Dragos,
There is now a change in thinking about the "cooling" of catadioptic scopes like Maks and SCT -
DON'T let them cool!
The problem that the cooling process with Cats creates is the primary mirror and the baffle that is connected to it act as a heat sink that stores heat, releasing it very slowly due to the nature of the materials these are made from. The tube of the OTA cools very quickly. The result is the air inside the OTA experiences a huge heat gradient between the cold tube and warm primary and baffle, so a heat plume is generated from the warm primary and baffle.
However, if the OTA is insulated, the metal tube isn't allowed to cool too fast, then the air inside the OTA won't experience the big temperature gradient between the tube and primary, and as a result a heat plume inside the OTA is not generated, and you can start using your SCT or Mak straight away! No cooling time is needed.
The scope is still cooling, but what has changed is the rate of heat loss - much, MUCH slower.
The added bonus of this is that the corrector plate stays warmer for longer, there by extending its dew-free period. Also, design the insulation around the OTA so that it extends out past the corrector plate so that it forms a dew-shield too.
I have done such an insulation mod to my 7" Mak. Last night I pulled the scope out from a warm room into the cool of the night, and I was straight away able to punch it to 340X and the only distortion in the image was that created by the so-so seeing conditions. I saw no heat plume from within the scope. This I verified when I defocused a bright star during my alignment process.
Below is a picture of the insulation/dewshield I made for my 7" Mak. It is made out of white Corflute - colour selection is critical as white resists dew formation best of all, whilst black sees dew from on it straight away! The Corflute is also carefully cut along every second corrugation, just through the first layer, so that it can be easily rolled around the OTA. I also put black felt around the inside of the roll where it extends past the corrector plate.
Now, with respects to your question about an Achro vs Apo, you shouldn't compare the aperture of an 80mm refractor to that of a 200mm SCT, regardless of the quality of the refractor. What you are wanting to actually do is have a scope of a much shorter focal length to compliment your SCT. This makes much more sense as you already have a competent high power scope, but its field of view is small, and a short focal length rich field scope is a perfect partner for such an instrument.
I've also explored this for the same reasons above. An SCT just won't give you rich and wide sweeping views. Only a rich field scope can. Sure an SCT will easily resolve Omega Centauri, but what it won't show you is the complex structures that surround it - the context of where Omega Centauri lies in within its patch of sky is totally lost. An SCT just won't show you the Rossete Nebula - it's just too slow in focal ratio despite its aperture grunt. but an 80mm f/5 or 100mm f/5 refractor will, and very easily at 15X magnification! And the giant open cluster, M7, an SCT will just not show you the entire cluster, nor the intricate network of dark nebulosity that criss-crosses M7, nor the mottled nature of the surrounding Milky Way that M7 lies in. But a modest 80mm or 100mm f/5 refractor will.
Achro or Apo?
As a rich field scope, f/5 is KING. But only YOU can decide is between the price difference between an achro or apo. Me, I'm very happy with my 100mm f/5 achro! I use it exclusively at low power which is what its strength is, and with a 30mm 82° eyepiece, it gives me a MASSIVE 5° true field of view! An SCT at best can give a touch over 1° TFOV. As Skysurfer says, an achro is not the best for high power stuff, and especially a fast f/5 one. LOW power is their strength.
And while looking for a fast refractor to compliment my then SCT, I too thought about 80mm or 100mm f/5
I tried both, and in the end the slightly bigger aperture of the 100mm edged out the 80mm despite the larger TFOV the 80mm offered. Comparing both scopes side-by-side the image of the 80mm just couldn't match that of the 100mm.
Yes, an apo may be sharper than an achro at the very edge of the field of view, but this area of the image you do not do any serious observation. You don't. No one does. The ONLY time you look at the edge of the field of view is when you are performing optical testing, and this is not what you do when out observing.
Now, if you are also doing photography with the refractor, then things change altogether, and an apo is your only option, along with the necessary field flattner that's matched to the scope.
But if you are only doing visual with the scope, it's more of a case if you can justify in your own mind the price difference between an achro or apo. Me I couldn't, and I'm stoked with the image my 100mm f/5 achro gives me.
Below are four sketches I've done using my 4"f/5 achromat. No SCT or Mak can give you these sorts of images. Left to right, Eta Carina, western edge of the Coal Sack, Omega Centauri, and the LMC which is a monster 12° in size!