Refractor or dob for viewing the bright planets

[Mike_S]

Sorry been discussing this in another post but thought best to do a new one for it. I have an F4.7  254mm diamater dob. Trying to improve my views of Saturn and Jupiter at the moment. Wondering if perhaps I would get a better view from a refractor and if so, is an F10 90mm Achromatic worth a go such as the SkyWatcher Evostar 90 or only if I spend more on a 100mm F9 Apochromatic like the SkyWatcher Evostar 100 ED DS Pro? I don't really want to spend money on an expensive EQ mount when I'm just doing visual and a bit of smartphone snapping. 

[Stu]

Whereabouts are you? UK based and suffering low planets?

If so then I would probably go for a 100ed f9 myself, or next best would be a 102mm f11. You don't need an EQ mount necessarily, a simple AltAz will do although you could pick up an EQ5 or Vixen GP fairly cheaply with a tracking motor to avoid the nudging. I find that helps  my observing quite alot.

[Sunshine]

Will a 100mm Frac trump  a 250mm dob in anything except imaging?

[Mike_S]

Yes U.K. skies suffering from low planets. 

What is a decent AZ mount for 100mm Apo’s? If I go this route it will have to be used especially as I’ll sell it if no improvement.

with an ordinary Achro I could just keep it as a travel scope as they’re only cheap and might tell me if my problems are relating to my other scope if I can see more detail or colour with it. 

[vlaiv]

Can I point to the obvious?

You have 10" dob - and one important aspect for planetary viewing is aperture - it provides resolution.

Smaller aperture can be less affected by poor seeing, so yes under some circumstances 90mm aperture will look more pleasing than 10" one, especially if you try to push magnification on dob over the limit of the seeing. Slower scopes have another advantage in poor seeing - they have wider critical focus zone, so any disturbance of the wavefront is less likely to produce defocus blur.

Having said all of that - why don't you just do an aperture mask for that dob - you can easily have 90mm F/13.3 scope that way that has zero chromatic aberration.

That is by far the cheapest option - you can make it out of some cardboard. Here is an example:

What exactly seems to be a problem with view thru that dob? Maybe rechecking collimation would be a good thing to do. What magnifications are you using?

[Mike_S]

Thanks I never heard of aperture mask. I don’t know a lot and thought more was better. Worth a go. I think my magnifications may have been too high so I’m going to try inbetween. Tried X 129, 240 and 342. I want to get more eye pieces to try 170-200. Problem is it’s just basically a bright ball of light for Jupiter. Saturn is okay, a bit bright still perhaps. Have a neodymium filter but will try a moon filter next time too.

[Mike_S]

I was thinking the slower F ratio frac would not suffer from chromatic aberration. I didn’t know that slower can also improve the view through reflectors to ED quality.

[vlaiv]

1 minute ago, MSammon said:

Thanks I never heard of aperture mask. I don’t know a lot and thought more was better. Worth a go. I think my magnifications may have been too high so I’m going to try inbetween. Tried X 129, 240 and 342. I want to get more eye pieces to try 170-200. Problem is it’s just basically a bright ball of light for Jupiter. Saturn is okay, a bit bright still perhaps. Have a neodymium filter but will try a moon filter next time too.

In principle bigger aperture is always better (all other being equal), but there are circumstances when smaller aperture provides more pleasing image. This happens in poor seeing when atmosphere is turbulent. Larger aperture is gathering light from larger part of atmosphere (larger cone) and it will be impacted by seeing more than smaller aperture (you can effectively imagine both scopes gathering light thru column the size of aperture - it is in fact cone but diverging so slowly and atmosphere is so thin in comparison to distance you are observing that it is effectively a column) - and already mentioned depth of focus / critical focus zone.

Don't attempt to go over x200 if seeing is not excellent. In poor seeing you will often be limited to about x100.

[vlaiv]

2 minutes ago, MSammon said:

I was thinking the slower F ratio frac would not suffer from chromatic aberration. I didn’t know that slower can also improve the view through reflectors to ED quality.

Ok, here is comparison that I did maybe a week ago. I used 8" F/6 dob and 4" F/10 achromatic refractor to observe Jupiter. Seeing was poor - at x200 or so it was obvious that image was heavily impacted by seeing.

Without filtering F/10 achromat shows purple halo around planet. Baader Contrast Booster filter removes that halo and image is almost color free - although with a bit warmer tone (due to lacking blue part of spectrum).

Image in frac was darker due to smaller aperture and there simply was no more detail than in dob. Image did look more stable than in dob, but image in dob showed more fidelity - although that fidelity was not in image of the planet - but rather seeing - I could see rippling and blurring effects of seeing more sharply with larger aperture. There was a bit more detail on the planet that was visible for split second occasionally when seeing was not as bad.

Both views pale in comparison to what the planet looks like in same 8" dob when the seeing is good and you can push the mag to x300.

From above you can see that even slow achromatic refractor is going to have some color on bright objects, but depending on how slow scope is - it can either be filtered out (in slow scopes) or really terrible (in fast achromats). Dob will give more than ED quality in terms of color correction - it is perfectly color corrected scope - there is no chromatic aberration present.

What you can do with aperture mask is: make view less bright if too much light bothers you (there are couple more tricks that you can use to help with that - higher magnification spreads the light and you get dimmer view because of that, so you can control brightness with magnification - not always an option if seeing is poor. Another trick to use is not to observe in full darkness - full darkness helps your eye get dark adapted and that hurts both detail seen and also makes you more sensitive to light - and planet looks too bright. When observing planets - keep some sort of "ambient" light on to prevent your eyes from dark adaptation), it lessens impact of poor seeing, and it makes focusing easier (this thing with focus in turn further helps with poor seeing).

[jetstream]

If your interested, from Greer

"Conclusion

Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio. The error in the hypothesis is that it was assumed that the same atmospheric distortion will cause the same shift in the best focus position in the two telescopes, and this is not true. While the high f-number telescope does enjoy a greater depth of focus, unfortunately the shift in best focus caused by turbulence is also greater. In fact, the two are locked together; the instrument with four times greater depth of focus also has a four times greater linear shift of the best focus position."

https://www.fpi-protostar.com/bgreer/seeing.htm

[Mike_S]

Are reflectors more affected by atmosphere turbulance than refractors? Is there something about a lens that has a viewing quality advantage over a mirror?

[vlaiv]

16 minutes ago, jetstream said:

If your interested, from Greer

"Conclusion

Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio. The error in the hypothesis is that it was assumed that the same atmospheric distortion will cause the same shift in the best focus position in the two telescopes, and this is not true. While the high f-number telescope does enjoy a greater depth of focus, unfortunately the shift in best focus caused by turbulence is also greater. In fact, the two are locked together; the instrument with four times greater depth of focus also has a four times greater linear shift of the best focus position."

https://www.fpi-protostar.com/bgreer/seeing.htm

That is good to know! I'll follow up reference and have thorough read on the subject.

[vlaiv]

8 minutes ago, MSammon said:

Are reflectors more affected by atmosphere turbulance than refractors? Is there something about a lens that has a viewing quality advantage over a mirror?

There is one thing that gives refractors advantage over reflectors in terms of performance (not related to seeing) - that is clear aperture. Reflectors have central obstruction and this central obstruction has an effect on resolved detail and contrast. For most instruments this difference is more theoretical than practical, but if central obstruction is large enough you will maybe notice two effects.

Medium sized details will have more contrast in refractor, and very finest details will be more resolved in reflector.

This diagram sums it nicely:

You can understand X axis as detail - close to zero - large scale detail, close to 1 very finest detail scope is capable of resolving. Y axis is contrast, but you need to understand that contrast is related to resolving detail as well as what we usually understand as contrast - if you have a feature that lacks contrast in comparison to background - it will be harder to spot.

Green is refractor "line" - unobstructed aperture, while yellow and orange represent same aperture with 25% and 50% central obstruction. You can see that central obstruction "removes" some of contrast in medium size details, but it actually "boosts" contrast of finest details.

Your dob has 25% obstruction. However, above graph represents relation of same aperture with / without central obstruction, so it is meaningful in comparing your dob with 10" refractor . When comparing 4" refractor and 10" reflector things are quite a bit different as green line is not going diagonally from 0,1 down to 1,0 but rather from 0,1 to about 0.4 on X axis. This means that even 50% CO on 10" scope will out resolve 4" clear aperture in all frequencies (will have better contrast across all feature sizes).

[Mike_S]

So what is the advantage in a 4 inch frac even an apo over a 10 inch dob? Is it just because they are easier to use on an EQ mount for imaging?

[vlaiv]

2 minutes ago, MSammon said:

So what is the advantage in a 4 inch frac even an apo over a 10 inch dob? Is it just because they are easier to use on an EQ mount for imaging?

Depends on scopes.

Above discussion is "all other being equal" type of discussion. In real life things are almost never equal.

Good 4" APO is very versatile instrument - it will show you planets up to resolution that is usable on most nights (up to x200 magnification). It will have very distinct rendering of targets due in part to clear aperture and in part that smaller aperture is less affected by seeing and finally due to overall contrast. APO scope will be polished and corrected to a high degree. Regular mass produced dob is not going to be either of those.

There are mirror makers that can make equally amazing quality mirrors that will give "refractor like" image, but that comes at a price. Mass produced items will be of "satisfactory" quality of figure (meaning you'll be hard pressed to tell the difference in most cases) and will have somewhat lacking polish - this creates light scatter and sky looks brighter than it would in refractor scope. This means contrast for DSOs (not as important for planets).

This is why many people feel that refractor gives better view of star clusters (especially open star clusters where you don't need as much light gathering and resolving as with globulars).

You will be able to use 4" APO for wide field as well because it is usually shorter focal length (like 600-700mm for 4"). It is easily transportable and easy to mount. It can serve as imaging scope.

In the end - it is virtually maintenance free.

Dob will show you plenty - more light gathering power, more resolution (in terms of aperture, when seeing allows), very easy to use (dob mount), but it's got bulk so not very transportable, it is large for storage, needs cool down time (mirror needs to get to same temperature as surrounding air - and this can be tricky for large mirrors if temperature is rapidly changing - like getting colder thru the night) and you need to do maintenance of it - it is open design so dust will collect over time (but you should not clean mirrors that often - once every few years), it will need collimation from time to time, and performance greatly depends on collimation.

There you have it - two different scopes for different needs/preferences and usage scenarios.

[jetstream]

More interesting stuff- this graph shows the effect of 1/4 wave spherical aberration (unobstructed) vs perfect optics with 32.5% central obstruction. It tells us that obstructed telescopes need good optics and that less than perfect unobstructed optics give no where near the performance expected. Note the similarities in where the sag occurs in the graph and corresponding frequencies.

All these things will "stack up" reducing performance available. Personally I like obstructions less than 25% and good optics from either obstructed or unobstructed scopes.

 

Edited June 25, 2019 by jetstream

[jetstream]

2 hours ago, MSammon said:

Problem is it’s just basically a bright ball of light for Jupiter. Saturn is okay, a bit bright still perhaps.

Just curious- what do you collimate your scope with?

[SIDO]

4 hours ago, Sunshine said:

Will a 100mm Frac trump  a 250mm dob in anything except imaging?

Yes indeed it will...

While one guy waits for his 250mm dob to equalize with the surrounding air temperature the other is observing with his refractor, this is true providing everything is equal mount wise and both individuals are utilizing none goto altitude azimuth mounts as goto and equatorial mounts require additional setup times.

A refractor will also not likely require collomation as a newtonian mounted dobsonian most certainly will and newtonians above 150mm most definitely will.

There are a few more areas of trumping but the formentioned are hands down trumpers so I'll leave it right there 😉  

                        Freddie.

 

Edited June 25, 2019 by SIDO

[Mike_S]

Thanks for very thorough replies. A lot to take in but I do like the science and maths side. It’s still tough to work out to me if a 4 inch frac of any quality will outperform my 10 inch dob. Perhaps if they were closer in size? My collimation might not be the best. I just do it with a collimation cap followed by a star test. I plan on trying a Cheshire eyepiece but I can’t see anything wrong with the star test so haven’t bothered buying new collimating equipment. 

I’m definitively going to try magnification of around X 170-200 but it was the same with much lower (X 120). I’m going to try moon filters as well. 

I feel like buying a cheap refractor for £150 just to try and that would rule out problems with my mirror or collimation if it’s the same. 

Edited June 25, 2019 by MSammon

[Sunshine]

1 hour ago, SIDO said:

Yes indeed it will...

While one guy waits for his 250mm dob to equalize with the surrounding air temperature the other is observing with his refractor, this is true providing everything is equal mount wise and both individuals are utilizing none goto altitude azimuth mounts as goto and equatorial mounts require additional setup times.

A refractor will also not likely require collomation as a newtonian mounted dobsonian most certainly will and newtonians above 150mm most definitely will.

There are a few more areas of trumping but the formentioned are hands down trumpers so I'll leave it right there 😉  

                        Freddie.

 

Those details seem to be apart from the question by the OP, I think he’s talking from a performance at the eyepiece POV, resolving power, yes, one can certainly load a 4 inch frac into a car a lot quicker but, if i had to choose between a final lifetime scope, a 4 inch frac or 10in dob, not much thinking to do.

Edited June 25, 2019 by Sunshine

[SIDO]

1 hour ago, Sunshine said:

Those details seem to be apart from the question by the OP, I think he’s talking from a performance at the eyepiece POV, resolving power, yes, one can certainly load a 4 inch frac into a car a lot quicker but, if i had to choose between a final lifetime scope, a 4 inch frac or 10in dob, not much thinking to do.

I answered your question in response to the way you framed it, the generalization of your question allowed this to happen. And yes vehicle load times are another area where small fracs trump larger dobsonians, thanks for throwing that one in there.

The op already has the 250mm f4.7 dobsonian, of course a 250mm dobsonian will outclass a 90mm f10 achromat visually for planets and that has been fully represented in responses thus far and I was not disputing this. But the refractor is the fastest scope on the planet outside of f ratio and on nights were time is limited due to responsibilities or impending cloud the achro can get the job done and be stored away before the dob is acclimated and ready for collimation...

        I was going to leave it right there and even said so...

                        Freddie. 

 

 

 

Edited June 25, 2019 by SIDO

[Stu]

1 hour ago, MSammon said:

So what is the advantage in a 4 inch frac even an apo over a 10 inch dob? Is it just because they are easier to use on an EQ mount for imaging?

This is where some will say 'none at all', and others (myself included) will say the 'quality' of the view.

Until you have looked through a decent quality 4" apo refractor then you will not understand the differences. Nice tight star shapes, no diffraction spikes, contrast etc 

I would not expect a £150 achro to outperform a 10" dob, but with the planets down low, a high quality apo will quite possibly do it, that's why I (keep!) suggesting a 100ED! With a cheap achro you may just miss the point and not prove anything.

@John has recently posted that he is preferring the views through his 4" Tak to his 12" dob on the planets this year because they are so low. I have found similar with an 8" Mewlon vs my 4" Tak; the larger scopes normally win but struggle down at 16 degrees or less where Jupiter and Saturn are currently.

[SIDO]

2 hours ago, MSammon said:

Thanks for very thorough replies. A lot to take in but I do like the science and maths side. It’s still tough to work out to me if a 4 inch frac of any quality will outperform my 10 inch dob. Perhaps if they were closer in size? My collimation might not be the best. I just do it with a collimation cap followed by a star test. I plan on trying a Cheshire eyepiece but I can’t see anything wrong with the star test so haven’t bothered buying new collimating equipment. 

I’m definitively going to try magnification of around X 170-200 but it was the same with much lower (X 120). I’m going to try moon filters as well. 

I feel like buying a cheap refractor for £150 just to try and that would rule out problems with my mirror or collimation if it’s the same. 

Best of luck of course and do work on collimation by laser or cheshire, you should be getting some pretty pleasing views of Jupiter with a 250mm dob and there must be a significant issue preventing that.

                       Freddie.

[SIDO]

1 hour ago, Stu said:

This is where some will say 'none at all', and others (myself included) will say the 'quality' of the view.

Until you have looked through a decent quality 4" apo refractor then you will not understand the differences. Nice tight star shapes, no diffraction spikes, contrast etc 

I would not expect a £150 achro to outperform a 10" dob, but with the planets down low, a high quality apo will quite possibly do it, that's why I (keep!) suggesting a 100ED! With a cheap achro you may just miss the point and not prove anything.

@John has recently posted that he is preferring the views through his 4" Tak to his 12" dob on the planets this year because they are so low. I have found similar with an 8" Mewlon vs my 4" Tak; the larger scopes normally win but struggle down at 16 degrees or less where Jupiter and Saturn are currently.

One reason being is that a 4" frac will not be as bothered by atmospheric dispersion as atmospheric dispersion correctors for visual are only suggested for telescopes 200mm and up, it's for this reason I think even 90mm f10 achro's will show some benefits but also have false colour of their own meaning Ed and Apo scopes will do better than achromats in this area.

For achromats f11 is where false colour tends to show less distinction but not all f11 achromats are equal and a better optical figure will often show less false colour, too bad the f11 and up achromats are harder to come by these days mine is 30 years old now and I still can't part with it...Was the one scope I had that saved the 2018 Mars opposition when the others failed me.

                           Freddie.

Edited June 25, 2019 by SIDO

[Mike_S]

Thanks. I’m holding back on an Apo purely because of the cost but also the mount as I don’t have one. I’m going to take a few months experimenting and eventually may  try a different telescope. See what comes up second hand. I almost bought an F11 Celestron Astromaster before the dob but they’re discontinued now. Nearest appears to be the F10 SW Evostar. I’m glad I got the dob, it’s just a steep but fun learning curve. I was wanting to upgrade it to a 12 inch as I do like the dob. Want to get better with my current one first though.