Refractor or dob for viewing the bright planets

[Stu]

4 hours ago, SIDO said:

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, I believe the Lyra Optics, TS and Altair 102mm f11 clones are all pretty good these days. I think that's what I would try in this position, or a Tal 100RS which can be picked up pretty cheaply and have good correction at f10 and have a good reputation.

[fiestazetecmk2]

I cannot see that waiting for temperature cool down for a dob is a reason to overlook having one.

My suggestion is have a look at the options don't just take anyone's advice.try before you buy. I'm sure there are plenty of people on here would state a case for both .its your decision at the end of the day .

[Mike_S]

16 minutes ago, Stu said:

Freddie, I believe the Lyra Optics, TS and Altair 102mm f11 clones are all pretty good these days. I think that's what I would try in this position, or a Tal 100RS which can be picked up pretty cheaply and have good correction at f10 and have a good reputation.

Have you got a link to these please? I thought they were Apo’s considering the price unless I’m looking at the wrong ones. 

[Alan White]

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.

[dweller25]

@MSammon

Are the planets very low where you live - if so any telescope is going to struggle to show good planetary detail at the moment.

You will have to wait until autumn 2020 if you live in the UK before a planet (Mars) is high enough for good results.

You have an excellent 10” planetary scope but as the planets are low then try to keep magnification below x150, this will give a very bright image in your 10” scope so you will need to use a neutral density filter - I used a 96-0.3 which gave great results in terms of improving contrast.

As already mentioned you could make a simple off axis mask to stop your scope down - this would give you a 90mm APO for no cost 😁

 

Edited June 25, 2019 by dweller25

[fiestazetecmk2]

Well said that man .

[Stu]

24 minutes ago, MSammon said:

Have you got a link to these please? I thought they were Apo’s considering the price unless I’m looking at the wrong ones. 

This is the f11 achro which is a very nice scope.

https://www.teleskop-express.de/shop/product_info.php/info/p1958_TS-Optics-102-mm-f-11-Fraunhofer-Refractor---Optical-Tube-Assembly.html

There is an ED version which is interesting, but perhaps missing the point.

https://www.teleskop-express.de/shop/product_info.php/info/p11262_TS-Optics-102-mm-f-11-ED-Refractor-with-2-5--RAP-Focuser.html

[Mike_S]

What is the advantage over a faster or slower apo? Maximum power?

i’m confused about these dob masks now. One of the links appears to suggest it’s a myth that it would improve the view. 

[vlaiv]

8 minutes ago, MSammon said:

What is the advantage over a faster or slower apo? Maximum power?

i’m confused about these dob masks now. One of the links appears to suggest it’s a myth that it would improve the view. 

There were two parts to explanation why smaller aperture gives more pleasing image under poor seeing:

- one part is that smaller aperture "look" thru smaller section of atmosphere and there is greater probability that small section of atmosphere will be "smoother" compared to larger section of atmosphere - that one is not a myth

- other part is related to critical focus zone - slower instruments have wider critical focus zone and rationale was that ideal focus position changes with disturbance in atmosphere (it does) and that wider critical focus zone will better compensate this focus shift due to seeing (myth - as focus shift due to seeing depends on speed of the scope and relative ratio of the two - width of critical focus zone and focus shift due to atmosphere remains the same).

There is another part to the story of using aperture mask - off axis one, that way you eliminate central obstruction and get clear aperture (see diagrams above on effect of central obstruction) - in effect you are getting unobstructed aperture with reflector. However there is no free lunch - you will be taking section of the mirror that produces coma (outer regions) - but this can be handled with coma corrector (many people use it on such fast newtonians).

As for fast vs slow APO, here are differences:

Fast APO:

- Will probably be more expensive because it needs exotic types of glass to keep color correction to high standard on very curved lens surfaces (faster the optics - more curve the is in surfaces). Harder to figure surfaces means higher price as well. There will be more field curvature (important for AP, but also for wide field observing if your eyes can't accommodate focus shift - that happens with age)

- More compact and therefore more transportable

- Easier to mount due to arm momentum

- Harder to focus - needs more precision and often requires dual speed focuser (but now days most high end scopes come with dual speed focusers anyway).

- Will require more expensive eyepieces to get the best views (correction for fast light beam, handling of said field curvature, shorter focal lengths and that makes eyepieces more expensive for comfortable viewing - decent eye relief). Very short FLs for maximum magnification

- Will provide larger field of view at low magnifications - good for observing large nebulae and star fields of milky way.

Slow APO - basically the opposite of above (to an extent)

- easier to figure and color correct

- longer and a bit more cumbersome to handle and transport

- Will need a bit sturdier mount and will suffer more of shake if under mounted

- Easier to focus

- Can get away with cheaper, less corrected eyepieces - max magnification can be obtained with relatively "longer" EP focal lengths

- Somewhat limited FOV in comparison to fast APO

 

[Stu]

1 hour ago, MSammon said:

i’m confused about these dob masks now. One of the links appears to suggest it’s a myth that it would improve the view. 

I have direct experience of this when looking through a 16" dob belonging to @Moonshane

We were observing under some fairly trying conditions as I recall, and observed Jupiter both at full aperture and with a 170mm aperture mask.

Put simply, in the rare moments of more stable seeing, the full aperture still showed more detail and colour, however, the aperture mask stabilised the view and made it much easier to observe. I think this is well worth trying so you can see some of the differences. 90mm mask will give an unobstructed 90mm apo type view, althoughbthings like collimation and cooling still need to be right of course.

[Mike_S]

This seems to go against the theory of the aperture mask? Or am so reading it wrong?

[vlaiv]

25 minutes ago, MSammon said:

This seems to go against the theory of the aperture mask? Or am so reading it wrong?

Check my answer above:

2 hours ago, vlaiv said:

There were two parts to explanation why smaller aperture gives more pleasing image under poor seeing:

- one part is that smaller aperture "look" thru smaller section of atmosphere and there is greater probability that small section of atmosphere will be "smoother" compared to larger section of atmosphere - that one is not a myth

- other part is related to critical focus zone - slower instruments have wider critical focus zone and rationale was that ideal focus position changes with disturbance in atmosphere (it does) and that wider critical focus zone will better compensate this focus shift due to seeing (myth - as focus shift due to seeing depends on speed of the scope and relative ratio of the two - width of critical focus zone and focus shift due to atmosphere remains the same).

Now read the first sentence of that article you posted:

Quote

Telescopes of equal aperture ....

Here we were discussing benefits of aperture mask over unmasked larger aperture - in poor seeing smaller aperture will give more pleasing view - part one of my reply, but two same apertures will give same quality view and speed of scope / critical focus zone - has no impact what so ever (part 2 of my reply and quoted article - well part two of my reply was indeed based on said article).

[fiestazetecmk2]

I'd like to ask is there any point in trying to state which gives a more pleasing veiw in poor viewing conditions?. we all like to veiw in fair to good conditions. But sometimes for me if it's that bad then I put my dobsonion back in my conservatory and wait for better nights.

Isn't that logical. 

[John]

1 minute ago, fiestazetecmk2 said:

I'd like to ask is there any point in trying to state which gives a more pleasing veiw in poor viewing conditions?. we all like to veiw in fair to good conditions. But sometimes for me if it's that bad then I put my dobsonion back in my conservatory and wait for better nights.

Isn't that logical. 

But if you want to do some planetary astronomy despite the poor positioning of the planets currently then the tendancy is to give it a go and make the best of it. Currently I'm finding that my refractors do make the best of it on the planets despite my 12 inch dob having a very good mirror, low % obstuction and curved secondary vanes to reduce the impact of diffraction.

If you just have one scope and it's a dob, it is worth trying the aperture mask. With my 12 inch dob I can get a 4 inch clear aperture with no secondary / secondary vane in the light path. The views are very, very similar to those that my 4 inch ED refractors deliver including coping better with unstable seeing than the full dob aperture does.

There is another issue with the dobsonian though, my cluttered southen horizon makes it difficult to actually get a really low target into the view of the dob. The refractors, on their tall mounts, make this somewhat easier because they seem to be able to peer over and between obstacles such as houses, trees and hedges

If the planets were any lower, I'd be peering between the legs of pedestrians and dogs !

 

[Mike_S]

John what mount do you use on your refractor for viewing planets?

[fiestazetecmk2]

But I'm northern hemisphere and the planets are low this time of year.I tend to look when there higher up in the night sky.

Edited June 25, 2019 by fiestazetecmk2
More details

[Mike_S]

Thanks for the brilliant replies. I think I’ve got enough to take this in now. Keep going with further thoughts if you wish 

[John]

17 minutes ago, MSammon said:

John what mount do you use on your refractor for viewing planets?

Now I use the same as I use for observing all targets - alt-azimuth, manually driven. A Skytee II (bottom pic) and a Giro Ercole (top pic). I did have a couple of driven equatorial mounts for a while but I prefer using the alt-az mounts so the eq's were gathering dust.

[John]

14 minutes ago, fiestazetecmk2 said:

But I'm northern hemisphere and the planets are low this time of year.I tend to look when there higher up in the night sky.

So am I - about the same latitude as you.

[Stu]

28 minutes ago, fiestazetecmk2 said:

But I'm northern hemisphere and the planets are low this time of year.I tend to look when there higher up in the night sky.

The planets are poorly positioned for a number of years. If it were just a matter of waiting a few months, this would not be such an issue. I'll post this again.

[Mike_S]

1 hour ago, John said:

Now I use the same as I use for observing all targets - alt-azimuth, manually driven. A Skytee II (bottom pic) and a Giro Ercole (top pic). I did have a couple of driven equatorial mounts for a while but I prefer using the alt-az mounts so the eq's were gathering dust.

What scope is this please John?

[John]

The bottom one is a Takahashi FC-100DL and the top one is a TMB/LZOS 130 F/9.2 triplet. The scopes that Stu has suggested earlier in this thread are similar in concept to the Takahashi but at somewhat less cost.

 

[jetstream]

8 hours ago, MSammon said:

Want to get better with my current one first though.

Great idea, I would concentrate on collimation and cooling- IMHO you need a cheshire, very easy to use. All scopes need to be equalized in temp - how long do you put your scope outside before observing?

When collimating with a star test very high mag is needed- just using the sec shadow isn't good enough IMHO.

The contrast loss from miscollimation and heat can be staggering..

[Mark at Beaufort]

I have often wondered whether a Skywatcher Evostar 120mm Achro when stopped down to 80mm giving a F12.5 would give an interesting view of Jupiter and Saturn. I only owned a 4" ED Frac F6.95 so never had the chance to experiment.

[Mike_S]

15 hours ago, Stu said:

This is the f11 achro which is a very nice scope.

https://www.teleskop-express.de/shop/product_info.php/info/p1958_TS-Optics-102-mm-f-11-Fraunhofer-Refractor---Optical-Tube-Assembly.html

There is an ED version which is interesting, but perhaps missing the point.

https://www.teleskop-express.de/shop/product_info.php/info/p11262_TS-Optics-102-mm-f-11-ED-Refractor-with-2-5--RAP-Focuser.html

Had a look at these and another look myself and came to the conclusion that you picked the best value! Maybe later in the year. The achromatic is only £492 here

https://www.astroshop.eu/telescopes/ts-optics-telescope-ac-102-1100-ota/p,58274#tab_bar_1_select

there are still some Celestron Astromaster 90/1000’s around too for under £200 or this looks better even 

https://www.firstlightoptics.com/evostar/skywatcher-evostar-102-eq3-2.html

Edited June 26, 2019 by MSammon