Anyone using the RST-135 (E) in Alt/Az mode? - Forums

Topic starter

Aug 25, 2022

I'm strongly considering to buy an RST-135 or RST-135E next year. Still watching the introduction of the new harmonic drive mounts of various other vendors but the 5 year warranty and the fact that it's already a tried and tested product, used to good effect by a number of users here at astrobin is more and more a strong argument for me. Main application will be photographing deep sky objects in equatorial mode. However, from time to time I may just go for some short sessions visually or for some quick shots of the moon and maybe occasionally the planets. I'm not aiming for maximum quality there, this is mainly just for fun. While polar alignment is not a big deal, just using the mount in alt/az looks like a nice option for someone who always has to set up and tear down his rig in each session. I know that the RST-135 offers this option, does anyone here actually use it to good effect? How hard/easy is it to switch between the two modes and is the goto of the mount accurate? (Still traumatized from my initial impressions with the AZ GTI which basically never hit any target with the SkyMax 127 - it's another price league though).

Thank you for sharing your experiences.

Clear skies
Wolfgang

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base16

Aug 25, 2022 · Edited: Aug 25, 2022

I don't know about SkyMax 127's weight & distribution once you add in eyepieces and such, but for very light payloads & short focal length astro imaging (think camera & lens), the AZ GTI might be more convenient.

Where RST would help is the versatility.

I use the RST-135 as you describe: 90% of my use on this mount is imaging/EAA these days, where the mount is in EQ mode. I occasionally use it for visual observation where the mount is switched to AltAz mode – switching between these 2 should take little to no effort and time (but you do need the provided Allen wrench & the hand-controller to switch modes). My current visual scope is a Skywatcher Quattro 8" F/4 Newtonian paired with televue 17 Ethos (the eyepiece alone weighs 1.5 pounds) + TeleVue NV device. The weight distribution is quite uneven depending on which angle the eyepiece is angled, so I prefer to use the RST-135 for visual with this setup than AP900. Note that AP900 has been my visual mount for large refractors (I've now sold the toa-150 so until I get another heavy ota, the AP900 will likely be sitting unused). The reason for preferring RST over AP900 is the lack of need for balancing on the RST… though AP900 is sometimes more convenient when viewing through a refractor (where weight is more deterministic than these giant unwieldy reflectors) because one can loosen the clutches and point anywhere.

As for accuracy, I wouldn't speak highly of pointing accuracy on the RST-135. But I know people are happy with it.. especially if you take the time to perform the 5-star alignment carefully. I use ASIAIR and an eyepiece camera to perform goto using plate-solving and don't rely too much on the star alignment & pointing accuracy of the mount. Platesolving is predictable and infinitely more convenient than the n-star manual alignment, especially at longer FL.

Best regards,
Arvind

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Torben van Hees

Aug 26, 2022

I‘ve used it in ALT/AZ for the first time last night. It‘s quick and easy to set up. I wasn‘t endeared with the pointing accuracy, though: I levelled carefully and used an electronic compass to point the mount north in the home position (the manual claims it is south, but that is only true in the northern hemisphere). I used a 980mm FL scope and a 8-24mm eyepiece and tried several 3-point and 5-point alignments. I didn‘t have a reticle eyepiece, but used the zoom at 8mm for creating the model. The objects were regularly ways off by several FOVs at 24mm. This is far worse than my Omegon Goto Dob and no comparison to the accuracy in EQ-mode, where initial pointing is usually (far) better than 10 arc minutes after a PA and sync. Pointing got a lot worse closer to the horizon, so maybe atmospheric diffraction is not accounted for in the pointing model. Considering the time it takes to set up a 5-point model, I‘ld recommend EQ mode for visual instead if you want goto.

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kuechlew

Topic starter

Aug 26, 2022

Thank you Torben, thank you Arvind!

Since this is certainly not the main purpose of this mount it's forgivable. Still a bit of a shame given it's sold as EQ & Alt/AZ mount. But you are right, the time it takes to perform star alignment is well invested in a polar alignment in eq mode.

Clear skies
Wolfgang

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