Visual image between frac and maksutov.

[tico]

Hello,
What difference exists between the visual image of a double star seen in a refractor and seen in a maksutov?
Thank you
Tico

Edited November 15, 2023 by Cornelius Varley
background colour to text removed

[vlaiv]

Depends on several factors.

First will be aperture size of both telescopes. Is it the same or does one scope have larger aperture than the other, and if so which one?

Second - optical quality of both scopes. Is refractor achromat or apochromat? How fast is maksutov / what is the size of central obstruction?

All those will contribute to differences / similarities of double star image between two scopes.

If we say - let's take academic case of perfect telescopes with the same aperture, then maksutov will have slightly brighter diffraction rings and slightly less pronounced central airy disk. It will be very small difference visually.

Ability to split stars will of course depend on observing conditions / seeing and difference in magnitude between two stars and their separation.

In theory, in some edge cases - ideal refractor will have slight edge over maksutov - if there is significant difference between intensities of double star components and stars are separated so that fainter star lands exactly on first diffraction ring of brighter star.

In all other cases - you should be equally able to split / not split pair with above two scopes (optically ideal, same aperture size, same viewing conditions).

[Stu]

I would add cooling and collimation to that list. I’ve used a few Maks, and often found stars were less well defined, a bit hairy vs a refractor. I always put this down to insufficient cooling, but was never really able to get to sorted satisfactorily.

[John]

The maksutov design that I've used that has produced the nearest to "refractor-like" views of double stars was a maksutov-newtonian. These have a smaller secondary size compared to the maksutov-cassegrain although the tube length is longer. The cool down time on that (a 152mm aperture) was quite long compared with a refractor though.

 

[lunator]

The general rule of thumb is a central obstruction of 20% or less will have virtually no impact on the appearance of the doubles compared to a refractor. 

My OMC250 has an obstruction around 30% and this will have a effect but the aperture means that I can resolve pairs much closer than any refractor I have owned. (All in the 70-120mm range)

If you are happy with a sub-150mm aperture I am sure a refractor will give super images but if you want to chase closer pairs then a mak of 150mm+ might suit you better.

Cheers

Ian 

[The60mmKid]

Really solid comments from all of the previous posters 💯 I'll add a personal account to complement the theoretical information.

I owned an Intes-Micro 5" maksutov-cassegrain with excellent optics and a ~30% central obstruction, which is quite typical. The impact of the obstruction was obvious when observing double stars. The diffraction rings were significantly more pronounced than through an unobstructed scope, which made an aesthetic difference at all times and a practical one when seeing was poor. Since poor seeing can result in wavy diffraction rings, the image was far less clear and stable than through the 4" refractor I'd often use alongside the mak. The mak also took several times longer than the 4" refractor to thermally acclimate, before which its performance was severely reduced.

I now own a 6" maksutov-newtonian with a sub-20% central obstruction. The diffraction rings around stars are only slightly more pronounced than through a refractor, and the image is as sharp as I'd expect through a top-tier refractor of the same aperture. This particular telescope also cools quickly and keeps up with ambient temperature changes, which eliminates the aforementioned thermal penalty that maks most often incur.

Edited November 15, 2023 by The60mmKid

[tico]

Thank you for All replies!! 

Sorry, I mean Maksutov-Cassegrain. 

Tico. 

[mikeDnight]

I love Mak Cass's!  Very clean star image when thermally stable. I think the most obvious difference between star images is that the Maksutov will have a noticeably brighter first diffraction ring, caused by the diffraction effect from the secondary mirror/ central obstruction. There's also no spider diffraction which is also a plus. The advantage of the refractor is that its also capable of wide field viewing, and rich star fields is something I'd miss if I only had a Mak Cass.

[Mr Spock]

I had a 140mm Mak many years ago. It was excellent on doubles. It also had high quality optics so the first diffraction ring wasn't so bad. It had to be kept in a cool place though and even then it still took a long time to cool down. On nights where the temperature was dropping rapidly it couldn't keep up and thermal equilibrium was almost impossible. The best wide field I could get was 1.3° which makes finding things difficult.

I prefer a 4" refractor these days for a couple of reasons. Firstly there's virtually no cool down time, even for a scope kept in the house. Secondly I can get 3.7° from the same eyepiece I used in the Mak. This means I can use it as a finder and get the double dead centre before putting the power on.
Most nights these are the four eyepieces I use:

I do have a 2.5mm eyepiece for x296 if I need it. The scope also has the advantage of almost perfect image quality.

I keep fancying a 180 mak for doubles, though I'm not sure the EQ5 would take such a heavy scope; it barely copes with the 8" f4 which is quite a lot lighter. That scope is intended for wide field DSOs and of no use for doubles despite its resolution advantage!

[bosun21]

6 minutes ago, Mr Spock said:

I keep fancying a 180 mak for doubles, though I'm not sure the EQ5 would take such a heavy scope;

Same position that I was in. I ended up getting the 150 mak which the EQ5 handles fine. I'm using it primarily for the planets at the moment. Nice and sharp.

[Nik271]

I use  the Skywatcher 180 Mak on EQ5 and it is fine, the scope weight is about 8kilos with finder and diagonal. But I have to agree the EQ5 is much happier with my smaller scopes. And with the unstable weather we are having recently  my lighter scopes get used the most. I'm starting to believe that for the UK seeing conditions  anything over 5 even 4 inches of aperture is hardly ever needed.

[lunator]

Hi Nik, generally a 5"scope will be good for most UK nights but when the seeing co-operates a well collimated 8" newt will split those doubles below 1" easily. It s just a rare occasion 🙂

Cheers

Ian

[Nik271]

2 hours ago, lunator said:

Hi Nik, generally a 5"scope will be good for most UK nights but when the seeing co-operates a well collimated 8" newt will split those doubles below 1" easily. It s just a rare occasion 🙂

Cheers

Ian

Absolutely.   It's just me being lazy: since 90% of the time the seeing is not good enough I end up setting up the smaller scope 100% of the time. And I keep saying, yes if the seeing is good I will make another trip to the shed for the big scope. Guess what happens after that - my big Mak stays cozy and snug in the shed all night 😞

Back on the topic of this thread. Most of the time I alternate setting up either my 100mm Svbony refractor or my 127 (actually 120mm)  Skymax. True the star images of the refractor are cleaner, the Mak has brighter 1-st diffraction rings when I look carefully. BUT in my case I have found that the extra 20mm aperture of the Mak beats the refractor by a little bit on double stars and planets. I can see deeper and split closer doubles. Granted the Svbony is not a Tak, not by a long way, but its still a decent refractor, similarly the Skymax is not a premium product, just a run of the mill scope, so the comparison is fair. My experience shows that a little extra aperture is actually worth more than the central obstruction penalty you have to pay with a Cassegrain design.  

 

[The60mmKid]

I think it's worth noting that much of the central obstruction debate relates to planetary performance (i.e., reduction of low-contrast detail relative to the CO size). Low-contrast detail isn't a consideration when observing double stars. In my experience, the way that CO comes into play when observing double stars is by exaggerating the diffraction rings (which may be a issue or a non-issue, depending on the observer's subjective preferences) and, relatedly, by making the telescope's performance more susceptible to poor seeing and poor thermal acclimation vs. an unobstructed scope. And I think the exaggerated diffraction ring "issue" only pertains to brighter doubles. I don't find it noticable or even perceptible when observing stars that aren't as bright. A run-of-the-mill 5" mak that I owned easily split closer doubles than any of my prized 60mm refractors could. But sometimes I'd prefer the cleaner (i.e., with more subdued diffraction rings) view through the refractor.

Edited November 18, 2023 by The60mmKid