Esprit 80mm Triplet APO first lights

[Joel Shepherd]

After a month of rain and cloud, the weather finally relented long enough for me to give my new Sky Watcher 80mm Esprit APO its first light. I'm coming to it after almost two years of using a 6" Newtonian and am hoping to use it for some wider-field color and narrowband AP. It's my first refractor.

Sunday night was clear (hallelujah), so I eagerly plopped the mount (an Advanced VX) on the spots marked out for it in the garden and didn't even bother doing a new star-align: just used the last alignment (from Jan. 10!) which worked out surprisingly well. Not sure whether the dots on the bricks, the AVX or simple luck is to credit for that. In any event, the first target I sent it to was M42.

I confess I was both a little underwhelmed and pleased. On one hand, M42 seemed dimmer or greyer: not sure if that was due to the smaller aperture or so-so atmospheric transparency. On the other hand, once in focus -- and the Esprit's dual-speed focuser is both solid and smooth -- the stars were crisp and bright, and I felt like I could make out more of the nebula even if it seemed less bright than usual.

With that, I swung it eastwards to the Beehive (M44) and yowza. Crisp bright stars, very noticeable differences in magnitudes, good contrast, pleasantly wide field. Almost a soft, warm feel to the light. Okay, off to M37. Less sure what to make of that. The light seemed weaker, but there also seemed to be many more fine pinpoints of stars. It seems like brighter things are brighter and dimmer things are dimmer, but everything is crisper.

The mount also sounds happier with the Esprit than with the Celestron XLT-150 reflector. The listed weights for the OTAs aren't that far apart (1.4 kg?): maybe it's because the more compact Esprit requires less torque to rotate around? Finallly I took a quick peek at Jupiter, through a tree, and was pleased with the detail of the rings. Oh, and I split Castor with -- I think -- a 25mm eyepiece, which seems on the low side, and easily distinguished the Eskimo Nebula. The Esprit isn't really made for visual, but it provided some pretty pleasing views nonetheless.

Last night was my first attempt at AP with it. My one goal for the night was to get the camera in focus. All this business about flatteners and spacers is daunting to me, especially after reading here about some of troubles early users had with it. My train was an Atik 460ex, Baader 29-46mm VariLock, Blue Fireball M48-T2 adapter and SkyWatcher's field flattener. I'm not sure quite where the extra 12.4mm is going, but racking the VariLock all the way down seemed to give the desired 54.9mm distance between the flattener and the sensor, so after aligning and PA-ing the mount, in the camera went.

I wanted a very clear target to do an initial focus on, so back to the Beehive I went. With the focuser racked far out the camera came into sharp focus (thank you, again, dual-speed focuser!), and with the locking lever pressed down it felt solid as a rock. The field flattener worked as advertised, the stars exhibited no diffraction spikes or other distortions.

So what's not to love? My main concern at this point is balance, especially in DEC. I spent some time collecting lights of NGC-2964, a small galaxy near Leo. At first, PhD2's guide graph for DEC was (by my standards) very good: less than 0.45 RMS error. RA guiding was flakier but still consistently sub-pixel errors. As the night wore on, however, and the Esprit steadily tipped towards the meridian, the DEC axis started to meander first one direction, then the other, in bigger and bigger swings. Eventually they got so big that PhD2 gave up. AVX's are notorious for having stiff DEC axes. I've eased mine off a bit, but the guider still seems to struggle with it: it has difficulty simply convincing the axis to turn against the error and when it finally does it has difficulty convincing it to stop.

My guess is -- and please correct me! -- that balance is playing a significant role, especially seeing how badly behaved the DEC axis got as the counterweight shaft approached horizontal.

But, the built-in Vixen foot on the Esprit 80 is short and doesn't allow much fore-and-aft adjustment. I have 2-3cm that I can shift the OTA forward, but that's it. And I haven't yet added the weight of a filter wheel to the mix, making the whole set-up even more focuser-heavy.

So what to do? Should I add weight to the front of the dew-shield (and what/how)? Is there a way to somehow position the OTA further forward on the mount?

The other, somewhat unexpected issue, is that with a lot more glass in between the target and the camera sensor, there are many more places for dust to land. Dust donuts galore. Not sure how to keep a lid on that either, but it seems a little more approachable.

I think that if I can get the DEC balance figured out then I'm going to have a good time with this scope. The increased FOV is already a joy -- the whole Leo Trio fit easily! -- and it seems to focus better, pull out more detail and -- when things are behaving -- guide more easily. Really looking forward to trying out the filter wheel (color is still a very new experience). Just need to work out the right spacers for the filter wheel and get the balance sorted.

Any tips/advice for dealing with balance on a short, back-heavy frac would be much appreciated!

-- Joel.

[Owmuchonomy]

Regarding your Dec guiding comments I had the best moments of success when I made that axis camera heavy irrespective of mount orientation AND most importantly I never guide in both Dec axes just North OR South ( or on good nights I switch it off).  I let PHD drift in Dec for a few minutes before deciding the best guiding direction for the night. So if it is drifting slightly South for example I just correct in North.

[laudropb]

Very enjoyable first light repot.

[Joel Shepherd]

Thanks Chris - I'll have to give that approach a try. It'd certainly be easier to keep the camera side heavy, since that's the direction it's trending now and will only more so with the filter wheel in place. Might also see if I can work some backlash out of the DEC gearing. Visually the RA seems worse w/respect to backlash, but it hasn't affected the guiding excessively: at least removing some of the backlash didn't noticeably improve the guiding. But I could see it being more of a problem if the mount was significantly out of balance as well.

[Joel Shepherd]

@Owmuchonomy: thanks for the DEC guiding suggestion. While not everything else went smoothly tonight, I set the DEC guiding for my fully-loaded and not well-balanced mount to south-only and it did a spectacular job. I don't know if I'd have stumbled on that solution on my own, so thank you very much for tip!