I managed to get a brief first light with the new Skywatcher Esprit 120 last night. As I learn more, I'll update this thread.
The Focuser
First things first, the focuser. The focuser is a really big deal on an imaging scope, and it's almost always neglected. This usually results in the purchase of expensive aftermarket focusers. Honestly I don't understand why manufacturers do this; either ship it with a usable focuser, or reduce the price and ship it without one at all.
Skywatcher claim that the focuser fitted to these as standard will handle payloads of 5kg while focusing, and 10kg locked.
My imaging train consists of a SX Trius 674 CCD, Orion Nautilus electronic filter wheel (with LRGB, OIII and Ha filters) and a focal extender. I think I weighed it all at about 2.5kg once, either way, well under the 5kg limit.
On unpacking one of the first things I did was stuck my ccd/efw on the back of the focuser and see if it would hold it without any tension applied. It did not; it immediately began slipping. Adjusting the tension/locking lever enabled me to halt the slipping.
Speaking of the tension/locking lever, it's quite awkward. I've never used a focuser with this type of locking mechanism and I'm not sure why they did it. On the bright side, it does seem to (without significant scrutiny) enable locking without shift.
In the field I was able to focus the CCD using the coarse-grained focus knobs, however inward adjustment with fine-focus knob didn't seem to work very well due to insufficient tension. You could say that's user error. I found that adjusting inwards with the coarse-grained control, then backing it out with the fine-grained enabled me to reach focus, even if it was a bit fiddly.
Another review I've seen mentioned the focuser having a series of tension screws that can be adjusted to enable smooth focusing with heavier loads. I'll be looking into making this adjustment and I'll report back.
Now something happened with the subs that I took last night, and I'm not sure what it is so it'll need further investigation before I can reach a conclusion on the focuser. After a few subs the stars got noticeably bigger, the images softer and the FWHM went up significantly. This could have been focus slip, or it could have been caused by user/guiding error: I forgot to turn the power up on the dew heater on the guide scope which caused some guiding issues. The dew heater on the Esprit was turned up well enough and was dew free. So it's possible there was significant focus slip during the subs, even with the focuser fully locked. I'll report back on this after the next clear night.
At this point I think I'll be able to adjust the focuser so that I don't have to look into a Moonlite/Feathertouch, but I'm uncertain. As I said, I'll try to adjust it and report back.
The Optics
In the past I've owned a Saxon ED100 doublet, Skywatcher ED80 doublet, William Optics GT-81 triplet, and a GSO RC8. I've not owned any premium refractors and barely used the GT-81 which is the only other triplet that I've owned.
I currently use SGP for imaging and have been using their statistics module to aid in focusing lately. SGP provides you with a Half Flux Radius (HFR) of stars across the frame. I've assumed this to be an average, but I'm not sure if that's the case. Anyway, we can basically say that a smaller HFR = sharper image.
With the ED100 the smallest HFR I saw was 1.47px. Last night, with the Esprit 120, I saw a HFR of 0.89. Now there's multiple variables at play here so we can't say that all of this is due to the optics, but I think it's safe to say that a significant amount is.
Now of course this is a totally unfair comparison. The ED100 is a doublet costing less than 1/3 of the price of the Esprit 120. The thing is though, it's this difference that I was looking for, it was this that would justify the price to me. How would this compare to a Takahashi? Who's to say.
One thing to note is that this analysis was done with a 11mm diagonal CCD chip and no flattener. And as you will see in the attached mosaic generated by PixInsight's AberrationInspector, the stars look pretty good across the entire field. Of course if you put this on a full frame sensor or even an APS-C sized sensor I wouldn't expect results as good as this. Note that this was generated from a single 430s frame, which was obviously subject to guiding/tracking fluctuations.
Speaking of the flattener, mine was missing from the box. According to my retailer, this has been known to happen with some of the Esprits because the flattener was packaged separately in some cases. I believe that info came from the local Skywatcher rep.
I have seen complaints in the UK that the Esprit were no longer shipping with the flattener, but apparently they should still ship with it over here. Mine is supposed to be mailed out to me shortly.
Everything else
The lens hood is retractable, which may help with transport but probably nothing else. It does have a locking screw (I think two actually, but my memory's terrible so don't quote me on that) which means it'll always stay up. That's a nice touch; my older GT-81 doesn't have one and relies on the tube rings being pushed flushed to the back of the hood to keep it up.
The lens hood does move awkwardly and doesn't feel very professional when compared to the smooth movement of the GT-81. But eh, it does the job and will stay up so I suppose that's what matters.
The scope comes with nice CNC tube rings and a Losmandy-D style dovetail, both of which are a big step up from the lower-end scopes and very welcome additions.
The scope also comes with a nice road case, complete with two wheels on one end, although I'm not sure how they'll go on typical terrain.
Also included is a diagonal, eyepiece and right-angle finder. As I don't do visual at all, these accessories will stay in their original packaging, never to see the light of day. Given the target market for this series of scope is mostly imagers, I question the inclusion of these accessories; they could be sold as optional extras as I imagine lots of imagers, like myself, would eschew finders in favour of platesolving, and probably never stick an eyepiece to it.
Conclusion
So far I'm happy with my purchase. I think going from a doublet to a triplet was a worthwhile investment, but if I were not into imaging I may not have been able to justify the cost.
I expect to be able to adjust the focuser so that it's realistically usable. A shame it didn't come like that out of the factory. If I'm unable to do so I'll report back.
Overall the mechanics of the scope (lens hood, focusing tension lever) feel a bit awkward and I'd hoped for better, but it's not really unexpected.
I just disassembled and tuned up an old WO 4" Crayford which was slipping in two places. The drawtube was slipping past the pinion - there's a tension screw over the pinion to apply extra pressure. After that the drawtube would still roll out until I added tension to the micro focuser - there's a nut that can be tightened to apply more pressure to the micro focuser bearing. The focuser now holds a 2" diagonal and the heaviest eyepieces.
Thank you for taking time to share your first impressions with your new telescope.
I am glad that you found optics to be very good, and it looks like you can successfully image with your camera without a flattener (although using one will probably make all stars in the corners perfectly round.).
I hope you will be able to adjust the focuser so it will work reliably. I actually expected that focuser might be potentially the weakest part of this scope, but as you said, it is still too early to say that with certainty.
I will be looking forward to your new, sharper, showing more faint detail and thus more beautiful images
I just disassembled and tuned up an old WO 4" Crayford which was slipping in two places. The drawtube was slipping past the pinion - there's a tension screw over the pinion to apply extra pressure. After that the drawtube would still roll out until I added tension to the micro focuser - there's a nut that can be tightened to apply more pressure to the micro focuser bearing. The focuser now holds a 2" diagonal and the heaviest eyepieces.
Cheers Tony. Mine will definitely need tweaking, but at this point I've found no diagrams / documentation on the process.
Quote:
Originally Posted by Slawomir
Hi Lee,
Thank you for taking time to share your first impressions with your new telescope.
I am glad that you found optics to be very good, and it looks like you can successfully image with your camera without a flattener (although using one will probably make all stars in the corners perfectly round.).
I hope you will be able to adjust the focuser so it will work reliably. I actually expected that focuser might be potentially the weakest part of this scope, but as you said, it is still too early to say that with certainty.
I will be looking forward to your new, sharper, showing more faint detail and thus more beautiful images
Cheers S :-)
I went up to the "obs" this afternoon to collect my larger battery for charging and thought I'd have a little look at the focuser while I was there. What I saw didn't leave me all that happy.
Turns out there's space for four bolts under the focuser, some or all of which presumably play a part in tensioning the focuser. While I have four holes, I only have three bolts in there. And while playing with the fine-focus control with the scope parallel to the ground I noted that it did not adjust the focus at all. So, whatever I thought I saw in the dark last night was either my imagination, or slippage. I did notice last night that inward travel wasn't working with fine-focus, but I thought outward was and had assumed this was just insufficient tension. Must have been insufficient tension causing focus to slip :p
The question is, how did this pass QA? That's a pretty large oversight.
The good news is that the focuser can almost certainly be made serviceable with adjustments similar to those I described above - most crayfords (excepting FT) have a somewhat similar construction and operate on the same principles. If you can get some info/pictures about how to adjust that particular focuser that's great, otherwise I'd suggest getting permission from the retailer to disassemble and adjust the focuser yourself - once it's apart you can see which screws do what. Failing that send the focuser only in to the retailer or Skywatcher for adjustment and ask them to adjust it to suit a heavy load - no need to ship the whole scope.
Nice one Lee. My Esprit has been giving me nice round stars with no halos, which is what I was looking for in an imaging apo.
Lee, what was happening with the ambient temperature while your subs were getting softer / going out of focus?
I've noticed that I need to refocus regularly during the first half of the night at AstroFest, and I suspected thermals. After midnight when the temperature became more stable I didnt need to touch it much. If the tube is contracting with dropping temperature, it surely can't be unique to this scope.
The focus lock is a bit quirky, but as you say there's no shift. My focused doesn't slip, but then my DSLR only weighs about 500g.
The good news is that the focuser can almost certainly be made serviceable with adjustments similar to those I described above - most crayfords (excepting FT) have a somewhat similar construction and operate on the same principles. If you can get some info/pictures about how to adjust that particular focuser that's great, otherwise I'd suggest getting permission from the retailer to disassemble and adjust the focuser yourself - once it's apart you can see which screws do what. Failing that send the focuser only in to the retailer or Skywatcher for adjustment and ask them to adjust it to suit a heavy load - no need to ship the whole scope.
Cheers Tony :-) I've hit up Skywatcher to see if they'll provide some details. I think all other crayfords that I've seen diagrams of usually have one clear grub screw that you adjust for tension, but these ones only have four symmetrically-placed bolts underneath so without opening it up fully or being provided with documentation, I'm not sure what to do.
Quote:
Originally Posted by Camelopardalis
Nice one Lee. My Esprit has been giving me nice round stars with no halos, which is what I was looking for in an imaging apo.
Lee, what was happening with the ambient temperature while your subs were getting softer / going out of focus?
I've noticed that I need to refocus regularly during the first half of the night at AstroFest, and I suspected thermals. After midnight when the temperature became more stable I didnt need to touch it much. If the tube is contracting with dropping temperature, it surely can't be unique to this scope.
The focus lock is a bit quirky, but as you say there's no shift. My focused doesn't slip, but then my DSLR only weighs about 500g.
Cheers Dunk :-)
Not sure about ambient temps, I wasn't measuring them. They were presumably dropping due to it being early evening, but I don't know by how much.
It's possible it was temperature related, but I have never seen such a shift on any one of my scopes, and I doubt the temperature shift was that much different. Given the other issues with the focuser, I think that's the likely culprit.
The main thing with buying a triplet for me is once you use parfocal LRGB filters with a doublet and a bhatinov mask, you'll see how out of focus the blue (probably, I guess that depends on your scope) light is. From there you think about how it's a spectrum rather than a discrete block and a significant amount of light in the spectrum isn't being brought to focus every time you're shooting L. This of course could cause colour halos, but I imagine other things could cause those.
It's possible it was temperature related, but I have never seen such a shift on any one of my scopes, and I doubt the temperature shift was that much different. Given the other issues with the focuser, I think that's the likely culprit.
The main reason I threw the temperature thing out there is because I'd spoken to a few others at AF who were using temperature-sensing motorised focusers to solve exactly the same problem...and mine was the only Esprit on the field
Yeah looks like the blue is usually sacrificed on a doublet, probably not the greatest if you want to capture some OIII
thanks for your first impressions, i look forward to more details. i am not sure how big the focussing issues were, but is it possible this is due to changes in temperature. I am not sure how much this affects refractors but it may be a component of the issue. when you get a chance i would love to see a colour image to see how well the CA is corrected. congratulations again on your new scope, you have to be very happy with the results.
The main reason I threw the temperature thing out there is because I'd spoken to a few others at AF who were using temperature-sensing motorised focusers to solve exactly the same problem...and mine was the only Esprit on the field
Yeah looks like the blue is usually sacrificed on a doublet, probably not the greatest if you want to capture some OIII
Quote:
Originally Posted by Somnium
thanks for your first impressions, i look forward to more details. i am not sure how big the focussing issues were, but is it possible this is due to changes in temperature. I am not sure how much this affects refractors but it may be a component of the issue. when you get a chance i would love to see a colour image to see how well the CA is corrected. congratulations again on your new scope, you have to be very happy with the results.
Yeah, I'm not sure. I have been imaging for a couple of years now from this location and I've never seen any measurable focus shift on any other scope due to temperature. I've been able to image for hours on end, always starting from the early evening, with the FWHM fluctuating but ultimately maintaining a horizontal trend.
Now it's possible that triplets are different in this regard, I've only ever used my other triplet once for real imaging, but I do use my GT-81 triplet as a guide scope and I've not noticed significant focus shift with that in the same conditions.
The other option is that this is a big beast, and maybe it hadn't fully cooled. I'm not sure if that would affect focus, but maybe? I would also expect the non-insulated aluminium composite panel box that I like to call an "observatory" probably doesn't have great insulative properties so I would expect the scope to be pretty close to ambient to begin with. I've never validated that theory with measurements though.
Anyway, probably seems like I'm saying with certainty that it wasn't temperature related, but I'm not actually ruling it out, I'm just leaning towards other causes based off previous experience (with different scopes!). I could be wrong, and time will tell :-)
I've contacted both SkyWatcher in the US and Tasco in Australia regarding my focuser issues and hopefully I'll have some helpful advice soon.
I guess the thermal behaviour depends on the construction, and carbon fibre tubes are featured in some other scopes to get around this. Also, I'm not saying with any certainty, but I noticed this last week and it was most noticeable in the evening when the temperature gradient was largest.
Keep us posted on the focuser issue, as this will no doubt be useful to those of us seemingly blazing the trail with these scopes
Carbon fibre may not be suitable for an APO scope. Aluminium tubes shrink and triplets will shorten their focal length as they get colder so to some degree theymatch each other and minimise focus shift. If it were carbon fibre unlike on a compound telescope which keeps everything the same at different temps it might actually make the scopes thermal behaviour worse.
This is from Roland Christen. I see Officina Stellare sell some carbon fibre APOs. I wonder how they perform in this regard.
If it makes you feel any better the FSQ106ED has a focus lock lever and it causes focus to shift if you don't partially engage then lock it all the way. Also there have been in the past numerous focuser flexure issues with it. I am not sure if this is still the case but it as for a while. So even Tak doesn't get it right when it comes to focuser every time.
The best focusers I have used have been AstroPhysics ones. Built like a tank and about US$600.
Then feathertouch (a bit lighter wieght though)
Greg.
Last edited by gregbradley; 17-08-2015 at 05:18 PM.
All the info i have read online points to CF tubes being NOT suggested for refractor. Open tube designs, yes, refractors definitely not. That came from Roland and Yuri plus I believe the guy that makes the objectives for CFF scopes.
I guess the thermal behaviour depends on the construction, and carbon fibre tubes are featured in some other scopes to get around this. Also, I'm not saying with any certainty, but I noticed this last week and it was most noticeable in the evening when the temperature gradient was largest.
Keep us posted on the focuser issue, as this will no doubt be useful to those of us seemingly blazing the trail with these scopes
Yeah, fair enough, it certainly does sound like a likely culprit.
Focuser issue is sorted! Well, the one with the fine-focus control. Focuser itself could use a tension adjustment, but I can work with that.
Attached shows the grub screw that, when tightened, enabled fine-focus control.
Imitation really is the sincerest form of flattery - not far from an FSQ-106ED clone
Skywatcher did originally make these as quads, but had coillimation woes, so redesigned them as triplets with seperate flattener. A few quads escaped into circulation and are collector's items now (though plagued with issues).
Do watch for diffraction spikes in the images - the split rubber rings SW use for the lens cells have been shown on occasion to impinge into the light path and create diffraction spikes (smallish). A company in Germany offers to fix the issue for a small cost, IF you have the issue: http://interferometrie.blogspot.com....-esprit80.html
So it's not a standard Crayford like the first Esprit scopes - it's more a rack and pinion. Provided the distance between the bearings on top of the drawtube is sufficient relative to the diameter of the drawtube, it should be a very good focuser once properly adjusted.
Looks like it, Tony, that sure doesn't look like the smooth axle you'd expect on a Crayford. They never refer to the focuser as either a crayford or a R&P and instead refer to it as a "linear power focuser", whatever that means.
Nice report. Looks like your system resolution is seeing-limited for your site - that's as good as it gets. definitely a keeper.
Maybe there could be just a very slight amount of image plane tilt? (left hand sharpness looks just a tad under that in the centre and right panes). If there is some, you can easily adjust it out with the camera.
Suggest that an add-on stepper drive for the very tidy looking focuser might be worthwhile - aluminium wanders about like nothing else when the temperature changes.
