Refractor for planets, mostly visual, 4" or 5"

[vlaiv]

I will be honest and express my doubt with claims of extraordinary sharpness of 4" instruments and ability to use over x300 magnification and such.

You can certainly use x300 magnification but what would be the point in doing so - when you can see all that detail with x100 magnification already. You'll just get blurrier larger image.

Also - 8" newtonian is going to walk all over 4" instrument. That is my belief anyway and scientific side of things agrees as well.

In fact - we can see this in practice rather easily. @CraigT82 recently did comparison between two lunar images - one taken with 4" instrument and another with 8.75". While I'm certain that his image is excellent (8.75" instrument) - mine was also described as at least fairly good - so I have no doubt that it is representative of decent 4" class instrument.

difference is more than obvious. By the way - image to the left - is like using x300 magnification. While you can use it, I see no point in doing so over perfectly sharp and nice about x150 for 4" instrument:

I also thought of very interesting experiment for anyone wanting to compare planetary performance of two different telescopes - one that excludes impact of atmosphere.

You need ~60 meters of clear space, and one of these:

Yep, it is a marble - about 10mm in diameter. Get nice colorful one with a lot of detail - to represent Planet X that you'll be observing. Observe during the day at distance of about 60m. That distance will be enough for spherical aberration not to cause any issues on most scopes up to 8" and also at that distance 1cm marble will have angular size close to Jupiter's angular size - about 45".

In fact, I just saw that you can get one of these:

That might not be a bad way to get regular planetary observing fix in times of prolonged poor weather

In any case - that will be good indicator of what particular instrument is capable of - without influence of atmosphere.

[jetstream]

I can say that my TSA120 hold its sharpness at high mag, eventhough detail is limited by aperture. My very best lunar /planetary views are with the 15" dob and there are very few images that match my low power views through the binoviewer on the moon.

I personally don't crank up the mag excessively most of the time.

[johninderby]

A 3” or 4” refractor can give very satisfactory views of the planets and can be very convienient to use however as vlaiv has said won’t have the resolution of a bigger scope. 

That’s why I like my CC8” which is roughly the equivalent of an f/12 8” newt in planetary performance. 🙂

[alex_stars]

Great idea with the marbles @vlaiv I definitely have to try that.

I want to take the opportunity mid discussion to thank all the contributors so far. Lot's to consider. I am still between the 4 and 5 inch class and here is what I currently consider, also reflecting on the eye sight I have:

I am used to exit pupils down to about 0.6 mm from my Mak, everything below and eye floaters take so much over that I can not observe anymore without loosing significant detail. That would mean for the 102 mm f/11 a magnification of about 160x (nice for Jupiter). However for Saturn and Mars that would be too little for my taste. The 125 mm f7.8 doublet though would make magnifications up to almost 200x possible, also for a readily available EP (Vixen SLV 5 mm) that I like. So I tend towards the 125 mm doublet. Unfortunately the TAK FC100 is out of my budget.... Else I would consider a high-quality 4"

[Sunshine]

You mean to tell me I've been languishing under clouded skies for weeks now and, all i needed was a bag of marbles!? to the local store I am headed this moment.

Edited January 30, 2021 by Sunshine

[jetstream]

On 29/01/2021 at 03:33, alex_stars said:

Since I recently learned that it actually has a 34% obstruction (MTF plots here), I am on the lookout for a new scope

The obstruction impacts things uniquely. It does not alter the fine detail you can see ie the spacial frequency range. For sure cool down will impact the views in a big way.

I would not expect a smaller refractor to give you more detail- but the improved cooling and smaller aperture (seeing friendly) might improve the views. One thing- my 90mm APO triplet has better optics than my SW120ED- and the 120mm shows more of everything on the moon and planets.

My advice- get as quality refractor as big as you can- the 120mm range is a sweet spot for me.Just don't be surprised when your 180mm shows more...

[johninderby]

Perhaps this discussion should be about the mininmum number of scopes you need? 🤔.  😁😁😁

[Highburymark]

If I had better skies and a better back, then I’d probably side with the “aperture is king” camp. But I went from owning an 8” SCT to a 100mm refractor, and honestly not once have I regretted the move. The excellence of the optics - much sharper than the SCT - together with a 10 minute cooling down time, ability to cut through average seeing and portability all contribute to the frac’s appeal. It easily goes up to 250x without loss of sharpness on good nights, and I feel I don’t need any more. Just my $0.02.

[vlaiv]

23 minutes ago, johninderby said:

Perhaps this discussion should be about the mininmum number of scopes you need? 🤔.  😁😁😁

Not sure if there is anything to discuss there - general consensus on that will be: "At least one more than I currently have"

 

[johninderby]

37 minutes ago, Sunshine said:

You mean to tell me I've been languishing under clouded skies for weeks now and, all i needed was a bag of marbles!? to the local store I am headed this moment.

I lost my marbles ages ago. 🤪🤪🤪

[vlaiv]

More I think about it - more I like marble observing idea.

I always wondered would I be able to see more detail if atmosphere was not interfering and would I be able to use higher power. I think that marble observing would allow one to also experience practical limits of their instrument.

Things that are at threshold of observability would no longer remain a mystery - did I or didn't I see that? Well - take a marble and inspect it - if feature is there - well, you saw it

[johninderby]

Perhaps one of these instead of a marble?

[JeremyS]

But do they make fluorite marbles?
 

[jetstream]

8 minutes ago, vlaiv said:

I always wondered would I be able to see more detail if atmosphere was not interfering and would I be able to use higher power. I think that marble observing would allow one to also experience practical limits of their instrument.

I'm off with rangefinder in hand, a marble and lots of enthusiasm lol! We'll check the 3 fracs

[JeremyS]

Binoviewers?

[Alkaid]

44 minutes ago, Highburymark said:

If I had better skies and a better back, then I’d probably side with the “aperture is king” camp. But I went from owning an 8” SCT to a 100mm refractor, and honestly not once have I regretted the move. The excellence of the optics - much sharper than the SCT - together with a 10 minute cooling down time, ability to cut through average seeing and portability all contribute to the frac’s appeal. It easily goes up to 250x without loss of sharpness on good nights, and I feel I don’t need any more. Just my $0.02.

I do like refractors also. Which one did you get?

[Tiny Clanger]

1 hour ago, vlaiv said:

I also thought of very interesting experiment for anyone wanting to compare planetary performance of two different telescopes - one that excludes impact of atmosphere.

You need ~60 meters of clear space, and one of these:

Yep, it is a marble - about 10mm in diameter. Get nice colorful one with a lot of detail - to represent Planet X that you'll be observing. Observe during the day at distance of about 60m. That distance will be enough for spherical aberration not to cause any issues on most scopes up to 8" and also at that distance 1cm marble will have angular size close to Jupiter's angular size - about 45".

In fact, I just saw that you can get one of these:

That might not be a bad way to get regular planetary observing fix in times of prolonged poor weather

In any case - that will be good indicator of what particular instrument is capable of - without influence of atmosphere.

Possibly stupid question ... is there any reason to use a 3d  sphere, or would a flat, printed picture do ?

Thing is, I have this image (which I used to use in school ) in mind ... , print out, cut out, laminate ( for rain proofing) then a quick foray under cover of darkness with a step ladder, stapler  and duct tape to stick them high on some fences and lamp posts I can see from my upstairs window  ...

The cloud situation looks really bad for the coming week round here, I may be that desperate for a viewing fix   

Heather

 

 

[johninderby]

24 minutes ago, JeremyS said:

But do they make fluorite marbles?
 

Fuuny you should mention that. 😁😁😁

 

Edited January 30, 2021 by johninderby

[JeremyS]

4 minutes ago, johninderby said:

Fuuny you should mention that. 😁😁😁

 

Excellent: looks like you can get either Tak blue or Tak green 👍🏻

[vlaiv]

13 minutes ago, Tiny Clanger said:

Possibly stupid question ... is there any reason to use a 3d  sphere, or would a flat, printed picture do ?

Thing is, I have this image (which I used to use in school ) in mind ... , print out, cut out, laminate ( for rain proofing) then a quick foray under cover of darkness with a step ladder, stapler  and duct tape to stick them high on some fences and lamp posts I can see from my upstairs window  ...

The cloud situation looks really bad for the coming week round here, I may be that desperate for a viewing fix   

Heather

 

 

No special reason to use 3d object - except for "authenticity" - well, there are a few drawbacks with printer version that you might be able to work around:

- paper version will usually be printed on white background - not sure how that will affect contrast, with actual marble - you can get darker background easily (of course - you can print black background - so that can be fixed)

- mind the scale to get proper size.

Enough distance needs to be between telescope and target for couple of reasons - first is focusing, telescopes are poor at close focusing, second is spherical aberration - you really need at least 60-70meters for apertures up to 8" to avoid introducing excessive spherical aberration because target is close.

However, even at 60m - Jupiter needs to be about 1cm in size in order to be approximately the same angular size as actual planet. Just make sure you have detailed image scaled to that size and higher printing resolution (well - most print at at least 300dpi and I guess that is more than enough).

Alternative is to display such image on smart phone as well - just use phone with high DPI count like Full HD or higher res devices that are 4-5" diagonal. That way, you can do it in night time as well.

 

[RobertI]

1 hour ago, alex_stars said:

I am used to exit pupils down to about 0.6 mm from my Mak, everything below and eye floaters take so much over that I can not observe anymore without loosing significant detail.

Another important point is that binoviewers reduce the impact of floaters significantly for most people. I have certainly found this to be the case with my BVs. 

[Stu]

3 hours ago, vlaiv said:

I will be honest and express my doubt with claims of extraordinary sharpness of 4" instruments and ability to use over x300 magnification and such.

You can certainly use x300 magnification but what would be the point in doing so - when you can see all that detail with x100 magnification already. You'll just get blurrier larger image.

There is no doubt that larger aperture gives better resolution given good enough skies, that’s just the laws of physics I agree.

I don’t however agree with you that there is no point using very high mag on a 4” refractor if conditions permit. With excellent seeing, my Vixen FL102S will take x300, and I was able to use this mag on Mars once or twice at opposition last year. I get that there is no more detail visible, but the additional image scale can help make it easier to see. The proviso is obviously the conditions. Most times there is little point going that high, often x150 is more than enough and any more gives a blurry image as you say. It is often a balance depending on the target; too much can wash out some of the more subtle, low contrast features. Under my normally very average seeing, I often prefer the more stable images I get through a 4” refractor than the more variable view through a larger instrument; you see more detail in the larger scope but have to wait longer for it.

I find my 8” f8 gives excellent images up to x360, again given excellent conditions and I prefer it to previous SCTs I’ve had due to the quicker cooling and generally more stable images. The lack of dewing up helps too.

Coming back to the original question though, a top notch 130 or 140mm refractor is certainly on my wish list at some point.... when I can afford one!

[Tiny Clanger]

2 hours ago, johninderby said:

Fuuny you should mention that. 😁😁😁

 

Last thing my mind needs is a vaccum cleaner ...

[mikeDnight]

An experience I had some time back may be of interest. I'd been loaned a lovely 8" SW Dob from paulastro to use alongside my then SW 120ED Equinox. I'd set both scopes up alongside eachother around 3pm on a spring afternoon so as to let each acclimatise. Around 6pm, another friend paid me a visit, and we just sat chatting for about three hours. As my friend got up to leave, I suggested we both have a look at the Moon and Saturn, both of which were high in the sky. I aimed the 8" Dob at the Moon and was in awe at the superb view of a detailed lunar landscape that was as steady as can be imagined. It was a terrific view, and in my mind I thought the 120ED had no chance of coming close to replicating such a view. When I aimed the ED at the Moon, I was again stunned, not by greater detail, but by the sharpness of the view. The superior sharpness of the 120 ED more than compensated for the greater resolution of the Dob. Neither of us could believe the difference would be so obvious or that the scales would tip so greatly in favour of the refractor.   Next it was the turn of Saturn, and again the 8" Dob gave a view that was nothing short of spectacular. Again I thought to myself that the ED wouldn't stand a chance of matching, let alone beating the view through the 8".  And again I was utterly stunned by the 120ED's planetary punch.  In the 8" Saturn was rock steady, showing all the major ring components with ease, so that the A ring, Cassini division, B ring and Crepe ring were all immediately obvious. The globe too showed the EQ and temperate belt along with dark polar hood easily.  The 120ED again delivered a significantly sharper view, which became most obvious when comparing the ring detail. The refractor hinted quite forcefully at fine grooves in the B ring that were not visible in the 8" Dob. The A ring easily showed the Enke minima and even hinted at a razor thin dark Enke gap close to the outer edge of the A ring. Cassini's division also showed a soft outer edge that almost bled into the A ring. Radial spokes were seen in the refractor but not in the Dob, which is another indication of the better definition of the refractor. The 8" Dob was a superb scope, but the 120ED had the edge on definition. 

Edited January 30, 2021 by mikeDnight

[HollyHound]

2 hours ago, RobertI said:

Another important point is that binoviewers reduce the impact of floaters significantly for most people. I have certainly found this to be the case with my BVs. 

I'm finding exactly the same now that I routinely use two Hyperion Zoom eyepieces on my Mak 127 lunar/planetary setup... much more detail and the floaters all but disappear at high magnifications 👍