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Thread: 4" refractors

  1. #1
    ch2718's Avatar
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    Default 4" refractors



    As I've gotten more and more into imaging, I've gradually been upgrading my kit, saving the OTA upgrade until last.

    I currently run a SkyWatcher EQ5 mount (<6.5 kg for imaging) with the SkyWatcher 130mm f/6.92 spherical Newtonian (4 kg), Altair GPCAM3 290C (color) CCD, and the Altair 60mm guide scope (1kg) for guiding.

    My kit is still very entry-level as things go, but I think it's at the point now where an OTA upgrade makes most sense, and I want to get something a bit nice.

    I'm really interested in getting a 100mm (4") refractor at about f/5.5 to f/7 or faster, so that's with a focal length in the region 700 - 550mm. I find my current 900-mm OTA's FOV is a bit too narrow for many deep-sky targets. The mass of a 4" refractor would also be compatible with my mount - anything bigger will strain it.

    My short list is:

    1. Altair Starwave 102 EDR
    https://www.altairastro.com/Starwave...Refractor.html
    This is the 'standard' range with FPL53 glass. It is has a doublet lens system so is not apo.

    2. Altair Wave 102 ED
    https://www.altairastro.com/altair-w...o-102-715.html
    This is basically the apochromatic version of the above with the triplet lens system, and seems top of the Altair refractor range.

    3. Esprit 100
    Sky-Watcher | Sky-Watcher Global Website
    This seems top of the 4"-refractor range in general, gets great reviews, is used by lots of serious imagers, and so on. It's also about the same price as the Altair Wave, and obviously apochromatic

    My questions are:

    1. Is the extra price for an apochromatic (over a doublet system) worth it? I know this is subjective, and must depend on many things, but I'm just interested to know your own views on this. If the Altair Starwave doublet system gets me 90% of the way there for about half the price, then there seems little point in buying an apo. But then, the Altair Starwave is still very expensive, and I don't want to regret not getting something better a year or two down the line.

    2. I haven't done any narrow-band imaging yet, but I'm sure I will do eventually. With a narrow band, am I right in thinking a doublet will perform just as well as an apo as there's only a narrow band of frequencies to focus. I understand that chromatic aberration arises when trying to focus a wide band of colors.

    3. If it's a choice between the Altair Wave 102 ED and the Esprit 100, which one to choose?
    I'm tempted by the longer focal length of the Altair Wave as I think with a reducer or Barlow, it will more comfortably straddle the range of FOVs I tend to look at. Any views on this please?

    Thank you!

  2. #2
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    Default Re: 4" refractors

    Imaging = APO
    C90 (USA Rubberized, 1996), C8 (2000), ES 102 Triplet (2018), Orion ST 80 w/2" GSO focuser (2018), Starmaster 20 x 80 (2015), Eagle Optic Rangers 10 x 50 (2004), Sirius AZ-EQ G PRO (2018), LXD75 (2" legs, 2007), SLT (2008), Manfrotto 055 CF (2007) , ZWO224, Panasonic Gx8 (8 second live view, 4 shot stacking in camera)
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    Default Re: 4" refractors

    1) Yes, with a doublet you will have CA (Chromatic Aberration). This is far more noticable in AP than for visual use.
    2) No, because the scope does not focus all the colors of light at the same point. To compensate, you have to refocus each filter and one or two of the filters will focus at a different distance. This changes the image scale on the sensor, making it more difficult to process.
    3) SW Esprit 100

    Can you do good AP with a doublet? Yes you can, it just takes more time and effort to process out the issues caused by the poor optics.

    You might want to consider a TS-Optics Imaging Star 102 mm scope.
    https://www.teleskop-express.de/shop...Telescope.html
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    Default Re: 4" refractors

    I disagree respectfully.

    Triplet is not an automatic pass into fully APO territory and neither are doublet a automatic fail for imaging. The explore scientific line of triplet without FCD100 is an example of triplets that people still complain about CA. Where as you'd find a lot of satisfied user of the Sky-watcher ProED and now EvoStar series which are doublets. Of course it's not a apples to apples comparison but clearly it's not a black and white situation between doublets and triplets.

    All said the Esprit is a good choice.

    As for your EQ5 and loads have a look on Astrobin for the user sky-watcher (johny) and what he puts on his copy of that mount, you might be surprised at what it can carry when wind protected and with an experienced user.
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    Default Re: 4" refractors

    gotak:

    I have noticed that some doublets are more expensive than ES triplets of the same aperture.

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    Default Re: 4" refractors

    Some doublets have fluoride elements. A well designed and polished fluoride can compete with the very best.
    Bruce
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    Default Re: 4" refractors

    Thank you so much for these useful comments - now I’m considering the Esprit and the Imaging Star, and figuring out what spacers I would need for the flatterers.

    gotak, I’m really impressed at what the EQ5 can do. I’m certainly happy with my results from it so far, but I was beginning to regret not getting something bigger to allow for equipment upgrades. But it seems that with care it has more capacity than I realised.

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    Default Re: 4" refractors

    Does anybody know why the SW Esprit is so much heavier (at 6.3 kg) than the TSO Imaging Star (at 4.5 kg)? The Esprit's FL is only slightly longer at 550 mm compared to the Imaging Star's 520 mm. They seem to be very similar telescopes otherwise. Are they a similar match in terms of performance?

    My EQ5 mount is just the EQ5 and not the HEQ5, so despite what 'sky-watcher (johny)' does with his HEQ5 on astrobin (see above), I think mass probably is a limiting factor for me.

    Also, I'm finding the spacers and adapters to be a real headache:

    - For the TSO Imaging Star, the field flattener ends with a M48 male thread. I need to take it down to a C-mount thread. Is the way to do this via a T2 thread and enough T2 spacers, and then step it down again to a C-mount via a T2 to C-Mount adapeter?

    - For the SW, it's even more confusing as the field flattener is a separate item and it's not clear which adapters it ships with. The draw tube ends with an M74 M thread, which I need to take down to M65 M for the field flattener, which has an M66 M thread on the camera side. Then I think I need to take that down to M48 M which gets me back to where I was with the TSO Imaging Star above, needing a M48 to T2 adapter, T2 spacers if required, and then a T2 to C-mount adapter before connecting to the camera.

    Am I right, and is it really this complicated!? This business with field flatteners, back focus and adapters is all new to me.

    Thank you!

 

 

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