SCT External Focuser: Real Difference or Total Overkill?
SCT external focuser vs internal focuser: I tested the PrimaLuceLab ESATTO 2" LP against the SESTO SENSO 3-SC on a Celestron EdgeHD 8 using NINA autofocus data. Many Schmidt-Casseg...
The Space Koala Astrophotography by Luca Bartek
Je n'entends rien à l'astronomie mais j'ai regardé cette vidéo avec intérêt
I've been using a 2" Moonlite Focuser for the past several years. And yes it does add quite a bit of weight to the rear of your optical train. So much so I had to add a counter weight to the front of my OTA so that I could get balance. Plus it adds complexity to your collimation process. As you have to center the focuser as well and the mirror. That is where I messed up with my Moonlite focuser. I now need to contact Moonlite so I can send it back to them to center the tube for me. I have no problems with my auto focus runs w/o the Moonlite focuser. Still get nice looking curves. To me personally, I'm not really seeing a vast improvement in my imaging. After I get the Moonlite Focuser fixed, I'll probably put it up for sale. And get rid of all the additional complexity. Certainly make it easier to travel with. ;0)
so are you thinking of going back to using a motor on the internal focuser then?
@the_space_koala yes, matter of fact I already have. My rig is a Celestron 8" Edge HD using a Celestron Focus Motor on the Edge HD. I was using a Pegasus Astro Scops OAG as I hated the need to go out and refocus my OAG every time I run an Autofocus routine. The nice thing about going back to stock focusing is I can replace the Scops OAG with the SVBony 238 OAG and once I get it focused I can just leave it be as autofocus routines will not affect the back focus. Plus the SVBony 238 comes with a filter drawer that I could not use with the Sops OAG as the OAG is 31MM deep. Which didn't allow for a filter drawere. Where the SV Bony does. So I'm happy with my decision so far. I did have to purchase a ZWO EAF as the Touptek Stellavita does not support the Celestron Focus motor. Only Celestron mounts are supported by the Stellavita. And I'm counting on the Steallvita to make my trips out to the field easier to accomplish. ;0)
it is very strange you had to refocus your OAG everytime. In general I focus an OAG to my main camera the very first night I assemble it and literally never touch the OAG focuser again. The only reason you should have to refocus the guider would be if you change the optical distance between the OAG and the main sensor, for example my swapping filters
Great video! I’ve been using a Moonlite external focuser on my Edge 8”, it takes up a lot of backspace making it difficult to get perfect collimation and doesn’t leave much focus travel. I use an eagle 4 so thanks to your study and video I think I will be switching to the Esato. One thing has been bothering me since adding the internal focuser, which way would you lock down the internal focuser nob, all the way clockwise or counter clockwise or somewhere in the middle or does that even matter? Thank you so much for all these great reviews and experiments. Clear Skies!
the way you should do it is have the external focuser halfway, measure the height and set the correct backfocus that way. Then, you focus with the internal focuser. Wherever the mirror lands is the correct position
@the_space_koalafantastic. That’s definitely the precise way to do it. Thanks so much! Clear Skies!
Appreciate the video. Can you discussing focusing with hyper-star in another video?
With the hyperstar you don’t have many options - you have to use the internal focuser!
Hello Luca! Ive seen you are a member of the hungarian astrophotographers associaton. Are you hungarian? :)
Yes and igen! :)
how did you calculate your step sizes?
you could use the mirror locks. just need to be sure which focuses you were using!
yeah, that would kind of defeat the purpose of automating this whole test if I had to stand next to the telescope all night and turn the locking knobs constantly
@the_space_koalafair, I just wonder if the mirror being locked for the esatto would have been beneficial? I have the bigger brother esatto on my edge 11 it seems a lot better than the zwo eaf I had on it before that adjusted the focus with the mirror.
Excellent analysis, particularly the part identifying the significance of outliers that drive the real statistical difference between predicted outcomes and actual results. My preference is always for an external focuser, which I will be using on my SCT remote setup (such a timely video for me); however, I have also motorized the internal focuser because it offers a much, much greater range. This will give me some flexibility in case I make changes in the future. The motorized internal focuser is new to me and I was pleasantly surprised by how little shift there is (this varies from OTA to OTA) and how excellent my curves were over two nights of testing. I was not able to do the painstaking side-by-side analysis you did. As you found, if one prefers to just motorize the internal focuser due to backfocus issues or wanting to lower the complexity of an already back heavy, complex setup, it can still be perfectly good at producing solid images. There will just some be some more random outliers. Good stuff!
As a user of more than 15 years of two SCTs, I have used both type of motorized focusers. First, the feathertouch focuser knob with the Microtouch motor focuser (top quality both elements). It produces almost perfect focus with the EdgeHD 9.25” when used at f2.3 with Hyperstar. For that kind of setup, the only solution is to use the internal focuser. What I observed through time was similar to your findings: at f10 with that scope, tight focus is viable but has variability because of the image shift. Both sides of the focus curve were not symmetrical. That’s why I moved to a MoonLite CHL focuser specifically designed for the EdgeHD. The difference is that almost every time focus is perfect (R^2=1.00). Stars are pinpoint. It’s the kind of solution that gives you the full focusing potential of the telescope. Of course seeing and tracking play a role, but the difference is there. As you found, maybe little difference, but it adds up. Thank you for this kind of analysis that show the devices performance. Regards form Bogotá, Colombia. Alfredo Beltran
R^2=1 is crazy work! we all dream of that :D
Wow, amazing scientific comparison :) I just added ESSATO 2" to my C9.25, but please make me a favour. Clean that corrector plate :D
such a large number of people are triggered by my corrector plates I may just clean them :D
Is there a case favouring motorising the internal focuser if there is no mirror lock available or does it make no difference in your opinion ? eg: in a Meade LX90. Thank you so much for your many informative videos. 😊
in this test I did not lock the mirror because I needed the other focuser available, so there's always going to be a benefit in stability
I enjoy your thoughtful approach very much as I am testing the same thing currently with my C11HD...but there is potentially a flaw in your comparison: You tested the esatto with the motorized manual internal focuser in parallel and not separately. Meaning: The EAF on the internal focuser does something for the esatto: It holds the mirror tight, especially with the right backlash. This is important to realize. I started to test with the external focuser, used the mirror clutches to "fix" the mirror...started with a manual focus (no eaf there) to focus the external focuser on the 146 mm backfocus as a start point. After a night of imaging with maybe two targets and a meridian flip, the EAF on the external focuser was 600 steps away from where we started in the beginning, step size was more like 30 steps. Basically, the mirror moves, the clutches do not hold the mirror steady. The focus and the backfocus changes quiet a bit. I guess the manual focuser moves as well because of the mirror weight. Therefore I installed another eaf to hold the manual focuser in place and build an additional resistance. I need another clear sky now to test the external focuser with this support. But as the mirror lock does not really lock (i read some people even use them lightly screwed in while focusing with an EAF and the internal focusser(!)), the esatto performed so well because of the other focuser in place holding the mirror tight.
it is normal that the position of the external focuser would changes by the end of the night due to temperature changes because the focus position changes. If the focus point didn't change throughout the night, you would not need to refocus at all
Great video. I am strictly visual astronomy, AP has no interest for me. In my current SCT, a 10 inch Meade, it has a Moonlight exterior focuser on it. It came that way. The 14 inch Meade that i will pick up sometime this year has a Feathertouch focuser on it. Personally for me the exterior focuser wins hands down as it makes life so much easier when changing eyepieces. The 10 inch is about 35 to 40 years old and the image shift in it is a very real thing. Thanks for the video and your viewing location is positively stunning.
This was a great review of both systems. I have the Esatto together with the Arco on my 9.25HD with the primary mirror locked. Focusing curves are just awesome.
Interesting conclusions and I had similar experiences. However, the backfocus design of the Esatto is problematic for some OAG configurations and leaves virtually no room for error. The focuser works highly accurately but with the reducer, some systems won't be able to find backfocus becuase of OAG designs. I have a video coming out in a few weeks time about this.
I don't follow what it has to do with the OAG. Is the focuser of the OAG touching the esatto physically?
@the_space_koalaWhere the OAG connects to the Esatto, the 1.25 barrel housing the guide camera won’t connect without at least 13mm of additional spacing because of the ‘flat’ design of the focuser. I would imagine some barrels will be slightly thinner but I doubt by too much. The solution for me was to remove the tilt plate which bought back 5mm so the distance to the sensor moved from 17.5mm to 12.5mm.
Hello @the_space_koala, thank you for this video. It comes at a good time because I bought a 8 inch EdgeHD and I’ve been wondering what’s the best way to autofocus. Question: can it be a solution to get a Baader Crayford focuser for example and attach a ZWO EAF to it ? Or it would increase the image train too much ? I’ve read that the internal focuser of the edgeHD is far better than the non edge SCT so i took an EAF at the moment but was wondering about the other options. Thank you 🙏
if you're thinking of the Baader steeltrack that everyone is getting for their SCT's you won't be able to use that together with the reducer for sure. You need to get a low-profile focuser. If you do that, you can of course mount an EAF on it and you'll be able to use it
@t@the_space_koalaank you! that’s what i though also about this product with the reducer.
Amazing content as always!
An lastly, the Edge HD telescopes as they come from Celestron don't have a fan cooling system which you need to deal with tube currents and mirror boundary layer issues. Deep Space Products sells the TEMP-est fan system to address this. By the time your done adding all the extras needed to turn the Edge into a serious imaging telescope, you've spent quite a bit more than what you paid for just the scope. All in all, I'm really eager to see how well the new SkyWatcher 8" F10 Mak's zero-mirror shift focuser and built-in fan works when that scope comes out later this year. I'm hoping it solves a lot (or all) of these problems. If it does, I'll sure be trading in my Edge HD for one.
Also you didn't talk about the difficulty with back focus. First nailing the 133.35 mm (F10) or 105 mm (F7) back focus by screwing things together and then extending the Essato draw tube just is a real pain. I have to remove my camera, stick my digital caliper through the Esatto until it hits then back of the baffle nut and adjust the draw tube until I get a value of 133.35 (back focus) - 56 (camera/FW/OAG/filters) + 3.5 (length of thread that screws into the OAG). It took quite a while fiddling with the focus motor and dealing with measurement error until I was happy. Now that you've got perfect back focus at the current temperature. The thing about SCTs is that they are very sensitive to temperature changes. Celestron specifies a tolerance of +/-0.5mm and that's only 5°C temperature change either way to be out of spec. That can be a real problem where there are large temperature swings during a night. Lastly, once you've got perfect draw tube extension for perfect back focus, you've now go to focus with the moving mirror and lock the mirror.... but locking the mirror moves the focus point. Again here your batinov mask comes in as you guess offsets from the perfectly focuses batinov mask focus point such that after the mirror movement caused by locking the mirrors makes your batinov mask image is perfect. That's a lot of trial and error and needs to be redone every ~5°C or so. All this struggle really makes me wish that Celestron would just fix the moving mirror focuser. John Hayes designed (and built) a perfectly stable focusing system for Celestron 10 years ago that he put on his personal C14.
How many image frames did you take per point in the focus curve? I have nearly this exact setup (Pegasus Astro Focus Cube instead) and I've found that by taking 3 images per focus point with 4 second exposures (to average the seeing variability) then the moving mirror focuser becomes a lot more reliable. Also, I've found that the moving mirror focuser sometimes doesn't actually arrive at the perfect focus point as found with a Batinov mask. On my system I need to add 3 focus steps to the calculated Hocus Focus result (which I think is caused by something mechanical). It would be very interesting if you could do an experiment with a Batinov mask to see what kind of reliability differences there were between the focusers by having longer focus exposures and more of them averaged together per focus point.