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Getting the best planetary views from a 120mm Sky-Watcher refractor

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#1 130st

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Posted 23 September 2019 - 03:02 PM

Hi there,

 

I recently purchased a preowned Sky-Watcher StarTravel120 OTA (120mm chromatic refractor, f/5) and trying to figure out what is the best configuration for planetary viewing.

 

I know this scope is not the best tool for planet viewing but still enjoy taking it out to my back yard once in a while...

This week I had some clear skies and managed to view both Saturn and Jupiter.  Since this is a short FL scope, using a 6mm EP (Zhumell Z series) I can only get a magnification of x100 and the visible object is very small undecided.gif

I don't have any decent EP below 6mm but tried some cheap Sky-Watcher 3.6mm MA EP that I've got with the scope but the visible image of Jupiter/Saturn is still very small.  I also tried using a 6mm Or. with a Barlow but the image was quite bad...

 

I already ordered a new dielectric diagonal (a SvBony clone form AliExpress) and now I consider buying a higher power EP.

 

What about a 2.5mm TMB?  How will it work with my scope?  Will it produce a "big enough" image of the planets?

 

Any suggestion about how to get better images from this scope?

 

Thanks

 

 



#2 vtornado

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Posted 23 September 2019 - 03:20 PM

For me, I think planets appear best at a 1mm exit pupil.  This is a balance between brightness and image size.

To calculate exit pupil, just divide the eyepiece focal length by the f number

For your scope f5 that is a 5mm eyepiece.

 

I just purchased this same scope from another member here an I really like it.  I was testing it on the gas giants just he other night.   It did OK with a baader fringe killer.  And I mean OK. 

 

For consumer grade optics I don't like to push them more the around 30x per inch of aperture.   For this scope is about

140x, even when CA is controlled. 

 

Some other things to consider ...

Don't view on a parking lot, driveway or patio, these soak up heat and release it over night creating thermals.

Trust me I just did this 3 nights ago.  I set up on black top, and had crummy views.  By moving the scope 30 feet

to grass my views were much better.

 

View the planets when they are at the meridian.  That is highest point in the sky.  Jupiter is at meridian right after sunset, and  it is still low.  Saturn a bit later.

 

The f/5 big achromat will have detrimental CA on Jupiter.  consider getting a yellow #8 filter, or if you have deep pockets a baader fringe killer.  These will remove unfocused light from the eyepiece.

 

For Jupiter I think you are going to get a better view out of your reflector, due to no CA.


Edited by vtornado, 23 September 2019 - 04:31 PM.

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#3 Barlowbill

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Posted 23 September 2019 - 03:31 PM

Saturn is somewhere between 746 million and 1 billion miles away.  Jupiter is somewhere between 365 million and 601 million miles away.  That is primarily why they are tiny.  Then there is your scope.  It is fairly small.  Don't expect them to get bigger through the eyepiece.  Ain't gonna happen.  Enjoy what you see.


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#4 clusterbuster

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Posted 23 September 2019 - 03:33 PM

You could make an aperture mask with a hole 100mm wide, i think that the f/5 is a one trick pony, and that is wide field viewing (which it excels at). The 100mm aperture mask would make it an f/6. not a big improvement, but it should be an improvement !

 Mark


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#5 MalVeauX

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Posted 23 September 2019 - 03:56 PM

Heya,

 

I would just leave it at 100x tops in that scope. I have that scope too. Definitely the opposite of what would be suggested for planetary viewing. It's a DSO scope meant for low power. But to view planets, it will be swamped in CA and the more magnification you push, the worse it will get. 100x will at least show discernible detail on the gas giants to differentiate them which is nice.

 

Instead of spending a bunch of money on eyepieces just to try and push this scope into places it wasn't designed for, you're better off just getting a small mirror that will handle planets without false color.

 

https://agenaastro.c...lector-ota.html

 

I would get that instead of a new eyepiece. If your mount can handle your 120 F5, it will handle that 6" F6.

 

Very best,


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#6 sg6

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Posted 23 September 2019 - 04:04 PM

The planets are not in good positions, just lowish and a long way away.

120 f/5 is 600mm with an achro that fast you are not going to get results above 100x, 6mm eyepiece. And an 8mm will give better but smaller views.

 

Concerning the 1mm exit pupil the easy way is that an eyepiece equal to the f number will deliver a 1mm exit pupil and a magnification equal to the aperture diameter.

 

Have a 102/600 and looking at Saturn with an 8mm was reasonable and did get a view with a 6mm - no real improvement however.

 

Really go get a 6mm and an 8mm and wait until July 2020. That is when Jupiter and Saturn are close again.


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#7 Sketcher

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Posted 23 September 2019 - 04:15 PM

A zoom eyepiece combined with your Barlow would allow you to fine tune your planetary magnifications.  If your magnification ends up being too low, you'll be able to gradually zoom in.  If it's too high, you'll be able to zoom out.  Soon you'll find that "just right" magnification for the telescope, the planet, and your sky conditions.

 

As for the planet looking too small, that's largely in the eye of the beholder.  You need to look for the fine detail that's hiding within that small image.  One of the best solutions is to make a large sketch of that tiny planetary image.  Take your time, and keep going back and forth between the eyepiece and the sketch as you attempt to see more and more detail.  It takes time to train the eye-brain system.

 

Here are a couple of sketches using a smaller, f/5 achromat.  The magnification was 105x, but the original sketches were made large enough to allow room to record the observed details.

 

ST 80 Jupiter   Sketcher

 

ST 80 Saturn   Sketcher
 
I even took notes on and sketched the observed chromatic effects.  They help make for a more "colorful" and "artistic" image. smile.gif
 
Jupiter and Saturn are rapidly growing more difficult to observe as they drop lower in the northern hemisphere observer's sky while their angular separations from the sun decrease.  These aren't the best of times for observing Jupiter and Saturn!

 

A short achromat isn't the best telescope for planetary observing, but that doesn't prevent a determined user from using one to observe the planets.  Too many people around here think you have to have and use the "best" telescope for this, that, and the other thing.  The reality is that any telescope can be used to observe any type of object.


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#8 SeattleScott

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Posted 23 September 2019 - 04:26 PM

100x may be as good as it gets. You can go higher but CA will be the limiting factor. Not sure if big and fuzzy is better than small and clear.

Scott
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#9 SeattleScott

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Posted 23 September 2019 - 04:58 PM

The planets are not in good positions, just lowish and a long way away.
120 f/5 is 600mm with an achro that fast you are not going to get results above 100x, 6mm eyepiece. And an 8mm will give better but smaller views.

Concerning the 1mm exit pupil the easy way is that an eyepiece equal to the f number will deliver a 1mm exit pupil and a magnification equal to the aperture diameter.

Have a 102/600 and looking at Saturn with an 8mm was reasonable and did get a view with a 6mm - no real improvement however.

Really go get a 6mm and an 8mm and wait until July 2020. That is when Jupiter and Saturn are close again.

Low is an issue right now. Close doesn’t really matter so much. Close is a huge deal for Mars. Not nearly as important for Jupiter and Saturn. They are so far away that their closest approach isn’t that much closer than their farthest point. It is a little like when the media makes a big deal out of a Super Moon. Yep, it’s about 10% bigger than when it is at its furthest point. Hooray.

But yeah I agree 80-100x is probably as good as it gets for F5 achro.

Scott
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#10 bobhen

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Posted 23 September 2019 - 06:21 PM

Consider a Baader fringe killer filter to reduce chromatic aberration

The Baader semi apo and Baader 495 Pass filter are also considerations

 

Wait for the planets and the moon to get higher. There is less turbulence the higher they are.

 

Make sure your telescope is acclimated to the outside temperature before observing

 

Check the Clear Sky Chart in your area for seeing conditions. HERE is a link to their homepage

 

Use the highest power you can while getting the steadiest image on that particular night. If the image is getting soft, back the power down. Some nights will be better than others.

 

Make sure your mount is as steady as you can make it and that you have a comfortable chair.

 

Bob


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#11 Joku_

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Posted 24 September 2019 - 06:15 AM

You could make an aperture mask with a hole 100mm wide, i think that the f/5 is a one trick pony, and that is wide field viewing (which it excels at). The 100mm aperture mask would make it an f/6. not a big improvement, but it should be an improvement !
Mark

Maybe even mask it further, to 80mm perhaps, which would be f/7.5? That should be decent regarding chromatic aberration, so maybe you could up the magnification to 150x -ish?

Edited by Joku_, 24 September 2019 - 06:15 AM.

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#12 SeattleScott

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Posted 24 September 2019 - 12:05 PM

Masking is a good idea. I would go 80mm if you are going to mask it. I made a 4” mask for my 6” F8.

Scott
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#13 MalVeauX

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Posted 24 September 2019 - 12:44 PM

Heya,

 

The 120mm F5 has a natural 52mm dust cap to stop the scope down to F11.5 which is APO pretty much (CA ratio of greater than 5.4, making it APO by Conrady standard even).

 

However, you can mask it with a custom mask to 70mm to achieve a decent aperture still, and get F8.57, which has a CA ratio of 3.11 which is minimal to no CA, so essentially APO level, by the Sidgwick standard. And there are lots of little 70mm APO's out there that cost $450+ that are common grab & go. So this literally does that, if you mask it to 70, with one scope. Yet can open up to 120mm for wide field with minimal CA at low power. Great versatility.

 

Very best,


Edited by MalVeauX, 24 September 2019 - 12:46 PM.

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#14 130st

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Posted 24 September 2019 - 01:28 PM

Thank you all for the wise tips waytogo.gif 

 

Next days I will try masking, yellow filter and won't go below 5mm...

 

Also adding to my wish list is a slower/longer refractor, I wonder what is considered slow enough to eliminate the annoying CA? f/7 and above?



#15 bobhen

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Posted 24 September 2019 - 02:12 PM

Thank you all for the wise tips waytogo.gif

 

Next days I will try masking, yellow filter and won't go below 5mm...

 

Also adding to my wish list is a slower/longer refractor, I wonder what is considered slow enough to eliminate the annoying CA? f/7 and above?

With a 4”…

F10 is decent
F12 is very good
F15 is best

But even at F 15 color error will still be there when compared to a true apochromatic refractor, however it is well suppressed.

 

I owned a 120 F5 and found that I liked filters better than stopping the scope down. Stopping down far enough to reduce the color error impacted the scope’s resolution too much. With a filter you get the full aperture and the scope’s potential resolution and brightness but you will have to deal with a monochromatic yellow tint to the image. It’s the price you pay with a short FL achromat.

 

These days you can get a 4” F9 ED refractor from Sky Watcher that shows very little if any color error and for a reasonable price.

 

Bob


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#16 MalVeauX

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Posted 24 September 2019 - 03:11 PM

 

 

Also adding to my wish list is a slower/longer refractor, I wonder what is considered slow enough to eliminate the annoying CA? f/7 and above?

Really, if you want color-free, it's better to just get a FPL53 based ED doublet. You can get one of those in the F7~F9 range and it will be color-free visually. For this, look to the Skywatcher PRO 120mm ED doublet.

 

To do the same with an achromatic doublet, would require a really really long focal-ratio which requires a significant mount to handle (and I mean long!).

 

From an achromatic doublet point of view, take a look at this chart (and again this is for achromatic doublets, not ED based doublets):

 

ChromaticAbberation_Chart.jpg

 

Anything with a ratio close to 3 or greater has minimal to no CA. You'll notice how very, very long these focal-ratios have to be as you go up in aperture (just look at 120 to 150 for example and the focal lengths have got to be very long which is not easy to use nor mount at all).

 

Very best,


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#17 havasman

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Posted 24 September 2019 - 04:46 PM

A great widefield observing tool like your scope can give good planetary views. But it's never going to fill the field with a sharp planetary image. When one buys a scope the choice of scope brings advantages but never in every style of observing. When a short focal ratio refractor is the choice, the owner has chosen to develop the observing skills required to pick fine details from a planetary image that is presented at lower magnification.

 

An alternative is using the scope in a manner that takes advantage of its strengths. Objects like M24, M31/32/101 and NGC7000 (with O-III filter) will present in your scope in a way that will make you very proud to have it.

 

Nobody buys a John Deere 5090GV to run flat track dirt racing but people constantly buy fine scopes and then focus on using them in ways that expose their weaknesses instead of their strengths.  shrug.gif


Edited by havasman, 24 September 2019 - 04:50 PM.

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#18 HellsKitchen

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Posted 25 September 2019 - 01:48 AM

Fact is, the 120mm F5 refractor is not an appropriate scope for planets. The CA serves to blur the image as magnification increases. These are rich field scopes best suited for low magnification wide field deep sky viewing, for which they are very good. There is a 130mm newtonian in your sig, use that for planetary. Because newts are CA-free, you will get a cleaner, sharper image and you will be able to use more magnification. You also wont be sacrificing resolution like stopping the refractor down will.


Edited by HellsKitchen, 25 September 2019 - 01:52 AM.

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#19 Joku_

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Posted 25 September 2019 - 02:39 AM

Really, if you want color-free, it's better to just get a FPL53 based ED doublet. You can get one of those in the F7~F9 range and it will be color-free visually. For this, look to the Skywatcher PRO 120mm ED doublet.

To do the same with an achromatic doublet, would require a really really long focal-ratio which requires a significant mount to handle (and I mean long!).


However, that Sky-Watcher 120mm ED is rather expensive compared to the Startravel. Not everybody is willing to invest that much money into their hobby. Also, the ED is less portable if you want to use it as a travel scope.


As for how to get better views from the Startravel, masking is probably the best idea here. According to the chart you posted, when masked to 80mm aperture, it would have chromatic ratio of around 2.5, which should be decent. If that's not good enough, *then* it's time to try a different scope.
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#20 aa6ww

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Posted 25 September 2019 - 03:38 AM

This is the worse year ever for planetary observing. Its not just your scope thats giving you mediocure views. its because the seeing conditions have been horrible most of the year, and at this time, the seasons are changing and Jupiter is past its prime for this season. One way around better planetary views is to get up high in elevation. It helps clean up the air making  your chances of better seeing more favorable.

I have a 6" APO and its been a horrible season for planetary observing. There have been a few good days but I havent seen one 5/5 seeing conditions all season. 

Id accept your scope for what it is and just use what you have and experiment. Remember also to give your scope at least a half hr to acclimate to the outside conditions before you start looking at the planets, so enjoy deep space also.

You really have an excellent scope for so much more then looking at boring Jupiter and Saturn this season. Even in the south where Jupiter and Saturn are right now,  there are so many messier objects your scope will just pull out just fine, that you would wonder why you even bother with the planets.

 

...Ralph


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#21 Jon Isaacs

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Posted 25 September 2019 - 04:05 AM

My two cents:

 

I see that ST130 has a 130mm F/5 which I figure is a Newtonian.  When properly collimated and cooled, this has the potential to be a better planetary scope than the 120mm F/5 achromat.  

 

Otherwise, a 120mm F/5 achromat is not designed for viewing the planets, it's designed for low power widefield viewing under dark skies.

 

Jon


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#22 130st

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Posted 01 October 2019 - 10:33 AM

Thanks again to you all

 

Yes, I also have the 130ST Newtonian however both OTAs share the same tripod (got a broken leg in one of the sets) and therefore periodically switch between them. 

 

I also consider a better higher mag EP to max out the views of Saturn and Jupiter , I was wondering how good the ES52 series 3mm will perform in my f/5 refractor?

 

I read some good reviews about the 3mm including reports of QC issues however it's very tempting given its current low price...

 

WDYT?



#23 howardcano

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Posted 01 October 2019 - 12:32 PM

I think the 3mm eyepiece will be WAY too much for a 120mm f/5 refractor.  Even 6mm was too much for me.  Try your 6mm with your Barlow first.

 

I found that best "planetary" performance was with an aperture mask of about 90mm.  For lunar, full aperture with a dark green filter was pretty sharp.

 

But it really isn't great at any of this.  Wide fields are where it shines!


Edited by howardcano, 01 October 2019 - 12:35 PM.


#24 130st

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Posted 01 October 2019 - 12:52 PM

I think the 3mm eyepiece will be WAY too much for a 120mm f/5 refractor.  Even 6mm was too much for me.  Try your 6mm with your Barlow first.

 

I found that best "planetary" performance was with an aperture mask of about 90mm.  For lunar, full aperture with a dark green filter was pretty sharp.

 

But it really isn't great at any of this.  Wide fields are where it shines!

I get quite good views with my Zhumell Z 6mm, the EP is very comfortable but the image is quite small.  I tried it with my Celestron Omni 2x Barlow but it was quite bad...

 

After reading some more reviews, I'm quite concerned from several reports of faulty ES52 3mm examples undecided.gif  I hope the 4.5mm production line is better, I really want a higher magnification, I believe it's possible when seeing conditions are adequate.


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#25 SeattleScott

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Posted 01 October 2019 - 01:32 PM

Paradigm 3.2?
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