5 replies to this topic

[james7ca]

There has been a discussion over in the refractor's topic ( __HERE__ ) about a rather unique 80mm f/10 refractor that is made by Long Perng that has a doublet with Ohara FPL-53 and Lanthanum glass. It's sold as an ED scope but given the quality glass and the rather long f/10 focal ratio it probably performs very similarly to most triplet APOs. In any case, someone who recently purchased this scope (as a rebranded StellaMira) was desiring to test the optics and thus I subsequently provided a list of close double stars ( __HERE__ ) that would be favorably placed for observation over the next month or two.

 

One of those stars, STF1932, peaked my own interest and on Sunday night I had a brief opportunity during late dusk to image this double star using a Celestron C90 Maksutov-Cassegrain spotting scope and a ZWO ASI178MM camera.

 

STF1932 is a true physical double that was first cataloged by Friedrich Georg Wilhelm von Struve in 1830 and which currently has a separation of 1.6 arc seconds. More interesting, however, is that this double consists of nearly equal 7th magnitude stars that during the month of June and early July pass very high overhead for many observers in the mid-northern latitudes (the double is located in in the constellation Corona Borealis). Thus, given these characteristic this pair of stars is a good candidate for testing the resolving power of small telescopes (at least for the next month or so). The 1.6" separation is just about the same as Dawes' limit for a 72mm refractor, but it should still provide something of a challenge for 80mm refractors and 90mm Maks.

 

Here is the link to the Stelle Doppie (Washington Double Star Catalog database) web page on STF1932:

 

  https://www.stelledo...?iddoppia=62338

 

It looks like the A component of this double MAY itself be a double with a separation of just 0.3".

 

STF1932 was also the double star of the month ( __HERE__ , June 2018) on the The Webb Deep-Sky Society website.

 

As for my contribution, I had good seeing and managed to image this double in RGB before San Diego's "June Gloom" marine layer arrived to end my night. Image capture was done in SharpCap Pro with image processing in AutoStakkert!, PixInsight, and Photoshop CC2022. I took five hundred subs for each color and then allowed AutoStakkert! to select the best 200 to produce each RGB channel. I didn't have time to refocus between filters (the clouds were very threatening), but I think that didn't matter as the C90 has good color correction and it seems that the ZWO RGB filters were close enough to being parfocal.

 

Unfortunately, I wasn't given time to take a reference image that would have allowed me to determine the actual image scale, but I expect that it was close to 0.4 arc seconds per pixel. In any case, north is up in this image.

 

So, below is my image of STF1932 as taken with a Celestron C90 and a ZWO ASI178MM camera.

 

Comments and criticisms (C&C) welcomed and thanks for looking.

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Edited by james7ca, 06 June 2022 - 08:31 AM.

[james7ca]

Another nice thing about this double star is that it is fairly easy to find as it is the brighest star along a direct line between the stars Alphecca (Alpha Coronae Borealis, at magnitude 2.2) and Psi Bootis (magnitude 4.5). In fact you should be able to center on Alphecca and then move 3° 39' west (RA) and have the double almost centered in the field of view. Then, 3° 6' further west will find Psi Bootis.

 

If you have a wide-field finder scope you could probably put Alphecca on one side and Psi Bootis on the other (along an east-west line) and find STF1932 almost at the center of the finder's field of view (within about 20 arc minutes). And again, it should be the brightest star in that center area (although only with a combined magnitude of around 6.5).

 

Below is what a 0.75° field of view would look like if you centered on the 2.2 magnitude star Alphecca (the brightest star in the constellation Corona Borealis) and then moved directly west in R.A. by just over three and one half degrees. Or, you could center on the star Psi Bootis (magnitude 4.5) and then let the field drift (without tracking) for 13m 51s and STF1932 should be fairly close to the center of your field of view. The latter should work even on an alt-azimuth mount.

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Edited by james7ca, 06 June 2022 - 07:48 AM.

[james7ca]

I also took a quick shot of the double star Izar, just the green channel as I was in a hurry to finish before the clouds rolled in.

 

North is up in this image.

 

However, you can see a color image of Izar that I took last month with this same setup (C90 and ASI178MM) at the following link on CN:

 

  https://www.cloudyni.../#entry11890284

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Edited by james7ca, 06 June 2022 - 08:32 AM.

[james7ca]

Last night I managed to capture STF1932 using an Astro-Tech AT72ED. Given that Dawes' limit for a 72mm aperture is 1.61 arc seconds I suspect that this is getting pretty close to the limit of what can be done with this size of telescope. I refocused with each RGB filter but I suspect that residual aberrations caused some loss of detail. In particular, the blue channel seemed a bit lacking so it's certainly possible that with a true APO and better seeing conditions one could get a bit more at this aperture.

 

I used a 3X drizzle in AutoStakkert! to get more image scale and then reduced that by 50% to get back down near to the critical sampling for the ASI178MM camera. In any case, that processing should have given a final image scale of about 0.4 arc seconds per pixel which is the same I had with the C90 (approximately, since I didn't measure the actual image scale with either setup). However, the session and processing with the C90 seems to have created a cleaner split.

 

Lastly, while 1.6 arc seconds may seem rather modest it should be remembered that aperture-to-aperture that represents about 0.8 arc seconds for a 6 inch scope.

 

As aways, C&C welcomed and thanks for looking.

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Edited by james7ca, 12 June 2022 - 07:14 AM.

[imtl]

You gotta love those Airy disks. I keep saying it but never enough. I just love the stuff you share here!

[james7ca]

imtl, thanks for the notice.

 

As for the Airy disk or pattern, I've yet to capture a full-color image of them that is really as good as I'd like. I had some in red light when using the AT72ED that look pretty nice but the RGB image just didn't come out very well.

 

The C90 does a pretty good job, but it has fairly broad and bright diffraction rings which I'd prefer to avoid.

 

What I think I need is a very well corrected (APO), long-focus refractor that has perfect collimation and those don't "fall off of the truck" very often (I certainly haven't found anything like that).