Obs session through an 80mm f/3.75 Achromat

Got a question on astronomy that you'd wanted to ask? Ask your questions here and see if the old timers can give you some good answers.
Post Reply
User avatar
Posts: 1170
Joined: Mon Jul 12, 2010 6:08 pm
Favourite scope: 94.5", f/24 Ritchey-Chretien Reflector
Location: Restaurant At the End of the Universe

Obs session through an 80mm f/3.75 Achromat

Post by cloud_cover »

The last few nights have been fantastically clear. Unfortunately between a couple of late shifts, sick kids etc etc... there's been no energy to move, set up and calibrate the SXD (which still takes about an hour from intending to move to ready for obs at not-so-dark site)

Unfortunately the Skypod cannot accept the weight of the VMC200L (tracking seems OK but it will refuse to slew unless I support the VMC's weight. Not good). So last night out of desperation of too many good nights going by, I set up my FM-80 finderscope on the Skypod and did some obs from my south facing window.
Unfortunately due to nearby blocks, the fact that it faces the rest of Singapore (including those laser lights) and non-removable rain shields, I only have a 6deg window from 20 to 26 deg altitude to observe, and about from south to just past west. Thankfully last night when the kids went to bed, Acrux, S. Pelaides, Eta carina and Sirius were nicely in this window :)

So, with all the rage on ED, APO triplets, Quads etc, how did a very fast achromatic doublet compare?
The first targets were Acrux and Sirius: bright stars included in the Skypod's database for alignment. Interestingly, I did not notice any CA when in focus! Not bad at all for a lens system designed as a finderscope. The EPs I used included a 23mm reticle plossl (the stock finder EP) and a Nagler 9mm T1. Unfortunately the Ethos 13 does not come into focus - it can't be shoved in deep enough. While the double star of Acrux was not split - I didn't try to nor expect to at 30x power, its smaller Mag 6.8 companion was nicely seen. This compares very well to my Nikon 10x50 which can generally see down to Mag 4+ in similar sky conditions.

After that, it was on to some DSOs, which I really like viewing. The first stop was NCG3766, a small open cluster with comprised of mostly Mag 9 stars. In the 9mm Nagler it appeared like a little ball of shiny dots, slightly twinkling. Very pleasant. Attempted to show it to the wife who said she saw nothing. Sigh....
On to the Southern Pelaides. The nice thing about using such a fast scope is even with a 9mm eyepiece, the FOV is still about 2.5deg, which is more than sufficient for a slightly spread out target like the S. Peliades. The stars were all nicely visible although unlike its northern counterpart, which returns to our skies later this year, there is no nebulosity about them so they look like.... stars. Interestingly for such a fast scope, there is minimal spherical abberation seen in the 9mm Nagler although in the 23mm plossl (with a TFOV of almost 4deg), the outer 1/3 stars look like lines.

Upwards and onwards to the Eta Carina area. Lots of nice stars, characterised by lots of apparent pairs (not double stars - these are easily seen using binos). No nebulosity though, even with a Lumicon deep sky filter. Light pollution is atrocious in this direction and it was just 20+ deg off the horizon.
Swinging around the Eta Carina area, NGC3324, an "Open cluster associated with nebulosity(Stellarium)" was again a very pretty object and brighter than the previouds cluster, since its stars are mainly Mag 6-7.

I tried the Deep Sky filter with various EPs here but came to the conlusion that as I was viewing starlight, the filter does not provide a better view at any exit pupil although the sky was somewhat darker.

The final target was NGC3532, a wider, brighter open cluster with nearby X Carinae. This was very pretty open cluster with nicely defined stars, well seen in direct vision, much like how a dispersed, thinned out and lemony version of Omega Centauri will look like. It was such a pleasure looking at this cluster that I kept my scope on it until the wife told me to pack up so the maid would go and sleep *grin*

A final slew over to IC2931 which includes some mag 3+ stars showed..... nothing. This was at 10deg altitude, demonstrating the horrible light pollution in that direction.
Overall, it was my first go at obs from my windows: As you can see the window for observation is small indeed from my house but the benefits of comfort and a quick set up and take down time were definitely appreciated.

While the view was definitely dimmer than in my 8" Cat, stars were actually better defined due to the absence of a 40% obstruction (caveat: I've never done 30x in my VMC so it maybe the stars are equal looking at that mag). While an 80mm scope is not a good idea for looking at nebula in heavy light pollution, even filtered, it is adequate for looking at stars and clusters. Interestingly, I expected to find significant Chromatic Abberation since this is after all a fast Achromat - and not even meant to be used as a "proper" telescope - but there was virtually none to be seen. Either this is a really well corrected optical system or I'm very CA insensitive. I'm beginning to think that the new crop of 6" achromats coming on may represent quite good value visually. For imagers, the addition of narrowband/LRGB filters will essentially eliminate CA causing blue bloated stars. Since these (filters) are quite important for local imaging, it may well be a viable alternative for the more cash strapped or value conscious folks looking for a larger refractor.
Thanks for reading (and staying awake!) :)

Posts: 1163
Joined: Mon Nov 03, 2008 4:45 pm

Post by superiorstream »

Yes,the nagler T6 are very suitable and correct CA in f/5 refractor-achromat--even up to 150mm f/5 achromat--very well.Thats why you dont see much CA.Another good series is the pentax XW.Try it.

User avatar
Posts: 311
Joined: Mon Dec 29, 2003 1:36 pm
Location: Upp Serangoon/Tampines

Post by hommel »

Cloud_cover that was a very nice ob report with an 80mm. I like observing with small scopes to see how much can be achieved with small apeatures scopes. Currently i'm using a 50mm 'scope' :) my personal blog here..

Post Reply