Neptune / Uranus Spectra

[physicus]

I'm working my way through the 'classic' low-res spectroscopy targets with my newly acquired SA200.  So nothing ground-breaking here to be sure, but here are my spectra for Uranus and Neptune.  I switched to the Esprit 100 (from the C9.25 SCT, see earlier post).  I'm still using a 25mm collimator eyepiece with a 50mm 1.4 Olympus Zuiko imaging lens, so 1100mm fl after magnification factor.  The CH4 lines are obvious for both planets, but the higher resolution on the Neptune chart struck me.  Wanton Googling of literature on outer planet spectra suggests I could be seeing several lines for Neptune not resolved for Uranus (if there).  The spectra were captured one after the other (Uranus 11.45 to 12:00,  Neptune 12:00 on).  Neptune had more subs: 10x120s vs 7 x 120s, and Neptune was higher (30 degrees vs. 25 degrees).  Neptune presents at 2.4", Uranus at 3.7" .  Processing was Nebulosity (stacking), Gimp (minor straightening), and BASS Project. 

Just wondered why the Uranus curve is smoother?  Is it the size of the source/altitude/other/act of god/who knows.

In other news I'm generally playing around with various optical configurations and figuring out how/whether/why to move to higher res. slit spectroscopy - and all that entails.  

 

[jnp]

Hi Tim,

Nice results. Don't underestimate the achievement of detecting gases from the outer planets, historically this was once thought to be impossible.  

If I understand correctly then you are using an eyepiece and 50mm lens to collimate the spectra, but without a slit? In this case the smaller angular size of Neptune could account for the better resolution despite the fact that Neptune is fainter. The reason for the differences could be with: shift in focus, staking/processing (check the individual subs for Uranus to see if more detail is there in some shots), different binning regions, change in 'seeing'?

You could improve the resolution of your existing setup by adding a slit. This will however create the new issue of how to get and hold the target on the slit. This is typically achieved using autoguiding with a reflective slit, adjustable slit or beam splitter.  A crude slit will allow you to take lunar spectra.

best regards

John

 

[physicus]

8 hours ago, jnp said:

Hi Tim,

Nice results. Don't underestimate the achievement of detecting gases from the outer planets, historically this was once thought to be impossible.  

If I understand correctly then you are using an eyepiece and 50mm lens to collimate the spectra, but without a slit? In this case the smaller angular size of Neptune could account for the better resolution despite the fact that Neptune is fainter. The reason for the differences could be with: shift in focus, staking/processing (check the individual subs for Uranus to see if more detail is there in some shots), different binning regions, change in 'seeing'?

You could improve the resolution of your existing setup by adding a slit. This will however create the new issue of how to get and hold the target on the slit. This is typically achieved using autoguiding with a reflective slit, adjustable slit or beam splitter.  A crude slit will allow you to take lunar spectra.

best regards

John

 

Hi John,

Thanks for your comments.  I've looked at a few other folks' plots and it looks like angular size of the planet may well be the main factor re resolution.  And yes, this is without a slit.  I've been messing with pencil sharpener blades etc, but guiding is the issue.  Commercial options, while relatively expensive, aren't completely out of the question, but I usually like to have a go myself first.  Latest brainwave is to build an angled slit from polished blades and catch the reflection in suitably modifed Celestron OAG (reversed).  Meantime, I think I will check out the moon as you suggest :-)

Regards

 

Tim 

[Owmuchonomy]

Very nice Tim.  Any particular reason you switched from the 9.25?  I guess for these targets the smaller angular size of the disc in the Esprit may have helped.  I am using my 9.25 with the SA 100. I have just started in this branch of the hobby.  In similar fashion, my second session included capture of Uranus.  I ran out of clear sky to go for Neptune though.