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Small Refractor Aperture Impacts on OC & Galaxies

Discussion in 'General Astronomy Chat' started by BillP, May 3, 2018.

Small Refractor Aperture Impacts on OC & Galaxies

Started by BillP on May 3, 2018 at 9:36 AM

6 Replies 1631 Views 4 Likes

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  1. BillP

    BillP Well-Known Member

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    BillP, May 3, 2018
    #1
    Tonight I decided to do a little open cluster and galaxy testing with my 152 ED Apo. I made 120mm and 130mm masks for it then went about observing to see how things differed with the different apertures. I know many of us lust after a 6" Apo, so having one thought I'd take a look at the differences real time.

    My location is a Yellow Zone, and I get SQM readings of 21.2 or a little more on darkest evenings. This was not one of those evenings, but the sky was still nicely dark and no Moon. So what is important about the results in the relative difference in the views as I have gotten better views of all these objects at darker sites with smaller apertures! So sky darkness is everything for DSO, but it is more of a luxury for most of us and something we only get on infrequent observing trips. Also note that all observations were fixed at 71x using Morpheus 17.5mm eyepieces in a WO Binoviewer through the 152mm f/7.9 ED Apo.

    First off, 120mm I didn't like at all. I was surprised. I felt it was too reminiscent of what my 102mm does. It was immediately obvious to me on some of the familiar open clusters I always visit with my 102.

    With the 120mm vs. 130mm it was surprising to see some fainter stars come into view when just moving from 120mm to 130mm. The galaxies I viewed were just smudges with slightly brighter centers with the 120mm mask. I viewed M104, M51, M65, M66, and NGC 3628. The latter was a difficult adverted vision long smudge. So I stopped using the 120mm mask, surprised how little improvement the 120 gave me over my 102.

    Now the 130 and 152 differences were interestingly different. The 130 showed open clusters clearly better for me than the 120, and the 152 was better still. They were filled more impressively with fainter stars. The 130mm showed galaxy cores vibrantly brighter, and the extent of the arms larger. A much nicer view than the 120mm mask. NGC 3628 was no longer that difficult to see and more direct vision now. I felt I could be quite satisfied with a 130mm compromise if I wanted to trade smaller footprint and better ergonomics over the larger 152. However, moving from 130mm to 152 was meaningful. M51 now showed some of the spiral arms (note that at a Bortle 1/2 site the 90mm I was using completely trounced the view I was getting from my Yellow Zone with the 152mm; I mean not even close!). M104 clearly showed the dust lane at 152 whereas it did not show at 130mm from my home site. M65 and M66 had cores that were now aglow and nice extent and shape to the stars around the central bulges so they looked like pretty little galaxies in the blackness. Finally, NGC 3628 was now quite easy to see and appeared to show some of the flared shape at the two ends.

    I personally do not find the resolution gains all that meaningful between a 130 and a 152 for planets as taking a smaller and more ergonomic scope out more often to have better odds of catching nights of best seeing I find more beneficial and productive. So I can live with smaller apertures for planetary because then it is no effort to bring out the scope so I will view them more. And the Moon is so rich in details that for me, I am basically satisfied with any aperture from as low as 80mm. But for DSO aperture is needed if you are not at a dark site if you want to see more than galaxy smudges. 152mm certainly shows wonderfully here (and a 140mm would as well I expect), but 130mm would be a good compromise and would not loose too much to enjoy the improved ease of use, mounting, and moving of the smaller and lighter refractor.
     
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  2. Mak the Night

    Mak the Night Well-Known Member

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    Mak the Night, May 3, 2018
    #2
    That's interesting Bill. I nearly bought a 120mm short tube achro recently, but in the end plumped for the 102mm. It's a matter of portability and usability for me as I'm physically disabled. I honestly thought the 120mm would be substantially better than the 102mm, particularly for fainter targets. A 4" OTA is easy for me to pick up with my left hand (my right arm and hand are partially paralysed), but 4.7" is trickier. Plus I wasn't sure if my Vixen Porta II/Hal-130 combo would hold the 120mm as stable as I wanted. I've always got my 235mm SCT for deep sky, but it isn't quite grab'n'go lol.
     
  3. BillP

    BillP Well-Known Member

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    BillP, May 3, 2018
    #3
    Yes...in the 4" it's a smudge...in the 120mm it stayed a smudge. That's what I saw. But the skies are so important for galaxies. My 152mm Apo here in my Yellow zone shows the two bright cores on M51, nice extent to the galaxies, and hits of the spiral structure. A 90mm at a Gray Zone showed me all the spiral arms bright and detailed, the bridge between the galaxies, it was like a picture. So much better than my 6" or even my 10" Dob shows me M51 from my Yellow Zone. So morale of the story for me is a nice light 102mm as some advantages as great for lost of targets in light polluted zones, and easy peasy to observe with, which means a pleasure to take to nice dark sites where it will perform better than the 10" at home! Makes a nice hassle free travel companion in the car. It is my favorite aperture for a refractor; it really is a joy to use and quickly becomes a trusted observing friend. Does not matter that is cannot do everything at light polluted sites as it is a trusted and reliable companion to explore the universe with making the journey very friendly and easy. I don't need to see everything. Seeing what I ca see is quite enough. If I want to see more then that is what books and research and imagination and socializing/sharing are for.
     
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  4. Mak the Night

    Mak the Night Well-Known Member

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    Mak the Night, May 4, 2018
    #4
    I live in the country so light pollution isn't a huge issue. I believe a 4" refractor is similar to a 6" reflector in some respects of light gathering ability. I have one or two filters which are specifically marketed at smaller apertures and their marketing blurb often claims they will be good on 80mm refractors and 130mm reflectors.

    IMG_20180129_133938.jpg

    I believe Patrick Moore stated that the lowest limit of a refractor to do serious lunar observing was 4" and 6" with a reflector. I didn't buy this (now slightly modified) ST102 for lunar/planetary particularly but I'm sure it will be capable if pushed. My 150mm Newtonian can be relatively easy for me to set up when I leave the EQ5 mount out covered with a tarpaulin in the months of the year of better weather (about one month then lol). I just take the OTA and a bag of eyepieces out and don't have to set up the mount every time. I have it at the extreme north end of my long garden. It's great for anything in the plane of the ecliptic, I can observe relatively comfortably, and the set up time is minimal. In the summer I'd keep looking up at the Summer Triangle though, which was difficult for me to access with the Newtonian/EQ5 and eventually I cracked and decided to get a 102mm refractor for my alt-az. I vacillated for a while and kept considering the ST120, I guess it was aperture fever or something. The 4" won though, for a variety of reasons, weight being a significant factor. I think I made the right decision after reading your findings. Well, it makes me feel better anyway lol.
     
  5. Dave In Vermont

    Dave In Vermont Well-Known Member

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    Dave In Vermont, May 4, 2018
    #5
    Most of those caveats about 'DON'T USE THIS FILTER WITH UNDER 8 INCH APERTURE...blah, blah, blah' is garbage. They're great to use - or at least try - on most anything I've got on hand. And I have most telescope-ranges covered.

    As well as enough filter's to choke a Giraffe...


    'Here comes that nut with a suitcase and a stick of butter!'.jpg
    "Help!! Here comes that nut in a cherry-picker with a suitcase and a stick of butter again!!"
     
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  6. Mak the Night

    Mak the Night Well-Known Member

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    Mak the Night, May 4, 2018
    #6
    In my opinion UHC filters can be used with small apertures if you have a wide enough exit pupil. I still got more detail on M42 using a 90mm Mak with a Baader UHC-S than with a Lumicon UHC. The Lumicon gave more contrast, but the Baader view was more nuanced. The Astronomik UHC-E was about half way between the two of them, which I more or less expected. I was using 50mm and 46mm (reduced) eyepieces. I think there's a lot of bollocks talked about filters and apertures. Conditions will dictate what will be needed on any given night. I've had a good 170x on the Moon with an ST80, yet people tell me it's impossible (I'm guessing they live in towns or cities). It isn't where I live, yet I struggled to get 150x the other night with an ST80. I often stack a Baader Fringe Killer with a Baader Neodymium for CA on planetary/lunar targets.

    NEOFK.jpg

    If the transparency isn't good I resort to a Wratten #8/TS Optics UCF1 stack; basically a standard #8 and a basic Moon & Skyglow filter, old school!

    UCF1 TS8.jpg

    Less subtle, but better contrast. Cheaper too lol.
     
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  7. Gabby76

    Gabby76 Well-Known Member

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    Gabby76, May 8, 2018
    #7
    A nice evaluation Bill and rather well timed as I have been looking for a long focus 130mm to fill the gap between my 115mm and 150mm. I have been masking the 150mm for some time and enjoy the views.
    I also see your 5" achromat thread on CN has resurfaced, your post #16 had me laughing as it sounds just as my opinion.
    When you exclude ES and Synta telescopes out of the mix it gets hard to find one.
     

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