DOB Vs Refractor for planetary viewing

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DOB Vs Refractor for planetary viewing

#1

Post by Yshire »


Hi all me and my friend are looking into buying a telescope but don't know which kind to get. We are looking at either a 150 APO or a 150p DOB. My friend is wanting to lean more towards planetary viewing and I always thought a refractor would suit more towards this? Could someone please outline the pros and cons between the two scopes and why one is better than the other.
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Re: DOB Vs Refractor for planetary viewing

#2

Post by yobbo89 »


Depends, what scope and doublet or triplet a corrected refractor should outperform the newt due to the central obstruction from the secondary mirror. Generally the dob is going to give you the best pay out due to price and what can be spent on a better and more corrected refractor can be added to purchase a bigger newt, which the bigger the mirror the better the angular resolution. Feel free to disagree or agree, I'm more bais to mirrors.
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Re: DOB Vs Refractor for planetary viewing

#3

Post by Yshire »


Sorry typo error it's a achromatic 150
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Re: DOB Vs Refractor for planetary viewing

#4

Post by stewe »


Achromats have considerable chromatic aberration (CA) on bright objects such as planets, especially if they have a small focal ratio (fast scopes). I _assume_ that the 150mm achromatic refractor you talk about is a fast one. If it is, then it is a "rich field" refractor intended to be used for low-power viewing of extended deep sky objects and stellar fields. But if it is slow, i.e., f/10 or slower, then it would probably give you nice planetary views with little CA, which you could further improve with a "minus-violet" filter. BUT such a scope would have a very long tube, and would probably cost you a lot of money to buy a mount that can hold it stably.

For viewing planets, you could consider a smaller-aperture semi-APO (cheaper) or APO (expensive) refractor. 150mm APO refractors cost a fortune, but if you have the money, go for it, they are the best for planets. If you don't then you can indeed go for a "slow" (f/8) Newtonian as well. It will have a long tube, so probably a Dobsonian mount would be the best solution.

The question is, do you anticipate to often have good (sub-arcsec) seeing? If yes, then you could often harness the larger aperture of the Newt for more resolution, but consider that it is probably more difficult to transport and that you probably have to collimate it before each viewing. If you have generally medium-to-poor seeing (1"-2" or worse), then you would scarcely get the advantage of the larger aperture, so the smaller refractor could be a better choice. The smaller refractor would also be easier to transport.

In case you prefer to have the 150mm aperture (i.e., more resolution on planets on nights with excellent seeing) in a short tube and also to have high contrast and you don't like to collimate every time you use your scope, then get a Maksutov-Cassegrain (Mak), which will give you enough aperture to view brighter deep-sky objects as well. Maks have virtually no CA, and they provide views almost on par with APO refractors with the same aperture for a fraction of the cost. Having said that, they are all around f/15, so their maximum true field of view is narrow, around 1 degree -- but if your main goal is viewing planets (and the Moon), a Mak will more than suit your needs.
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Re: DOB Vs Refractor for planetary viewing

#5

Post by Richard »


100% a Mak is probably the best for planets given the size but there are not many to look at , one can only look at Saturn, Jupiter and Mars (sometimes) nice and the rest is so so , so unless you are fixated with these 2-3 planets then look at a more useful scope like a Dobs
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Re: DOB Vs Refractor for planetary viewing

#6

Post by helicon »


It's really a decision that comes down to your personal preference on what kind of objects you wish to view. My 6" achro does decently well on the planets (f/6.5) but also does well on the deep sky objects. It actually offers a slightly better view on Jupiter, Saturn, the moon and Mars than my 10" Dob. As mentioned above, a Mak can also provide excellent views of the planets (better than an achro) while being much cheaper than a 6" apo. Good luck with it.

P.S. Keep in mind that a 6" achro is not quite as portable as a 10" Dob and can require two people to set up.
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Re: DOB Vs Refractor for planetary viewing

#7

Post by Lady Fraktor »


helicon wrote: Thu Jan 23, 2020 3:22 pm Keep in mind that a 6" achro is not quite as portable as a 10" Dob and can require two people to set up.
Well I will give an opposing view on this, I have a modified 150mm f/8 refractor that weights 30lbs (the complete mount weights about 75lbs) and I do not require a second person to help set it or the mount up.

For planetary viewing with a refractor a longer focal length is best as there is less noticeable chromatic aberration.
A larger Newtonian will give you a slightly larger image scale and let you see fainter objects but the refractor will still give the most contrast between the two telescopes.
Contrast is what enables you to see all of the fine details.

Another thing the refractor will be able to do is give a wider field of view, a 150mm f/8 refractor is capable of a 2.3° FOV and a 10" f/8 Newtonian only give a 1.35° FOV.
You would need a 10" f/5 Newtonian to get a comparable FOV. A 150mm f/5 refractor will give a 3.7° FOV so still wider than a comparable Newtonian though at short focal lengths refractors are not really good planetary telescopes, they are great for widefield viewing though.
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Re: DOB Vs Refractor for planetary viewing

#8

Post by notFritzArgelander »


Richard wrote: Thu Jan 23, 2020 2:55 pm 100% a Mak is probably the best for planets given the size but there are not many to look at , one can only look at Saturn, Jupiter and Mars (sometimes) nice and the rest is so so , so unless you are fixated with these 2-3 planets then look at a more useful scope like a Dobs
Only is way too strong! I'm very happy with DSO views with my MK66 full aperture corrector Rumak Mak at f12 and my sub aperture corrector f10 VMC110L. The littler one takes a 0.5x focal reducer very nicely too and is an excellent grab and go kit for general viewing.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: DOB Vs Refractor for planetary viewing

#9

Post by Voyageur »


notFritzArgelander wrote: Thu Jan 23, 2020 5:55 pm
Richard wrote: Thu Jan 23, 2020 2:55 pm 100% a Mak is probably the best for planets given the size but there are not many to look at , one can only look at Saturn, Jupiter and Mars (sometimes) nice and the rest is so so , so unless you are fixated with these 2-3 planets then look at a more useful scope like a Dobs
Only is way too strong! I'm very happy with DSO views with my MK66 full aperture corrector Rumak Mak at f12 and my sub aperture corrector f10 VMC110L. The littler one takes a 0.5x focal reducer very nicely too and is an excellent grab and go kit for general viewing.
I interpreted the "only" as referring to "the only planets of real interest" rather "the only objects."

The OP mentioned being primarily interested in viewing the planets. A lot of newcomers say that, without any real understanding of what to expect. What to expect is exactly what Richard said: Jupiter and Saturn are wonderful, Mars sometimes okay. I'll add that Venus is a white blob that shows phases; Mercury is a rarely-seen starlike object (except during the very cool recent transit when it was a black dot on the Sun); Uranus and Neptune are tiny faintly colored dots if you are skillful enough to locate them.

Point being that it is good to consider how a scope can be used to observe the very large number of interesting objects that are not planets.
Last edited by Voyageur on Thu Jan 23, 2020 6:25 pm, edited 1 time in total.
Scopes: Vixen VMC200L, D=200mm, F=1950, f/9.75; Televue 2" Everbright diagonal. Coronado PST; AstroTech EDT 80mm, F=480, f/6.
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Re: DOB Vs Refractor for planetary viewing

#10

Post by helicon »


Lady Fraktor wrote: Thu Jan 23, 2020 5:37 pm
helicon wrote: Thu Jan 23, 2020 3:22 pm Keep in mind that a 6" achro is not quite as portable as a 10" Dob and can require two people to set up.
Well I will give an opposing view on this, I have a modified 150mm f/8 refractor that weights 30lbs (the complete mount weights about 75lbs) and I do not require a second person to help set it or the mount up.

For planetary viewing with a refractor a longer focal length is best as there is less noticeable chromatic aberration.
A larger Newtonian will give you a slightly larger image scale and let you see fainter objects but the refractor will still give the most contrast between the two telescopes.
Contrast is what enables you to see all of the fine details.

Another thing the refractor will be able to do is give a wider field of view, a 150mm f/8 refractor is capable of a 2.3° FOV and a 10" f/8 Newtonian only give a 1.35° FOV.
You would need a 10" f/5 Newtonian to get a comparable FOV. A 150mm f/5 refractor will give a 3.7° FOV so still wider than a comparable Newtonian though at short focal lengths refractors are not really good planetary telescopes, they are great for widefield viewing though.
Well, I can set it up myself but there is always that moment when I sling the OTA into the dovetail with one hand and then tighten the locking knob with the other hand (Twilight II). I almost dropped the scope once. So if I have folks over for dinner I usually set up the scope, hold the OTA with two hands and have someone tighten the knob for me. (sister, nephew or whomever). Otherwise I do it myself.
:)
-Michael
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Re: DOB Vs Refractor for planetary viewing

#11

Post by notFritzArgelander »


Voyageur wrote: Thu Jan 23, 2020 6:10 pm
notFritzArgelander wrote: Thu Jan 23, 2020 5:55 pm
Richard wrote: Thu Jan 23, 2020 2:55 pm 100% a Mak is probably the best for planets given the size but there are not many to look at , one can only look at Saturn, Jupiter and Mars (sometimes) nice and the rest is so so , so unless you are fixated with these 2-3 planets then look at a more useful scope like a Dobs
Only is way too strong! I'm very happy with DSO views with my MK66 full aperture corrector Rumak Mak at f12 and my sub aperture corrector f10 VMC110L. The littler one takes a 0.5x focal reducer very nicely too and is an excellent grab and go kit for general viewing.
I interpreted the "only" as referring to "the only planets of real interest" rather "the only objects."

The OP mentioned being primarily interested in viewing the planets. A lot of newcomers say that, without any real understanding of what to expect. What to expect is exactly what Richard said: Jupiter and Saturn are wonderful, Mars sometimes okay. I'll add that Venus is a white blob that shows phases; Mercury is a rarely-seen starlike object (except during the very cool recent transit when it was a black dot on the Sun); Uranus and Neptune are tiny faintly colored dots if you are skillful enough to locate them.

Point being that it is good to consider how a scope can be used to observe the very large number of interesting objects that are not planets.
Maybe your interpretation is correct. I'm kind of used to the idea, often stated, that Maks are no good for DSOs.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: DOB Vs Refractor for planetary viewing

#12

Post by Voyageur »


notFritzArgelander wrote: Thu Jan 23, 2020 6:28 pm
Voyageur wrote: Thu Jan 23, 2020 6:10 pm
notFritzArgelander wrote: Thu Jan 23, 2020 5:55 pm

Only is way too strong! I'm very happy with DSO views with my MK66 full aperture corrector Rumak Mak at f12 and my sub aperture corrector f10 VMC110L. The littler one takes a 0.5x focal reducer very nicely too and is an excellent grab and go kit for general viewing.
I interpreted the "only" as referring to "the only planets of real interest" rather "the only objects."

The OP mentioned being primarily interested in viewing the planets. A lot of newcomers say that, without any real understanding of what to expect. What to expect is exactly what Richard said: Jupiter and Saturn are wonderful, Mars sometimes okay. I'll add that Venus is a white blob that shows phases; Mercury is a rarely-seen starlike object (except during the very cool recent transit when it was a black dot on the Sun); Uranus and Neptune are tiny faintly colored dots if you are skillful enough to locate them.

Point being that it is good to consider how a scope can be used to observe the very large number of interesting objects that are not planets.
Maybe your interpretation is correct. I'm kind of used to the idea, often stated, that Maks are no good for DSOs.
Yes, I've seen that too. Gross overstatement.
Scopes: Vixen VMC200L, D=200mm, F=1950, f/9.75; Televue 2" Everbright diagonal. Coronado PST; AstroTech EDT 80mm, F=480, f/6.
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Re: DOB Vs Refractor for planetary viewing

#13

Post by Kingofthehill »


I prefer the Dob route, because there is a decent chance you'll be able to sit in a chair while viewing.
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Re: DOB Vs Refractor for planetary viewing

#14

Post by Bigzmey »


Slow scopes (F8-F15 ratios) with CA-free design are best suited for planetary observations. So, in your price range I would go with 5"-6" catadioptric (SCT, Mak, etc.). If you can pay more I would step up to 8"-11" catadioptric.

Why not achro refractor? You need aperture to resolve fine planetary details and at larger apertures achro refractors suffer from CA, which degrades resolution at high powers.

Why not large Newt or DOB? They are fast scopes and in fast scopes image quality degrades fast when you go to high powers due to optical aberrations inherent to fast optic design. Also, they are more affected by bad seeing due to the short depth of focus.

Why not APO refractors? Price. If you can afford 6" APO, go for it! :)

Here are more details on how to observe the planets.

app.php/article/how-to-observe-the-moon-and-planets
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
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Re: DOB Vs Refractor for planetary viewing

#15

Post by Refractordude »


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Re: DOB Vs Refractor for planetary viewing

#16

Post by Bigzmey »


Refractordude wrote: Thu Jan 23, 2020 9:45 pm 90mm aperture collects plenty of light for the bright moon and planets. Right click the images.

https://www.firstlightoptics.com/all-te ... 0-az3.html

https://www.firstlightoptics.com/books/ ... -book.html

https://www.firstlightoptics.com/books/ ... -book.html
Don't get me wrong. You can get rewarding planetary views with small scopes, or scopes of any design. However, if you like to dig dipper and resolve fine details you need combination of resolution (large aperture) and sharp, aberration free optics.
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
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Re: DOB Vs Refractor for planetary viewing

#17

Post by AntennaGuy »


I got yer compact, lightweight, inexpensive, 6" aperture without CA issues right here:
https://www.highpointscientific.com/cel ... -91010-xlt
* Meade 323 refractor on a manual equatorial mount.
* Celestron C6 SCT on a Twilight 1 Alt-Az mount
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Re: DOB Vs Refractor for planetary viewing

#18

Post by helicon »


AntennaGuy wrote: Thu Jan 23, 2020 10:10 pm I got yer compact, lightweight, inexpensive, 6" aperture without CA issues right here:
https://www.highpointscientific.com/cel ... -91010-xlt
$398 is a good deal for that scope.
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Refractors: ES AR152 f/6.5 Achromat on Twilight II, Celestron 102mm XLT f/9.8 on Celestron Heavy Duty Alt Az mount, KOWA 90mm spotting scope
Binoculars: Celestron SkyMaster 15x70, Bushnell 10x50
Eyepieces: Various, GSO Superview, 9mm Plossl, Celestron 25mm Plossl
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Re: DOB Vs Refractor for planetary viewing

#19

Post by Lady Fraktor »


A 100mm refractor is an excellent size for planetary viewing, the majority of my planetary telescopes are 100mm to 105mm of varying focal lengths.
I think a lot of people confuse resolution and contrast.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
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Re: DOB Vs Refractor for planetary viewing

#20

Post by Richard »


notFritzArgelander wrote: Thu Jan 23, 2020 5:55 pm
Richard wrote: Thu Jan 23, 2020 2:55 pm 100% a Mak is probably the best for planets given the size but there are not many to look at , one can only look at Saturn, Jupiter and Mars (sometimes) nice and the rest is so so , so unless you are fixated with these 2-3 planets then look at a more useful scope like a Dobs
Only is way too strong! I'm very happy with DSO views with my MK66 full aperture corrector Rumak Mak at f12 and my sub aperture corrector f10 VMC110L. The littler one takes a 0.5x focal reducer very nicely too and is an excellent grab and go kit for general viewing.
Yes any scope can be modified for other uses but in my opinion a Mac is a planetary scope just like a long focal length refractor like my Tasco ,good for planets, double stars and globs but without modification (reducer etc) not good for DSO, but honestly most people will not see DSO due to the LP in their area, probably 95% , few of us live in a low LP area
Reflectors GSO 200 Dobs
Refractors None
SCT C5 on a SLT mount
Mak 150 Bosma on a EQ5
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