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4" vs. 5" APOs Under Bortle 6 - 7 Skies

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#1 RobM

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Posted 13 July 2019 - 11:17 AM

Wondering out loud whether or not there would be any tangible advantage to having a 17lb. (including 2" diagonal and various 1.25" EPs) 125mm APO doublet versus a similarly tricked-out 10 lb 102mm APO doublet (both in the f7-f7.5 FL range) if living under consistently Bortle 6-7 skies approx. 35 miles directly SW of NYC.  For the sake of discussion, let's say the scopes in question are the Stellarvue Access 102mm FCD100/lanthanum and the Stellarvue SVX125D Premier Refractor with the FCD100/lanthanum doublet.  I don't want this to turn into a flame war about the merits of Stellarvueslaphappy.gif   The discussion could easily revolve around Takahashi 4" vs. 5" doublets.  Said scopes would be securely mounted on the vintage Vixen Polaris mount pictured below riding on a 1" Azek weatherproof board, affixed to a 10" square pressure-treated wooden pier with 3/8" leveling bolts, with the pier sunk 4-1/2' into the ground (with concrete footing).

 

20190706_114340-01.jpg

 

Given the fact that on a clear night of approx. 40-50% humidity when I look towards the zenith near Vega, I can just barely make out the constellation Lyra, would there be any merit in a 5" vs. a 4" for visual observing?  I can see Polaris, but not the rest of the Little Dipper.  Viewing is limited by the aforementioned Bortle 6-7 skies to the Moon and planets, plus the rare DSO.  The light dome from NYC is easily visible to the northeast and my neighborhood is illuminated by those pesky orange mercury-sodium streetlights. 

(Must...not...buy...pellet...gun  mrevil.gif )

 

Thoughts?  The more pragmatic side of me says go with the 4" APO and buy a 7" Orion Mak OTA.

 

 

 



#2 Astrojensen

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Posted 13 July 2019 - 11:28 AM

Increasing the aperture always means you can see more objects/details, regardless of the light pollution. 

 

How you're going to get more aperture is up to you. wink.gif

 

 

Clear skies!
Thomas, Denmark


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#3 Jond105

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Posted 13 July 2019 - 11:29 AM

I mean, I switched from a 4” to a 120mm in a bortles 9 zone with no regrets. I think I was being greedy though. Look the 4” does a lot of nice work, but to me, I could see the faint difference in the extra glass. Whether it be clusters, double observing or detail on planets. So my advice, if you want to spoil yourself a bit, grab the bigger one. If you could be content with the 102 never thinking “what if” grab the 102mm

#4 rolo

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Posted 13 July 2019 - 11:43 AM

You may consider a 4" scope and try to observe from a dark site if possible. For low power deep sky you don't really need an APO unless you'll be imaging.


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#5 k5apl

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Posted 13 July 2019 - 12:14 PM

I think the extra aperture is good if its in your budget and physical capability.  My sky at home is much like you describe.  But, some nights are much better than others, and the extra aperture will be a plus.  

One night my friend with his 12 inch Dob and my 130mm refractor were observing under poor transparency, and objects looked about the same in each scope.  But when it cleared, he pulled Way ahead.  

And, if possible, find a dark site fairly close by to make another big difference.  And, if 120mm-130mm scopes are too much for your budget, look at quality 6 inch MAKs.  I have a TEC MAK6 that performs about equal to my 130mm refractor at about 1/2 the cost, while still getting refractor-like views.   I do have a 102mm refractor but I seem to gravitate toward the 130mm refractor most of the time.



#6 Esso2112

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Posted 13 July 2019 - 12:23 PM

Both a 4” and 5” refractor will show a lot of detail on the moon and the planets. The 5”, more so and your sky brightness won’t matter for those objects. Another advantage to the 5” would be that you could use a LPR, or similar filter, without dimming the object too much.  I love my 4” scopes, though.



#7 junomike

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Posted 13 July 2019 - 12:27 PM

IME

For Lunar/Planets the extra 25mm is always beneficial.

For DSO's I'd pass on both and go for a 120 F5 or better still 150 F5 Achromat.


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#8 markb

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Posted 13 July 2019 - 12:55 PM

Aperture is always good, and my initial comment would have been so oriented (my 6"fl f5 Jaegers delivers more than the 5" at somewhat greater mass and bulk) but maks and large SCTs also bring focal length and image scale, but reasonable mass amd size, to the table. When I saw your parting comment it resonated with me.

I'd seriously consider the 7" mak (but no smaller) solution, and keep the 4 apo.

When I got a reasonably sharp C11 and a really great C8 the image scale and long fl (eliminating the need for super short eps on the Jaegers f5's) really got my attention. The planets in both are satisfying, and the 11 easily delivers Jovian moon discs even in average seeing.

A 7" mak will deliver quite a bit of of that, but note I found a 5"mak to just be insufficient in aperture.

Cooling will be an issue, but the Reflectrix insulation threads seem to address that problem.

I have found combining a moderate refractor and a much larger SCT or mak make a very satisfying pairing. The compound scopes can perform at least some of the medium f-ratio functions with a telecompressor, certainly in your current f7 ballpark.

I reread and noticed your sodium light pollution comment, Neodymium based filters like the Baader Moon and Skyglow are the only ones I have found visually satisfying here in eastern Long Island, in part due to the amount of sodium lights in use. None of my regular bandpass filters worked for me, visually.

Worth a try if that is your primary light pollution type. I tried it in a sodium-only village a bit N of here in a 'big estates' area, and the difference was amazing.

Other areas, Phoenix comes to mind, are using broad spectrum lighting and all filters seem minimally effective at best.

Edited by markb, 13 July 2019 - 01:08 PM.


#9 BarrySimon615

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Posted 13 July 2019 - 01:33 PM

One thing that has not been addressed is the capability of your mount.  It is adequate for a 4" such as what you presently have.  Any advantage of a 5" would be partially offset by your mount being undersized (somewhat) with a 5", particularly with a more robustly build 5".  If keeping the 4" and complementing it with a 180 mm (7") Mak, that scope would overwhelm your mount.  Given the base you have built below your mount, I would suggest  something on the order of a Celestron CGem, SkyWatcher EQ-6, Orion Atlas or IOptron iEQ45.  Anything less and a 180 mm (7") Maksutov would be undermounted.

 

Barry Simon


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#10 bobhen

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Posted 13 July 2019 - 02:51 PM

If your goal is seeing the most deep sky detail from that location you should consider adding some form of EAA. That can be an inexpensive astro video camera or a more expensive Night Vision Image Intensifier. With EAA, whether it’s a 4” or 5” refractor won’t matter except in regards to image scale.

 

For the moon and planets, high-quality optics are as important as aperture. So if you move up in aperture make sure the optics and your mount are up to the task.

 

Bob



#11 Jon Isaacs

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Posted 13 July 2019 - 03:01 PM

I have both a 4 inch apo and I reacquired a 120mm Eon just yesterday.  The mount I use is sturdy enough for the 120mm.

 

I think the question is, what do want to look at?  If it's planetary/double star then I think the added aperture is definitely worthwhile.  That's why I reacquired the Eon, the added aperture makes it a better scope at high magnifications.  Those are my primary interests under light polluted skies, somewhere around Bortle 7.  

 

Jon



#12 CHASLX200

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Posted 13 July 2019 - 04:41 PM

I don't even know what a 6 or 7 bortle sky means. I still go by Mag.


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#13 agmoonsolns

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Posted 13 July 2019 - 04:43 PM

For me, a very definite yes. A nice compromise between the 100mm and the 130mm is the 120mm. While only a 20mm difference in aperture with the 4", it shows enough additional detail to be very much worth any extra time setting it up.


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#14 BarrySimon615

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Posted 13 July 2019 - 08:25 PM

ht

I don't even know what a 6 or 7 bortle sky means. I still go by Mag.

A Bortle 6 sky allows viewing to a magnitude limit between 5.1 and 5.5.  A Bortle 7 sky will give you faintest stars between 4.6 and 5.0.  On the best of nights I can reach down to about 4.6, so I am mostly under Bortle 8 skies being just 5 miles straight line from downtown New Orleans.  Our dark sky site where we do the Deep South Star Gaze will get us under Bortle 3 skies with stars as faint as magnitude 6.6 seen at the zenith.

 

Barry Simon



#15 Ihtegla Sar

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Posted 13 July 2019 - 09:17 PM

Assuming two telescopes of equal optical quality, more aperture is going to let you see more, even under light polluted skies. I live under Bortle 7/8 skies and my larger aperture scopes definitely show dimmer objects but they rarely get used because of set up and cool down time. The question is whether the additional cost, weight, size, set up time, and cool down time of a five inch are worth getting the bigger scope. Taking all this into consideration I opted for a light weight four inch florite doublet rather than a five inch refractor.

But its an individual choice and the best scope is the one that gets used. I typically don't have a lot of time for viewing due to work and the vagrancies of the weather in the Pacific Northwest, and I am always tired, so fast and easy set up and cool down were the most important factors to me.
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#16 dscarpa

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Posted 13 July 2019 - 09:26 PM

 The 125 for sure . Plus being the Premier your optical quality is going to be higher. I've got a WO ZS110 and the extra aperture vs a 4" gives it a edge.  David



#17 db2005

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Posted 14 July 2019 - 06:30 AM

Moon, planets and double stars will always benefit from more aperture regardless of light pollution on account of better resolution alone. Clusters also look much better with more aperture, even in the presence of light pollution. However, I do find that most DSOs don't look great even from my own Bortle 5 skies from which I currently do perhaps 90-95% of my observing, so I spend most of my time on targets that still look nice in the presence of light pollution.

 

I know this might be heresy... but if significantly darker skies are within your reach it might be worth considering getting a much smaller, very portable scope to bring along to a dark location. I'm thinking something like a 2.5-3" quality APO on a light alt-az mount and tripod.


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#18 Spikey131

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Posted 14 July 2019 - 07:12 AM

On topic:  5”>4”, as long as you can handle the weight.  Would a bigger, heavier OTA discourage you from setting up?

 

Off topic:  Have you considered a GOTO mount?  They make finding things much easier in light polluted skies.


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#19 Jon Isaacs

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Posted 14 July 2019 - 07:24 AM

On topic:  5”>4”, as long as you can handle the weight.  Would a bigger, heavier OTA discourage you from setting up?

 

Off topic:  Have you considered a GOTO mount?  They make finding things much easier in light polluted skies.

 

A proper GOTO mount for a 4 or 5 inch that's solid will be substantially heavier than a simple ALT-AZ mount and thus has that same issue, will the longer setup time and more effort required discourage the scope's use?

 

Jon


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#20 Spikey131

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Posted 14 July 2019 - 07:58 AM

A proper GOTO mount for a 4 or 5 inch that's solid will be substantially heavier than a simple ALT-AZ mount and thus has that same issue, will the longer setup time and more effort required discourage the scope's use?

 

Jon

 

Good point.  But I was addressing separate issues.  I’m not sure how much the size of the scope affects the OP.

 

He was describing his light pollution.  It is hard to star hop when you can’t see the stars!  This is where a GOTO mount can really help.



#21 CHASLX200

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Posted 14 July 2019 - 08:39 AM

ht

A Bortle 6 sky allows viewing to a magnitude limit between 5.1 and 5.5.  A Bortle 7 sky will give you faintest stars between 4.6 and 5.0.  On the best of nights I can reach down to about 4.6, so I am mostly under Bortle 8 skies being just 5 miles straight line from downtown New Orleans.  Our dark sky site where we do the Deep South Star Gaze will get us under Bortle 3 skies with stars as faint as magnitude 6.6 seen at the zenith.

 

Barry Simon

I don't know all this new stuff on how skies are judged. I just know on the best nites i can just make out all of Ursa Minor, but can't hold all the stars in the handle and the two dimmer stars in the bowl, they come and go.  I am right on the gulf so north and west is my darkest sky. Most nites i can just see Polaris and the two brighter bowl stars.  Kinda like Canis Major, i can see one of the brighter stars in the dogs head, but only just. My south sky has bad sky glow from the Clearwater area.



#22 spencerj

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Posted 16 July 2019 - 12:15 PM

The Tak TSA 102 I used to own was the best 4" refractor I have ever seen.  Optically it was essentially perfect.  I used it on a heavy Alt-Az mount on a solid tripod.  My current AP130 outperforms that TSA 102 on everything.  Always. Under any sky conditions including my suburban backyard.  It is a better planetary scope, better double star scope, better DSO scope . . .   

 

My AP130 is an EDFS model from around 2003.  The OTA weighs about 15lbs and at F6 it is not super long.  Pretty reasonable scope to handle.  I use it on a heavy Alt-Az mount on a solid tripod.  Like the TSA 102, it is a 2-trip, out-the-door setup.  So basically the same effort to use as the TSA 102.  It travels with the same effort and only slightly larger footprint than the TSA 102.  Everyone's situation is different, but for me handling a 5" refractor was really close to a 4" refractor.  The view through the eyepiece is not as close.  Optical quality being equal and hassle factor under control, aperture is your friend.


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#23 jag767

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Posted 16 July 2019 - 12:26 PM

I literally just moved up to a 120 from a 106, as in the is still on its way. I was deciding between 120 and 150, but after considering mounting, the 120 won. Im not expecting a world of a better view, but 14mm is 14mm. Taking into account the 120 is the same weight as the 106, only longer, i don't see it being used any less, and quite possibly more if the view is more pleasing!


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#24 SuperSirLink

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Posted 16 July 2019 - 04:41 PM

I too was in a similar situation... I opted for a 120 after heavily considering a 150...  

I know this might be heresy... but if significantly darker skies are within your reach it might be worth considering getting a much smaller, very portable scope to bring along to a dark location. I'm thinking something like a 2.5-3" quality APO on a light alt-az mount and tripod.

 

For this reason... I "downgraded" from an 8" SCT on a GOTO mount that I was less likely willing to transport to a dark site due to weight and setup time.  My focus has been about keeping portability in mind but still staying with a much aperture as I can.  For me though I live on the very fringe of Bortle 5, and have mostly Bortle 4 around me, although I can drive about 1.5hours to access Bortle 3 skies.  So getting as big of a scope as I could while prioritizing portability/storage was a must...


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#25 Richard Whalen

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Posted 16 July 2019 - 04:52 PM

From a 4" I would go to a 7" MCT. Then your talking about a significant difference in resolution and image brightness. Also a 7" MCT should be as easy to mount as a 5" APO if not easier. 




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