Skywatcher Esprit 100 APO vs RASA 8

I own the RASA 11 and love it. However, it is not without it's quirks and issues.

• If you are shooting mono, the lack of a filter wheel is a big pain point. You're locked into shooting one filter per night, or babysitting your rig all night to swap filters manually. And because your filters are in a drawer/slider system instead of a sealed wheel, you can't reuse flats and need to reshoot flats after every filter change.
• Shooting flats is also a bit of a PITA, since your cam hangs off the front of the scope. This is easily solvable with a dew shield and flat panel, but is worth mentioning.
• Tilt is also a real issue. The CFZ on the RASAs is tiny, and you will need to tune tilt in order to get decent FWHMs across the frame. This needs to be done anytime you rotate the camera or take the camera off and put it back on.
• f/2.2 comes with other challenges in addition to tilt. If you're shooting narrowband, you will need special filters to account for the 'bandshift' effect with faster optics. Options exist (Astronomik's MaxFR filters are my recommendation) but are not cheap.
• The 8 also has it's own problems, in that it's backfocus distance is really small, so fitting stuff like filter drawers and tilt adapters in the train is problematic. Baader has a nice 'UFC' system that works, but is again, expensive.
• It's a big beast of a scope and that has it's own concerns. You're not going to be imaging in even light wind unless you're in a dome. Even with a solid mount and a reasonable image scale, winds above about 1.5mph wreck my guiding. It also takes ages to cool down, even with the fan running.
• Anything made for the RASA has a ton of markup. A commercial dew shield (unheated!) for the RASA11 is over $200, for example.

Triplet fracs like the Esprit are -way- easier to use and get good results with than the RASAs. I have one incoming next year (waiting on a buddy to get a Tak off backorder) and am really looking forward to having a rig that's a little less 'work'.

In your situation, I'd recommend just grabbing an EAF and setting up autofocus on tempshift/HFR change in NINA. $200 for the EAF and free for NINA solves your focus issue and doesn't saddle you with an awesome but fiddly scope.

SemiPro's message reminded me about filter size issues too! f2 means you need big filters close to the sensor to avoid vignetting. You probably want 2" filters even for APS-C, unless you machine your own adapters to get the filters closer to the sensor than is possible with off the shelf components. 36mm filters will vignette hard enough that flats have a hard time correcting it. (I'm using 36mm NB filters and it's painful)

If the primary concern is focus drift due to temperature change throughout the night, a RASA is not the solution. In fact, it will likely be worse.

The best solution is to use an electronic focus motor and software such as NINA which allows you to automatically refocus, for example, every hour.

With a RASA your optics are much more sensitive to collimation and tilit. You also cannot use filter wheels which makes mono imaging practically impossible. A good triplet like the Esprit is much less painless to get a good image out of. With reducers and modern highly sensitive CMOS cameras, the F2 speed of the RASA is much less of an advantage.

Georg N. Nyman:
Thank you all for your excellent answers and suggestions - maybe more are coming, but so far, I think, I deal with my smaller problems with the Esprit and keep it and be happy with it....

Thanks a lot,

CS,

Georg

That would be wise, IMO. The aperture advantage isn't as great as the square root of the ratio of the square of the diameters would suggest, either.

The best solution is to use an electronic focus motor and software such as NINA which allows you to automatically refocus, for example, every hour.

I would agree with this.  I tend to do an automatic refocus on each filter change and then make sure that the number of exposures on any one filter doesn't exceed one hour.  It works really well.  The ZWO EAF 5V autofocusser is a cheap and good solution as all it needs is a standard USB 2.0 connection to power it.

Andy Wray:

The best solution is to use an electronic focus motor and software such as NINA which allows you to automatically refocus, for example, every hour.

I would agree with this.  I tend to do an automatic refocus on each filter change and then make sure that the number of exposures on any one filter doesn't exceed one hour.  It works really well.  The ZWO EAF 5V autofocusser is a cheap and good solution as all it needs is a standard USB 2.0 connection to power it.

Better still you can also configure filter offsets and there will be no need to refocus on filter change. There's even a NINA plugin that automatically calculates the offsets for you.

I would lean towards buying an autofocuser if I were you, just as suggested above.  If you are happy with the quality of the images from your Esprit, then getting a new, unproven telescope is much riskier (and more expensive) than getting an autofocuser.   I just picked up a PegasusAstro FocusCube 2 for my Esprit, and if the sky ever clears out (new equipment curse in full effect), I'll finally be able to sleep without thinking about focus.

FWIW, I also have an Esprit 100ED, and I am not so happy with mine.  I've had endless tilt problems since I got it in July 2020. Nonetheless, my friend had a complete RASA-8 system available for sale last year, and I didn't care for the shape/size/quality of the stars in the images he showed me.  I could have bought it from him locally at a good price, but I decided not to.  He sold it to upgrade the filters that go with his refractor.

Jason dain:
Refractors are bulletproof (just about) and the optics are very good.

I'd flip the assertion and go the other way. Reflectors are bulletproof and I wouldn't trade a reflector properly made with a midget scope with lenses.

I'd like to add that a small reflector (i.e., a newtonian) can be efficiently be used as replacement for a wide field apo at a fraction of ownership cost. As an example a 130 mm f/4 short newt can be used with a small mount in the same way as on of those 3"-4" f/6-ish chinese doublets or triplets that are flooding the market. In the same vein I recently tested a low budget 6" f/4 newt (metal tube) to efficiently replace a 120mm APO(i-ish) refractor on a lower end mount (GEMtwentyeight). Just for the heck of it I let it run unattended for 5 hours in sub-freezing temperatures without ever touching the focuser. It didn't flinch a bit. At the end of the run it was covered in frost but the main mirror was remarkably free of condensation.

Hi Georg,

I went through *exactly* the same dilemma as yours.  I started out with an Esprit 100 and bought an RC8 about a year later.

I love both.  The GSO RC8 is relatively straightforward to use and collimate.  With the right tools and procedure.  ( I use a Takahashi collimator and remove the entire 1ary mirror baffle ). The RC8 holds collimation really well (for months), but I only move it 1 metre from pier to shelf in my observatory.

It holds focus really well - I rarely have to focus during a night - unless temperature drops by more than 10C.   Being f/8 helps.  I really don't mind the "slow" beam when it comes to integration time, after all I can always bin up to match seeing.  The small read noise penalty is small by comparison to the sky noise - even in Bortle 3 skies.   The 1600mm focal length and CMOS pixel give a image scale about 2-3x too small for ideal sampling at my place - so no penalty in lost resolution by binning.  

In many respects the RC8 is a fantastic scope for galaxies and the moon - where the focal length with Barlow and lucky imaging at 0.001sec  allows me to beat my lousy seeing.

However, in my view the RC8 has two big limitations.  

1) The field is curved.  I think a field flattener is mandatory for all sensor greater than 1inch in size.   
2) Even with a field flattener, the corrected, unvignetted field of view is limited - particularly if you are use a larger sensor.  This is where the slow beam hurts.  Indeed I don't really think of f/8 as slow - but rather narrow field.  Ideally there would be reducing flatteners for RCs.  But there aren't.  There are some - but there are designed for larger focusses and you are stuck with a 2inch focusser.

So, I you love the larger field of the Esprit 100, you will be frustrated by the smaller field of the RC8.  Its great for galaxies and solar system, but I found it frustrating for nebular work.

After a very happy years observing - and 100 or so images - I bought a TS Hypergraph 8.  A 200mm Newtonian at f/3.2 with carbon fibre tube.  It was a brute to collimate, but once done it has held its collimation very well.  At f/3.2, even the Carbon fibre tube needs regular focussing (every 1.5 hours) during typical nights here [where the temperature typically falls by 5-10C].  But collimation and re-focussing is the price you pay for a wide-field, low f-ration astrograph.

I looked at a number of other options for a fast large aperture telescope, but discounted the RASA (due to the filter wheel, cable management issue) and more expensive options like the RH telescopes - a large central obstruction giving rather ugly star halos.

Anyway, those are just my thoughts.  Good luck with the decision.  You can't go wrong with the RC8.  Its a great scope for what it does.  

CS

Brian

Later edit:  oops sorry I misread RASA8 for RC8.  Please ignore my rembl8ngs.

Caleb Kam:
Fellow Esprit 100 User here.

Per some have already outlined earlier, RASA comes with its own challenges, namely a very tight CFZ and the lack of ability to do mono with filter wheel (i.e. automation). I think the Esprit 100 is the absolute go-to and I believe it is by far the easiest scope to use both to carry around and for imaging. At F/5.5 you dont need high speed filter but it is plenty fast, at 550mm it is wide enough for most nebulae targets. All in all, Esprit will def be the solution with less headache attached to it.

Hello Caleb,

Thanks for your encouragement, I shall of course keep my Esprit 100.... I was tempted by the aperture and the speed of the RASA but after thinking about all those opinions and reflection from users, I am more than before convinced to keep my Esprit and automate it...

CS,
Georg

Marc:
So yeah, I'd stick with the APO... Just because I know how soul crushing it can be to spent your clear nights with an allen key dancing around your scope and trying to get it to work the way you want.

lol.. there is that i suppose..

@Georg N. Nyman 
Are you a tinkerer?
Or do you just want it to work from the get go pretty much
Clear nights doing any sort of equipment fettling isn't everyones idea of fun...