Planetary observations with 4" apo refractors

[Darth Wader (Wade)]

I've decided to concentrate on planetary observing for now because most of my observing sessions tend to be brief tours of the planets and moon in my backyard and are becoming few and far between, plus it's my main interest anyway. It's a bit difficult to centre a planet in my 8" dob at 250x and I really do need something with quick cool down times, and I like the sharp images of a refractor. Looking at a few reviews on CN and the net in general the Sky-Watcher 100mm ED APO looks like a decent scope at the higher end of my budget. Anyone have any experience with 4" APO refractors for mainly planetary obs?

Cheers
Wade

[Coen]

Is another option to mount your 8inch on a motorised EQ5? It is on the upper end weight wise.

Works a treat for me and might be a cheaper course - the 8 inch does give better resolution than the 4 inch (assuming everything is lining up).

[Satchmo]

Originally Posted by Coen

Is another option to mount your 8inch on a motorised EQ5? It is on the upper end weight wise.

Works a treat for me and might be a cheaper course - the 8 inch does give better resolution than the 4 inch (assuming everything is lining up).

Thats a good call. You can get a motorised 8" F5 on EQ5 for less than a 4" ED tube , and if you have a good mirror , in good seeing it will leave the 4" ED for dust.

[Darth Wader (Wade)]

It's a collapsible model so can't really mount in on an EQ. Plus, I don't think reflectors are for me - heavy, too large, collimation is a PITA.

[Coen]

Fair enough. Just got a reflector myself after many years of refractors so still getting used to it.

While I do not have experience of an ED 100mm, a friend of mine has a base model 100mm and another has a 150mm both of which give nice views although the tubes do start to get long. There is also the 150mm Mak to consider but is at around $1000 for the OTA; the views are very good and the long focal length excellent for planets and double stars.

Does your budget include the cost of the mount itself or focussing on the OTA only?

I've found that an EQ3 is adequate for a 100mm but an EQ5 is better. The difference is quite noticeable especially when changing eyepieces and waiting for the oscillations to damp down.

[mbaddah (Mo)]

Why dont you go for a Skywatcher 120mm ED instead of a 100? I have used both for quite a bit of time, the 20mm aperture increase was very noticeable on DSO and planets. Both are 900mm in focal length, very portable (for me).

[casstony]

Wade, since you live in the city it should be fairly easy to get a look through a 3 or 4 inch refractor at a public night. If not, just buy what you want second hand so you can sell at a modest loss if it doesn't suit. ED100 prices take a big hit on the used market.

For quick looks an ED80 on an alt/az mount might suit. If you can put the scope out to cool before observing a 6" Celestron SCT is a compact unit with good optics.

[Tandum (Robin)]

I saw a 2nd hand Takahashi Mewlon 210 somewhere recently for around $2K. That would be the bees knees for planetary.

[Darth Wader (Wade)]

Originally Posted by Coen

Fair enough. Just got a reflector myself after many years of refractors so still getting used to it.

While I do not have experience of an ED 100mm, a friend of mine has a base model 100mm and another has a 150mm both of which give nice views although the tubes do start to get long. There is also the 150mm Mak to consider but is at around $1000 for the OTA; the views are very good and the long focal length excellent for planets and double stars.

Does your budget include the cost of the mount itself or focussing on the OTA only?

I've found that an EQ3 is adequate for a 100mm but an EQ5 is better. The difference is quite noticeable especially when changing eyepieces and waiting for the oscillations to damp down.

Originally Posted by mbaddah

Why dont you go for a Skywatcher 120mm ED instead of a 100? I have used both for quite a bit of time, the 20mm aperture increase was very noticeable on DSO and planets. Both are 900mm in focal length, very portable (for me).

Sadly the 120mm is way out of my budget. I'm looking at the 100mm ED100 PRO (Gold OTA, superseded model, the only difference I can tell is a 2 speed focuser in new models) which I can pick up for $899 new, plus a basic EQ-5 mount for $399. I think $1200 really is as far as I can go for the meantime. The Maks are a little out of my range, and I'm not too familiar with them at all. Do they require collimation/any other maintenance? Are they heavy? I did read on CN that planetary images can be affected by long cool down times too.

Originally I was looking at a Sky-Watcher 150mm Achromat but its f/8.3 ratio had me a little worried about chromatic aberration, otherwise it would be perfect for a planetary grab n go at $849.

I'm a member of the Macarthur Astronomical Society so might poke around and see if anyone will give me a peek through their 4" scope.

[Steffen]

Originally Posted by Darth Wader

Plus, I don't think reflectors are for me - heavy, too large, collimation is a PITA.

Many years ago I was thinking the same - I ended up getting a 6" Mak (Intes MK67). That seemed to me (and still does) to be the best of both worlds.

Cheers
Steffen.

[Coen]

CA in a 150mm f/8 is no big deal. I know someone who has one, had it for quite some time and makes no mention of CA issues. More of an issue is the length of the tube and being low when the object is high.

I was using a 120mm f/5 refractor for quite some time and found it passable on planets although on the bright stars the purple glow got people interested as to why; a filter can help cut down its intrusion.

[Darth Wader (Wade)]

Originally Posted by Coen

I was using a 120mm f/5 refractor for quite some time and found it passable on planets although on the bright stars the purple glow got people interested as to why; a filter can help cut down its intrusion.

Hmmm... food for thought. I briefly considered the 120mm f/8.3, but again the dreaded CA monster scared me away. If you can use an f/5 and find it OK for planets then I shouldn't have a problem with an f/8.3 120mm. This option is looking like the best yet for planetary work within my budget.

This may be a dumb question but how does the filter work? I did read about Baader Fringe Killers - do they just screw into the eyepiece barrell like others?

Cheers
Wade

[Coen]

Chatted today with the friend who has the 150mm f8 and he indicated that yes CA is present but only really (but not annoyingly for him) noticeable on Jupiter, Moon, Venus, Rigel, Sirius - generally use a filter anyway so no big deal. He bought the scope for the planets and enjoys it, in particular Uranus and Neptune plus Saturn moon spotting.

You can get a fringe killing filter (basically cuts the purple down). I had that particular filter living in the diagonal on the 120mm most of the time (took it out a few times to show people the halo also took it out when I was the only one using it i.e. not using it to demonstrate). The filter just screws into the thread at the barrel of the eyepiece/diagonal. Most filters and eyepieces have standard threads although there are a few that do not. Check one of your eyepieces out and you'll see that the barrel is threaded and ready.

Filters for planets work a treat, for example I found a light blue one for Jupiter was good as it helped my eyes bring out the cloud deck detail. Others find light green or light yellow work well. The red needs lots of aperture as it cuts a lot out. You'll find for the moon you'll need a filter but I suspect you already know that from the 200mm dob you have.

BTW you do not necessary need to go expensive with filters to begin with, some basic colour ones to learn and appreciate what works for you.

[Satchmo]

Everything looks pale yellow with the MV filter. I still prefer the view without.
If i want to see serious detail on the planets I would use at least an 8" Newt.

[chris lewis]

A 100 mm / 4 in. is really the minimum for good planetary observations. I used a Orion 100 ED for several years. The images esp. on the brighter planets was very satisfactory. Subtle details could be seen on Jupiter and Saturn under steady seeing conditions. Maximum magnifications of 200x - 240x could be used - more then that and the image dimmed. A good 4. in will give you many years of rewarding viewing. I did go up to a S.W Equinox 120 ED about a year ago - mainly as I got great trade in deal. Yes the 120ED certainly gives you a brighter image, more detail and can go to 300x on steady nights. It gives you more 'headroom' to play with. Having said that I still would be happy to use my 'old' 100 ED.

Chris

[Wavytone]

You might also consider a 6" f/12 - f/15 Mak, this will match a 4" APO for sharpness and the image will be a bit brighter.

[ausastronomer (John Bambury)]

I would agree with Mark, for serious planetary observation an 8" newt is minimum IMO.

I have used some of the worlds best 4" and 5" APO refractors including Tak FS 102, Tak FSQ 106, Tak TOA 130, Televue NP 101, Televue NP 127, an Astrophysics 130 and a TMB 130 triplet. These are all clearly better performers than any of the doublet ED refractors coming out of China and Taiwan. So they should be, some of them are 5 or more times the price for equivalent aperture. None of them could equal a good well cooled and collimated 8" newtonian. The Tak TOA 130 which Rocket Boy owns, gives absolutely exquisite planetary views. However, you can't defy physics. A 5" telescope simply does not have the same resolution as an 8" telescope. Aesthetically sometimes the view is more pleasing but because of the lower resolution you cannot resolve the same amount of detail with the smaller scope. If you're happy to accept less detail for convenience, then a smaller scope like a 4" refractor or a 5" or 6" Maksutov on a driven mount may well be the answer you are looking for.

In regard to a couple of specific scopes that have been mentioned. The 150mm/F8 Synta OTA is no grab and go scope. It's an over the shoulder job IMO. In addition in many of its packaged mount configurations it is clearly undermounted for high power planetary work which IMO is somewhat academic as the scope does have a lot of CA at higher powers on bright targets. In respect of the 120mm/F5 short tube refractor I can offer the following. Clearly some other peoples expectations of a reasonably performing planetary scope are different to mine. A 120mm/F5 achromat on pure physics alone cannot be a good planetary perormer and that is something it was never designed to do. I would avoid purchasing that scope for your stated purpose.

If an 8" newt isn't for you and you are prepared to accept less detail and are somewhat budget constrained, go with a 4" ED APO or a 5" or 6" Maksutov

Cheers,
John B

[Starkler (Geoff)]

Originally Posted by Darth Wader

[SIZE=2] It's a bit difficult to centre a planet in my 8" dob at 250x

Only if you haven't had a proper go at fixing the motions. These cheap chinese dobs are never optimal as they come from the shop and huge improvements can be made fairly cheaply. Its so much cheaper to get one of these scopes working well than to give up and buy something much smaller on an eq mount for the same $$.

I recall seeing Mars at the snake valley camp in newts of 8", 10" and 12", with each size jump giving a distinct leap in brightness and resolution. When all else is equal, aperture always wins.

I would back a 12" skywatcher dob against any portable refractor you could set up in the field.

[dannat (Daniel)]

keep in mind the 150mm f8 scope has a plastic cap that comes off to reveal 4" of aperture, effectively giving you a 4" f12 - if you are worried about CA

[ausastronomer (John Bambury)]

Originally Posted by Starkler

Only if you haven't had a proper go at fixing the motions. These cheap chinese dobs are never optimal as they come from the shop and huge improvements can be made fairly cheaply. Its so much cheaper to get one of these scopes working well than to give up and buy something much smaller on an eq mount for the same $$.

I agree Geoff and that's the route I would be taking.

My 10"/F5 GSO dob, which my son still uses, tracked pretty ordinarily out of the box and it was a task to keep it tracking smoothly at anything over 200X. Replacing all the pads with virgin teflon and rebuilding the base with ebony star laminate improved it immensely, to the extent where I can hand track pretty easily at anything up to 500X and still easily keep the target centred. On numerous occasions I have observed the moon and Saturn at 625X (5mm Pentax XW in the 2.5X powermate) and still had little difficulty tracking with it. I couldn't have hoped to track with it at those powers as it came from the factory.

Cheers,
John B