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Triton (and a mystery star) with a 90SLT

Astrophotography DSLR Maksutov Celestron Moon Planet Observing Report Astrometry
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#1 Nicole Sharp

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Posted 12 November 2021 - 06:13 AM

I only used the Celestron NexStar 90SLT once before returning it, so I had only a few hours to capture some photos and decided to try for Neptune, which is the only planet I've never been able to photograph before (I was able to photograph Uranus without GOTO during the Venus-Uranus conjunction).  Unfortunately, I didn't know which "star" was Neptune, but the GOTO was accurate enough (after a Solar System Alignment on Jupiter) to still put it within the field of view.  Overall, I was impressed with the performance of the 90SLT, and was disappointed that I had to return it due to the fact that it did not have sufficient clearance to go above 77 degrees in altitude angle without the optical tube assembly hitting the tripod legs (the 90SLT needs a pier extension or wedge to go to zenith when used with a DSLR camera).

 

I couldn't find any photos online of Triton taken with an SLT mount, so I figured I should share it here:

 

https://www.flickr.c...57720128436529/

 

https://www.flickr.c...p/tags/neptune/

 

Neptune can be identified as belonging to the "asterism" of four bright "stars" in a line, the third "star" (from lowest to highest in altitude angle) being Neptune.

 

Triton is most clearly visible at 33% zoom.  Increasing the image scale past 33% washes out the signal of Triton with the increased visibility of ISO grain and altazimuth field rotation.  This was taken using a 30-second exposure at ISO 51200, so there is up to 8 pixels of trailing (proportional to the angular separation from the optical axis).  The offset of Neptune from the optical axis is because I didn't know which "star" was Neptune in the DSLR liveview after GOTO Neptune.

 

Here is the photo scaled to 34% (1365 x 2048), in which Triton makes Neptune appear like a tight double star:

 

https://live.staticf...c5ac0f766_k.jpg

 

I had no idea that Triton could show in a 90-mm Maksutov-Cassegrain with a DSLR camera, and I actually thought it was a background star at first, until I looked on Stellarium.

 

It should be noted though that I "cheated" --- I used my Omegon 1000/90 (f/11) Maksutov-Cassegrain instead of the Celestron 1250/90 (f/14) Maksutov-Cassegrain.  The faster focal ratio makes it much more suitable for long-exposure astrophotography on an altazimuth mount.

 

This is a single-shot JPEG photo (no stacking or processing) taken with an un-modified Canon EOS Rebel SL2 (200D) DSLR camera at prime focus, using the Farpoint low-profile T-adapter and the Explore Scientific low-profile T-ring on the Omegon MightyMak 90 with the Celestron NexStar SLT (90SLT) mount.

 

datetime = 2021-10-27T23:05:13-04

(phi, lambda, h) = (+39D:42M:23.84S, -78D:38M:34.82S, 323 m)

(T, P) = (+3.9 deg C, 1013.21 mbar)

 

focal length = 1044 mm = (1000 mm) + (44 mm)

sensor = 6000*4000 px @ 22.3*14.9 mm^2

pixel scale (at 6000*4000) = 0.734"/px

aperture diameter = 90 mm

Rayleigh resolution at 502 nm = 1.40"

 

Triton altitude angle = +44.1203 deg

Triton apparent magnitude = +13.65

Neptune apparent magnitude = +8.02

Neptune-Triton elongation = 15.3"

Neptune angular radius = 1.17"

Triton angular radius = 0.065"

 

However, I am posting this to the DSO forum instead of the Solar System forum because of a stellar curiosity.  If you look on the center left edge of the photo, there is a small asterism of three stars, with the brightest of the three stars on the very edge of the photograph.  Of these three stars, Stellarium says that this one should be the dimmest, with an apparent magnitude of +14.34 versus +13.74 and +13.09 for the other two stars.  However, in the photograph, the star at magnitude +14.34 appears to be brighter than the star at magnitude +13.09.  So I am wondering if maybe the magnitude in Stellarium is wrong, or if this is a variable star?  Unfortunately, Stellarium does not give names to any of these stars.  How do I get Stellarium to provide names for every star?  What would be the best way to find out the name and correct magnitude of this mysterious star?

 

Note that there may be some stars missing from the photo since I had automatic noise cancelation enabled (this is the infamous "star-eating").

Attached Thumbnails

  • IMG_9576-small3.jpg
  • Neptune.png

Edited by Nicole Sharp, 12 November 2021 - 06:30 AM.


#2 Nicole Sharp

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Posted 12 November 2021 - 07:29 AM

I don't think it could be a camera or lens artifact causing the increase in apparent brightness.  All of the other stars appear to match the patterns shown in Stellarium.  This is a Maksutov-Cassegrain, so there is no significant coma or field curvature.

 

I took two photos at each ISO setting and exposure time.  Here is the second photo at ISO 51200 for 30 s, though it appears to have a little bit of a tracking error since there is more trailing:

 

https://www.flickr.c...57720128436529/

 

I won't be able to find this particular star again without GOTO.  The difference in apparent magnitude relative to the two nearest visible stars should be significant enough to see either visually or photographically, but a much larger aperture would be needed for visual observation than for photographic observation.  Stellarium gives an apparent magnitude of +14.35 within an hour of culmination at latitude +40 degrees, so you might need at least 235 mm of aperture (such as a C9.25) for visual observation.

 

The star in question is located within the triangle formed by HIP 115720 (+9), HIP 115892 (+7), and HIP 115953 (+7) in the Constellation Aquarius.

 

Coordinates @ 2021-10-27T23:05:13-04:

 

(RA, DEC) = (23H:30M:01.26S, -4D:48M:07.0S) = (352.50522, -4.8019) deg

 

J2000 (RA, DEC) = (23H:28M:54.81S, -4D:55M:13.4S) = (352.22836, -4.9204) deg

 

I know some variable stars can change by this amount of magnitude (from +14 to +13), so I am wondering if this might be the case.  Otherwise Stellarium appears to have the wrong magnitude.

Attached Thumbnails

  • IMG_9575-small3.jpg
  • stellarium-007.png

Edited by Nicole Sharp, 12 November 2021 - 08:26 AM.


#3 Nicole Sharp

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Posted 12 November 2021 - 08:15 AM

I can't find anything online to identify stars with.  Do I need to buy a star atlas?  If anyone has one to look up this star, let me know the name.  I wish that Stellarium could give the star names :-( .


Edited by Nicole Sharp, 12 November 2021 - 08:16 AM.


#4 Nicole Sharp

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Posted 12 November 2021 - 11:58 AM

I found the star in the NASA SkyView Digitized Sky Survey (DSS):

 

https://skyview.gsfc...112693819_1.jpg

 

https://skyview.gsfc...nt/cgi/query.pl

 

https://skyview.gsfc...me-find-a-star/

 

This image was obtained during the survey some time between 1983 and 2006.

 

I still don't know the name of this star, but in the DSS, it does appear to be dimmer than the nearby magnitude +13 star, in accordance with magnitude given on Stellarium.

 

Assuming my photograph is correct, then this would appear to be a variable star?  It would have gone up a whole magnitude between when the DSS photo was taken and 2021.

 

I suppose I can email the AAVSO about it to see if they might have any information.

 

Are there any star atlases (preferably digital instead of print) that have named stars down to magnitude +15, with indicators on which stars are variable?


Edited by Nicole Sharp, 12 November 2021 - 12:00 PM.


#5 Nicole Sharp

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Posted 12 November 2021 - 12:35 PM

The General Catalogue of Variable Stars (GCVS) doesn't have any variable stars listed within 1200 arcseconds (20 arcminutes) of "23 28 55. -04 55 13." (J2000).

 

http://www.sai.msu.s...13.&radius=1200

 

http://www.sai.msu.s...-bin/search.htm



#6 Nicole Sharp

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Posted 12 November 2021 - 03:15 PM

I think that Star UCAC2 30293949 might be the mystery star at "23 28 54.81 -4 55 13.4".  I just really wish I could get these star names into Stellarium.

 

https://simbad.u-str...uery&CoordList=

 

https://simbad.u-str...simbad/sim-fcoo

 

I am not sure how to read the data provided by SIMBAD.  It looks like it provides a visual magnitude of +11.760 but that seems to be too bright for the apparent magnitude?  If it is the absolute magnitude, that also doesn't make sense, since a parallax of 2.1109 milliarcseconds would then indicate an apparent magnitude of +20.138, which is much too dim.

 

If in fact the apparent visual magnitude is +11.760 (not counting atmospheric extinction), then the apparent magnitude given in Stellarium is incorrect.

 

But if the apparent magnitude given on Stellarium was formerly correct (maybe as of 2006), then that would still indicate that this might be a variable star?



#7 Keith Rivich

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Posted 12 November 2021 - 03:16 PM

Can you annotate your image to show the exact star?

 

SkyTools shows a 15.77 magnitude star almost at the position you indicate. The name of the star is listed as J233000.6 -044807



#8 Nicole Sharp

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Posted 12 November 2021 - 03:48 PM

Can you annotate your image to show the exact star?

 

SkyTools shows a 15.77 magnitude star almost at the position you indicate. The name of the star is listed as J233000.6 -044807

 

I think it is UCAC2 30293949 (also known as Gaia DR2 2633229953663541248).

 

I tried uploading the original photo to Astrometry.net, and there is a star that appears to match what I photographed (brighter than what is shown in Stellarium or from the DSS) with the identifier 2633229953663541248.

 

https://nova.astrome...54465#annotated

 

SIMBAD gives a number of additional names for this star.

 

If that is the star, then the question is why does it appear that the apparent magnitude has changed since 1983/2006?  Where did Stellarium get it's magnitude from if it doesn't match SIMBAD?

Attached Thumbnails

  • IMG_9576-small3-labeled.jpg

Edited by Nicole Sharp, 12 November 2021 - 04:11 PM.


#9 Keith Rivich

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Posted 12 November 2021 - 04:33 PM

I was able to match your image to the correct star field in SkyTools. The star you have circled is listed as GSC 05247-0170. 11.8 Magnitude. Alternate ID is: J232856.1 -045607. 

 

ra 23 28 56.2   dec -4 56 08


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#10 Nicole Sharp

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Posted 12 November 2021 - 04:49 PM

I was able to match your image to the correct star field in SkyTools. The star you have circled is listed as GSC 05247-0170. 11.8 Magnitude. Alternate ID is: J232856.1 -045607. 

 

ra 23 28 56.2   dec -4 56 08

Apparent magnitude +11.8 agrees with SIMBAD at +11.760.  SIMBAD gives this star also as DENIS J232856.1-045607 or PPMX J232856.1-045607.

 

So that's good to confirm the identification of the star.

 

I am wondering though if the apparent magnitude of +14.15 given by Stellarium might just be outdated as opposed to outright wrong, since the star does appear dimmer in the DSS photos from 1983/2006:

 

https://skyview.gsfc...112693819_1.jpg

 

I am not sure how to access any other resources that might confirm that this star used to be dimmer than it is now, possibly by two whole magnitudes.

 

SIMBAD gives the temperature as somewhere between 5878.50 and 6557.10 kelvins.


Edited by Nicole Sharp, 12 November 2021 - 05:02 PM.


#11 Keith Rivich

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Posted 12 November 2021 - 05:11 PM

Looking at a DSS image the target star appears to be the correct relative magnitude compared to other stars around it. At least in this image it is not fainter then it is now. I will annotate an image later. You can see of this jibes with what you think is going on. 



#12 Nicole Sharp

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Posted 12 November 2021 - 05:49 PM

Looking at a DSS image the target star appears to be the correct relative magnitude compared to other stars around it. At least in this image it is not fainter then it is now. I will annotate an image later. You can see of this jibes with what you think is going on. 

 

The closest star in the photograph to UCAC2 30293949 (Gaia DR2 2633229953663541248) is given on Stellarium as apparent magnitude +12.90 (excluding atmospheric extinction) at J2000 coordinates "23 28 45.28 -4 52 52.0".  But no stars within 2 arcminutes are given on SIMBAD for these coordinates.

 

https://simbad.u-str...uery&CoordList=

 

On Astrometry.net though, this star is listed as Gaia DR2 2633231259333598208.

 

I think on their chart, the G (green) magnitude corresponds most closely to visual (V) magnitude.  In that case, they give an apparent magnitude of +11.66 (compared to SIMBAD's value of +11.76) for 2633229953663541248 versus +12.94 (compared to Stellarium's value of +12.90) for 2633231259333598208, which corresponds to what I see in my own photograph (2633229953663541248 is brighter than 2633231259333598208).

 

However it looks to me in the 1983/2006 DSS photo that 2633229953663541248 might be slightly dimmer than 2633231259333598208, unlike the photo from 2021.  Historical astrometry of the magnitude though would be more accurate than eyeballing the photo to guess which star is brighter.

 

Though this still doesn't explain the wrong magnitude given on Stellarium.  I use the stellar magnitudes on Stellarium to determine the limiting magnitudes in photos (to help determine targets for future photos), but if they aren't accurate, that makes it more difficult.  If there was a way to display the star names on Stellarium, it would be easier to double-check the magnitudes, but using Astrometry.net appears to work okay for that.


Edited by Nicole Sharp, 12 November 2021 - 06:02 PM.


#13 Keith Rivich

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Posted 12 November 2021 - 07:10 PM

The closest star in the photograph to UCAC2 30293949 (Gaia DR2 2633229953663541248) is given on Stellarium as apparent magnitude +12.90 (excluding atmospheric extinction) at J2000 coordinates "23 28 45.28 -4 52 52.0".  But no stars within 2 arcminutes are given on SIMBAD for these coordinates.

 

https://simbad.u-str...uery&CoordList=

 

On Astrometry.net though, this star is listed as Gaia DR2 2633231259333598208.

 

I think on their chart, the G (green) magnitude corresponds most closely to visual (V) magnitude.  In that case, they give an apparent magnitude of +11.66 (compared to SIMBAD's value of +11.76) for 2633229953663541248 versus +12.94 (compared to Stellarium's value of +12.90) for 2633231259333598208, which corresponds to what I see in my own photograph (2633229953663541248 is brighter than 2633231259333598208).

 

However it looks to me in the 1983/2006 DSS photo that 2633229953663541248 might be slightly dimmer than 2633231259333598208, unlike the photo from 2021.  Historical astrometry of the magnitude though would be more accurate than eyeballing the photo to guess which star is brighter.

 

Though this still doesn't explain the wrong magnitude given on Stellarium.  I use the stellar magnitudes on Stellarium to determine the limiting magnitudes in photos (to help determine targets for future photos), but if they aren't accurate, that makes it more difficult.  If there was a way to display the star names on Stellarium, it would be easier to double-check the magnitudes, but using Astrometry.net appears to work okay for that.

I think the guy that owns Stellarium is responsive to questions. You may ask him which catalogue he uses for these faint stars.



#14 gzotti

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Posted 17 November 2021 - 11:06 AM

Note that Stellarium uses several catalogs, as described in the User Guide, which thin out beyond a certain magnitude. Faint stars have no name or catalog numbers stored with them, to keep the program reasonably small and fast.  There seems to be no guarantee for completeness in star catalogs, though, so there are stars missing which are brighter than others which are included.  This explains that photographs easily show more stars than Stellarium. The star in your photograph is indeed missing from Stellarium's catalog, like certainly many more.  The process to identify faint stars in your photographs with Stellarium is still easy, though:

 

Switch on the DSS layer

Identify your suspect star

Shift-click to set a marker

Select the marker (click on it)

F3 search window, SIMBAD tab. Fetch info.

 

 

 

As you have found already, your star identifies as:

 

UCAC2  30293949 (Star)
Distance from query: 17.04 arcsec
Other Identifiers:
- UCAC2  30293949                                   
- DENIS J232856.1-045607                            
- DENIS J232856.1-045606                            
- 2MASS J23285614-0456076                           
- PPMX J232856.1-045607                            
- RAVE J232856.1-045608                             
- SDSS J232856.14-045607.7                          
- TIC 301328605                                     
- UCAC3 171-283928                                  
- UCAC4 426-133861                                  
- USNO-B1.0 0850-00633919                           
- WISEA J232856.13-045607.6                         
- WISE J232856.12-045607.5                          
- Gaia DR2 2633229953663541248

 

HTH, G.



#15 tdfwds

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Posted 17 November 2021 - 12:56 PM

With respect to the magnitude 'issue', when I checked the source site for this software I got the impression (I didn't read the manual in full) that in this instance NOMAD was used for the star in question.  NOMAD is a mixed bunch of totally inhomogenous astrometric catalogues, a non-critical compilation of mostly if not completely USNO catalogues that they just lumped together, and the photometry is of varying kinds and all over the shop, as well as inherently poor for the source catalogues that are from digitisations of survey plates.  A lot of people liked it because it was a convenient one stop shop, all sky and large magnitude range, but it is apocryphal often, and will probably be replaced in all vendor sources eventually with some trimmed down GAIA alternative for the mag range it covers.  There'll be some intertia on this as people will have already made a big download to get the faint stars for the software, and the new one might be twice as big (although you can make GAIA data quite small with reasonable magnitude cutoffs and ditching most of the data which is quite irrelevant, leaving just position and one passband mag, or pm and plx too if you want to be careful.

 

Anyway, when I looked up your mystery star in multiple VizieR catalogues most had it as the same brightness, or within a magnitude of that, across multiple optical passbands

 

Except NOMAD and SDSS.  For some reason the SDSS g' mag, which is a blue to visual wide passband, has it as less than mag 14 value.

 

Nomad had the magnitude you quoted for the blue magnitude, but brighter in the others.  The source reference column showed it came from the YB6, which is completely unpublished and an USNO inhouse catalogue.  I think it might have been used for the SPM catalogue  It means something like "Yale blue", and is a bunch of old photographic plates and a more recent epoch, from the days when observatories did proper motion surveys by taking plates of the sky and then coming back decades later and doing it all over again, with the same plates.  These have been used for the SPM.  YB6 is a blue plate special, so to speak, and the magnitudes imported into NOMAD for it are therefore from old photographic plate scans, as I believe they are the first epoch plates that are used.  Astrometric surveys didn't worry much about the photometry as long as it was approximately okay, as it was a means of double check more than anything, if used at all.  Incidentally, YB6 is NOT anything to do with the Yale BSC (Bright Star Catalogue).

 

The blue mag in NOMAD is the same value as the faint magnitude you state.

 

Finally, it is a bit weird that both NOMAD YB6 "blue magnitude" and SDSS g' have it as faint, whereas many other surveys, quite a few also possessing g' and blue magnitudes, have it with magnitudes similar in visual or red and blue.  After all, the star isn't very red in colour, in fact it shouldn't even be orange, likely a yellow star.  The amplitude would also be a bit large, possibly a record breaker, for an eclipsing binary caught twice, once by each of these surveys.  In the SDSS case if all the different passband magnitudes are contemporary, ie same night, then it cannot possibly be an eclipse as only u' and g' are faint, and u' can have issues in SDSS data.

 

A fair bit of this is done from memory, including the vizier search when I first looked at it once someone described what star it was because unfortunately I couldn't understand either your charts or your description in the charting package I use.

 

This link will show you all the catalogues in VizieR within one arcmin of this object's position, the ones with smallest r in the lefthandmost column, which is in arcminutes will likely be it, but beware it doesn't appear in all catalogues, whilst other catalogues go so faint several nearby stars are included that would have been too faint for you to image.  Beware link wordwrap, if any.

 

http://vizier.u-stra...&!-4c=Find Data

 

BTW, it is nice of Triton to be just visible in average sized 'scopes, ain't it? ; )  And Neptune's finally climbing North again!



#16 c2m2t

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Posted 17 November 2021 - 04:37 PM

I think it is UCAC2 30293949 (also known as Gaia DR2 2633229953663541248).

 

 

Hi Nicole!

I believe your best guess is correct. Before reading your guess, I tracked back to UCAC2 30293949 using Aladin. Your image is not rich in stars but there was enough to bring me to this conclusion as well.

 

See attached image for more labels and data pertaining to this star.

 

Cheers, Chris.

Attached Thumbnails

  • UCAC2 30293949.JPG



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