Refractor or dob for viewing the bright planets

[Mike_S]

Thanks, yes the moon is incredibly sharp with it even with my highest power eyepieces. I might try get to an astronomy club this weekend to look through others.

[jetstream]

16 minutes ago, MSammon said:

Thanks, yes the moon is incredibly sharp with it even with my highest power eyepieces. I might try get to an astronomy club this weekend to look through others.

Its most likely just the position of the planet, and local thermals then...

[dweller25]

8 hours ago, MSammon said:

Is this from northern hemisphere and this time of the year? What magnification please?

x130 - x150 at the moment but you must use a neutral density filter.

Edited June 27, 2019 by dweller25

[cletrac1922]

On 25/06/2019 at 20:22, Alan White said:

As an owner of a good ED Vixen 103s and a 250mm Orion Optics UK Dobsonian, I think I should offer my view on this one as well.

My Dobsonian is my favoured winter tool for longer deep sky sessions and dark sites,
My 103s ED is my favoured winter and summer tool for all other observing.

The reason, poor seeing and LP at home, the 103s copes better with this.
As others have said no or minimal cool down, unlike the Dob.
The Vixen somehow at the magnifications I use at mainly 200x or far less, provides a better contrast image.

My answer is you need two scopes for differing things.

If I had to cull my scope further than I already have the Dob and my 8" Celestron would go,
the Vixen ED103s would stay or be replaced with a 100mm Tak.

 

I agree with Allan

I tend to use my 10" Flextube Dob as first weapon of choice

I am out a couple of times per month with my club doing presentations primary schools/scout groups

With the Dob, easy 5 minute set-up, when have 10+ 7-9yo's asking can they see

With my ED80, takes longer

Set up tripod, southern hemisphere, north leg faces south

Use compass to align tripod

Then have to wait until dark, to do 2 star alignment, using SynScan Controller

Never worry about Dob cooling, as even in winter, average temp does not go below 10 deg C at night

Recently had to do school presentation mid week

Rushed home from work, pack scopes and eyepiece case into car

When got there, realised had left  jump start battery pack at home, to power my EQ5 

Trusty DOB, got me out of trouble

With Dob, easy to jump from one object to another

Saturn, Jupiter, Jewel Box in CRUX, Orion Nebula

John   

Edited June 27, 2019 by cletrac1922

[John]

35 minutes ago, cletrac1922 said:

 

I agree with Allan....

 

I don't know John but you seem to be saying something different to Alan

Alan says that, if pushed, he would let his dob and SCT go and keep the 103mm refractor. You seem to be saying that your dob is always your 1st choice over your refractor. Unless I've misunderstood

Anyway, the OP already has a 10 inch dob.

[Mike_S]

It’s the mounts that are putting me off a refractor especially as I don’t really do astrophotgraphy. I do like the dob mount. I’m going to order another filter just to try. Perhaps a variable polarising one. I will get a 7mm eyepiece as well. Also a Cheshire collimator and maybe a new focuser as the rack and pinion thing is awful. Got lots to try to get the best our of this scope first. Thanks everyone for the replies. I will let you know if I get to look through a refractor or if improve my view with the dob. 

[SIDO]

7 hours ago, MSammon said:

It’s the mounts that are putting me off a refractor especially as I don’t really do astrophotgraphy. I do like the dob mount. I’m going to order another filter just to try. Perhaps a variable polarising one. I will get a 7mm eyepiece as well. Also a Cheshire collimator and maybe a new focuser as the rack and pinion thing is awful. Got lots to try to get the best our of this scope first. Thanks everyone for the replies. I will let you know if I get to look through a refractor or if improve my view with the dob. 

I hear you on the mount issue, I run an alt az mount with slow motion controls for my 90mm f11 refractor and to be honest upgrading to a 102mm f11 likely would be a bit shaky on my 20# capacity gso mount due to the overall length/weight of the scope and it's corresponding moment arm effect though at some point an Ed doublet of some shorter focal length but similar aperture will be upgraded eventually as I'm not getting any younger lol... My 90mm weighs in now at 8 pounds due to a new original style synta focuser and light weight dovetail & diagonal replacing the heavier items I had in place for some years due to the disability thing coming about, the scope only pushes about 120x in average seeing so I have the crosshairs in the 50mm right angle finder removed so that larger targets can be viewed without obstruction at widefield and due to the limited mag of the telescope crosshairs are of little need to an experienced observer. The scope has for years provided a quick and easy alternative to my larger scopes and has complimented them well.

For a filter idea I would suggest a neodymium coated moon & skyglow filter as it's just a great filter for Jupiter and Mars providing enhanced views of the grs and other details of those two worlds, if I could only keep one filter for those two planets it would be the Moon & Skyglow.

When observing the planets the meridian will mark their highest altitude and this marker can be enabled in Stellarium or similar planetarium software, when observing planets I stick to one hour before and after the target passes it but not all observer's have the unobstructed southern views I have and I get that.

When using my 200mm dobsonian for Jupiter, Saturn and Mars I do incorporate a Dispersion Corrector and do fully see an improved view using the device and have not peered through a larger scope without one installed while visually observing bright planets in some time and inturn have lost the ability or need for reliable comparison. Some swear by them others see no advantage I just use them and do not promote them as alot of variables are in play observing the bright planets and these devices can be very confusing and overly complicated for those just starting out.

Do try the Moon & Skyglow though if you haven't already, I think you will like it...

                          Freddie.

                     

Edited June 28, 2019 by SIDO

[Alan White]

My mount choice is a Giro type alt az for my refractors and SCT.
Easy quick set up, no power needed, no alignment needed, all nice and simple.

Manual Alt Az same as Alt Az on the Dob, all simple like me.

The only power needed is for the dew heaters at certain times of the year and if a long session at a dark site takes place.

Don't let the mounts scare you MSAmmon, they can be your friend.
My Alt Az sets up almost as fast for the Refractor than the Dob does.

At present I use a Altair Astro Sabre, but I did have a Skytee 2 and a Giro Ercole before this.
Favourite was the Ercole, but gear itchy feet got the better of me and it was sold on, miss that one a lot.
 

[Mike_S]

Never seen good quality AZ mounts before besides on dobs. Will check them out thanks.

[John]

I had my 130mm refractor on a Giro Ercole mount last night. Very simple to use - "dob like" really !

The views of Jupiter were poor though last night even with this scope which is one of the best planetary refractors in it's aperture class

Also tried my 12 inch dob on the same target - even worse really.

 

[Stu]

I agree! The Ercole is a great mount, very stable. Any wobble is likely to be down to the tripod so it is worth making sure that is decent, much like John's Berlebach. 

This is a biggish grab and go setup I used in the past, C925 and FC100 with a Gitzo tripod.   

[Mike_S]

This article has just blown my mind. Right I tried a 9mm eyepiece tonight (X 133, 1.9mm exit pupil) and I got my best view of the bands. It was just very small. I will try a 7mm eyepiece when I get one.

anyway so it seems that exit pupil is a lot more important than I realised. So if I had a 12 inch F5 dob instead I could see the same detail perhaps with a 1.8mm exit pupil but it would be at X 166. 

But people are still getting better views with a 100mm apochromatic refractor?

 https://medium.com/@phpdevster/how-telescope-aperture-affects-your-view-24507147d7fc

Edited June 29, 2019 by MSammon

[Mike_S]

Also this article.

https://medium.com/@phpdevster/help-i-cant-see-detail-on-the-planets-ac27ee82800

so it seems my main issue was actually atmospheric disturbance as it really looked like the example he shown with poor atmosphere. It was higher when I viewed it tonight so that explains things.

it would be interesting to try a refractor sometime but even more aperture with a larger dob would help instead. 

[jetstream]

1 hour ago, MSammon said:

This article has just blown my mind. Right I tried a 9mm eyepiece tonight (X 133, 1.9mm exit pupil) and I got my best view of the bands. It was just very small. I will try a 7mm eyepiece when I get one.

anyway so it seems that exit pupil is a lot more important than I realised. So if I had a 12 inch F5 dob instead I could see the same detail perhaps with a 1.8mm exit pupil but it would be at X 166. 

But people are still getting better views with a 100mm apochromatic refractor?

 https://medium.com/@phpdevster/how-telescope-aperture-affects-your-view-24507147d7fc

Your path will lead to the discovery of this: from Peach and thats why I started posting MTF graphs...

 

"115/Aperture (mm.) For example, a 254mm aperture telescope has a dawes limit of 0.45" arc seconds. The dawes limit is really of little use the Planetary observer, as it applies to stellar images. Planetary detail behaves quite differently, and the resolution that can be achieved is directly related to the contrast of the objects we are looking at. A great example that can be used from modern images is Saturn's very fine Encke division in ring A. The narrow gap has an actual width of just 325km - which converts to an apparent angular width at the ring ansae of just 0.05" arc seconds - well below the Dawes criterion of even at 50cm telescope. In `fact, the division can be recorded in a 20cm telescope under excellent seeing, exceeding the Dawes limit by a factor of 11 times!. How is this possible?."

Seriously read and understand this article . Your 10" is a super planetary scope.

http://www.damianpeach.com/simulation.htm

ps I can see the Enke division on Saturn when the planet is high and the seeing good, in my 10" dob.

Edited June 29, 2019 by jetstream

[John]

Stu and I had some good results on Jupiter last night - Stu was using a 210mm cassegrain and I was using a 100mm refractor. Both rather good ones though !

Here is Stus report:

https://stargazerslounge.com/topic/337329-jupiter-better-than-expected/

Here is mine:

https://stargazerslounge.com/topic/337349-another-jupiter-report-from-28-29-june/

I think the key last night was the seeing conditions which were pretty steady for a prolonged period. And we observed for quite long periods as well. The planet is still low of course and the atmosphere has a impact. If we had been observing Jupiter when it was much higher in the sky under these conditions the results would have been stunning I reckon

Stu is in the home counties and I am in the SW of the UK but our latitudes are pretty similar I think.

[Stu]

If we get another good night I might even put the Mewlon and Tak side by side on the Ercole for a direct comparison....     

[John]

I did have a short glimpse of Jupiter with the 12 inch dob last night and it was much better than the previous night. The dob did not show anything that the Tak 100 was not showing though. The contrast of the belts was better with the refractor.

 

[Stu]

Do you see much colour in the Tak John? To me it is visible in GRS quite clearly but the belts and zones are much more pronounced with more aperture. I've always thought my eyes are not that sensitive to colour but again experience at the eyepiece brings lots more out. In the Mewlon it is right there though.

[Geoff Barnes]

I do wonder if it is more to do with the eyepiece than the scope, with particular emphasis on the coatings applied.

The other night I was swapping various eyepieces on Jupiter and settled on the Baader Morpheus 6.5mm, which not only gave me the sharpest view but definitely gave the most intense colour in the bands and the GRS. The Baader Zoom was a close second and obviously utilizes the same Phantom coatings as the 6.5mm, the Skywatcher Planetary 4mm and Explore Scientific 4.7mm did not reveal nearly as much colour to my eyes.

I think Baader's Phantom coatings are really quite special for colour rendition.

[Mike_S]

So I have learned quite a bit here. When it comes to the bright planets, it seems that light gathering is not important but aperture is for resolution which brings out the detail. 

I’m a bit confused on contrast. How to improve contrast with a reflector? Can they ever beat refractors for contrast? 

[Mike_S]

Is there a calculation for determining at what point a reflector aperture size beats the contrast of a refractor of a given size? So say a 100mm refractor. Considering typical central obstruction in the most common Chinese reflectors, what would the required aperture size be to match the contrast of the refractor? Is there a rule of thumb? Obviously don’t want to go too big as field of view comes into it.

[Stu]

Interesting @Geoff Barnes, there may well be something in that. The orthos I use are very high transmission and low scatter, seem to do a great job. I have BGOs too and a Nag zoom, could compare those to see if there is a difference but would expect them to be similar.

[Ricochet]

27 minutes ago, MSammon said:

Is there a calculation for determining at what point a reflector aperture size beats the contrast of a refractor of a given size?

I don't think contrast works like that. You are fighting diffraction from the secondary obstruction so the only way to beat a refractor would one to have a negatively sized secondary, not something that you can achieve in the real world. All you can do is reduce the secondary to the size where it isn't really noticeable. From memory I think it is about 20%. 

40 minutes ago, MSammon said:

How to improve contrast with a reflector?

The best mod I ever made to mine was to add a baffle to mask the edge of the primary mirror. You lose a couple of mm in aperture but you lose the part where the mirror is hardest to figure and the coating quality is worst which removes scatter. 

[John]

4 hours ago, Stu said:

Do you see much colour in the Tak John? .....

Yes, but I agree that aperture (normally - these are not quite normal times for the planets) does seem to enhance the colours more. Under good conditions with Jupiter higher in the sky, the range of colours that my 12 inch dob shows can be breathtaking with greys and blues as well as amber, rust, orange and shades of cream.

These tints are still there but muted by the thicker atmosphere that we are viewing through I think.

 

[John]

3 hours ago, MSammon said:

Is there a calculation for determining at what point a reflector aperture size beats the contrast of a refractor of a given size? So say a 100mm refractor. Considering typical central obstruction in the most common Chinese reflectors, what would the required aperture size be to match the contrast of the refractor? Is there a rule of thumb? Obviously don’t want to go too big as field of view comes into it.

I don't know about a formua but when I was comparing, a few years back, the performance of my ED120 refractor with a good Russian 150mm mak-newtonian (18% obstruction) that I also owned, I felt they were more or less equal on planetary performance but the larger aperture mak-newt pulled ahead a little when deep sky objects were the target. I guess a good 150mm F/8 newtonian might be getting close to the performance of the 150mm mak-newtonian so my conclusion from these comparisons (over a period of a few months I should say) was that a 4.7 inch ED refractor is roughly as good as a good, on planetary and lunar performance as good quality, modestly obsructed newtonian of 6 inch aperture.

Your mileage may vary though, as the saying goes