First light report

by Teri Smoot

Warning, long E-mail...

I've had my TEC-180FL out once for its first light (about a week ago). I am planning to try it for imaging next full moon (in about two weeks). My first light experiences were only visual -- kind of getting used to the telescope and the mount together before I tried using it for imaging. I had started to write up the first light report but hadn't finished it yet.

I've got the TEC-180FL mounted on a Losmandy G11/Gemini. It worked very well for visual use -- slewed all over the sky; put objects in the FOV of an 8mm eyepiece; never overheated the motors; etc. There is a bit of vibration when I "tap" the tube though. I probably need to go through and tighten up all the bolts on the G11 and then be careful when imaging not to touch the scope (wind is usually not a problem here). I have used a TEC-140 on the G11 (primarily for imaging with a DSLR) for two years and haven't needed to get into guiding. I'm hoping for the same for the -180 since it too is F/7 and the focal length is not too much greater. IF I need to get into guiding, though, I probably will have to remount the TEC-180FL.

Now, for a simplified first light/observing report.....

I took the TEC-180 out just after the start of a new lunar cycle. The moon went down before I started and was not a factor. Seeing was average and I observed under semi-dark skies (about magnitude 5.0). As I said above, I am primarily an imager (you can see my stuff at http://tas.digitalastro.net/ ). I did all of my observing using a Televue 8-24mm Zoom eyepiece (usually at 8mm). I find this eyepiece a great one to use to get the Gemini aligned and PAC'ed. I really don't have any "good" eyepieces.

My observing targets were mostly the Messier objects that are now visibile in the sky. I selected the ones I did because I have recently looked at many of these with the TEC-140. I started about 8:00 pm local while there was still some light in the sky and observed till around 10:00.

I started by taking a peek at Jupiter. It was pretty low on the horizon and not a good target. I could tell this because Jupiter's shape was really "boiling" (i.e., circle to square to circle, etc.). However in those infrequent times when the atmosphere settled down a bit, I was really pleased by the sharpness of the EQ belts at least. I liked the image scale and think that Jupiter and Saturn (and Mars) will be fun as they get higher. Later in the evening, I went over and observed Uranus (again with the 8mm setting on the TV zoom EP -- THIS IS NOT A VERY GOOD PLANETARY EP). I was pleased to see the disk (albeit small). I may also have just observed Titania (mag 13.9) with averted vision. If so, that is probably right at the limit of seeing in my skies.

After viewing Jupiter, I started looking at several of the different Messier objects. My notes follow:

M8 -- Can't quite see color in the nebula (maybe there just at the limit). Can see the dark lane separating the nebula and the adjoining star cluster (in images, its not so much a lane as a less bright area). Definitely a better view than the TEC-140, but ?DSN??? (i.e., dark site needed to really get the best results). Observing before sky is totally dark!

M20 -- Again, observed before skie is totally dark. Nebula barely there but probably can also see trefoil in nebula.

M22 -- Beautiful. Fills the field of view. Lots of stars. Cotton candy.

M28 -- Dim but still nicely there. Just 8:00 pm and sky not completely dark and Saggitarius in the west!

M17 -- Again nice. Can't see color but can see the swan. More nebula than the -140 but...

M16 -- Cluster very nice. Just a hint of the nebulosity. Still somewhat light though.

M11 -- The first "Oh Wow" object. This just seemed to draw the eye further and further into the cluster. The more you look, the more you see. I believe the extra aperture showed enough more stars to make this object really nice in the -180.

M15 -- Very nice. Just at the level of resolving individual stars. Kind of like sugar.

M2 -- See M15.

M31 -- Certainly more galaxy visible than in the TEC-140. Some arm structure visible. No "oh wow" on this object though.

M27 -- Barely see color. Some structure visible. Maybe see central star with averted vision.

M57 -- Very nice. Almost another "oh wow". Some structure visible in the rings.

Double Cluster (NGC869/884) -- This was another "oh wow" object like M11. It seems that the more you look, the more you see. Very nice.

Other DSO's attempted but not really seen: M33 (too much sky background), NGC 891 (ditto), and NGC7293 (ditto). All may be at the limit of seeing using averted vision again.

Double stars:
Tried to see if I could split Antares (didn't expect to). When I looked, Scorpio was in the skyglow and pretty low. Maybe sensed a green companion at just the level of visibility (think it was around the 5:00 position). If so, very marginal observation.

Alberio -- Obviously very easy to split. Very nice colors. This pair looked nicer at the lower magnification (i.e., 24mm).

Epsilon Lyrae -- Easily split both stars with the 8mm eyepiece.

"Star test":
I also observed Vega to see if I could see any false color. None, nada, zip. This was the only object where I carefully examined things both sides of focus. When I do this with the TEC-140, the diffraction rings are little black circles. With the -180, the circles were noisy. I believe this is characteristic of some thermal instabilities. Yuri had published some formulae for cooldown times. Using these for the -140 and -180 yields cooldown "time constants" of 0.903 min/deg F for the -140 and 2.697 min/deg F for the -180. My telescopes are stored in my unheated garage but still cooldown about 20 deg F at the start of my observing runs. For the -140, this yields a cooldown time of 18 minutes (one "time constant"). By the time I wheel out the telescope, hook up the cables, do a polar alignment (using the PAS), and then the Gemini Alignment and PAC, about 20-30 minutes has gone by. Enough for the -140 to probably get most of the way to thermal equilibrium. For the -180, this time will be more like 54 minutes. So, the "noisy" diffraction rings were almost certainly thermal.

SO, IN SUMMARY.....

1. The TEC-180FL is happy on the G11/Gemini for visual use. Imaging tbd and may be marginal.

2. On star like objects (open clusters, globular clusters), the -180FL gives views that I characterized as "Oh Wow" Very nice sharp pinpoint stars and with a sensitivity that just draws the eye farther and farther into the object. I would calculate the -180FL should go about a half magnitude better (65%). This is easily seen.

3. On planets, I really didn't have the best test subjects (Jupiter the only evening planet and it very low). But, based on a limited amount of viewing, I think this will be a very nice planetary scope (not a Mak Cass, but nice). Here, the resolution should be 29% better as should the image scale.

4. On the extended DSO's (nebula, galaxies), I think I will need to go to a dark sky site to really appreciate the difference. [Alternately, I'll need to image -- I can remove lots of the background when processing this type of object and, hence, get a good image with less than perfectly dark skies.]

5. Yuri's calculations for cooldown time are probably right (or maybe a bit optimistic). The -180FL will see a larger thermal transient effect than the -140.

6. And, what eyepiece would people recommend?

Bottom line. The TEC-180FL will be a very nice scope. However, the TEC-140 provides about 60% (in terms of gathering photons) - 78% (in terms of resolution) of the performance of the TEC-180FL for about 30% of the price! My sense that the -140 occupies an perfect price/performance point was reinforced.

Teri Smoot