography, but the 102 mm would be able to replace my Skymax 102 and would let me get up to around 100x on planets (I am restricted to exit pupils of at least 1 mm). Probably I need both, but couldn't justify that many telescopes!

Ags, I was *this* close to owning the TS 70ED - ordered and everything - but the delivery got pushed out another couple of months, and with an upcoming holiday to darker skies I ended up cancelling the order and plumping the extra cash for a William Optics Z73. I can vouch for how nice doubles look in a small frac as I was observing Albireo and Epsilon Lyrae in the last few days.

I'm very, very pleased with the Z73 but I did kinda fall in love with (at least the thought of) the TS 70ED, so if you do go for it I'd love to hear how it turns out!

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