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  #1  
Old 06-12-2018, 09:04 PM
D44bond (Dragos)
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Visual observing achromat or apo

Hi, im thinking about buying a 80mm-100mm refractor since my sct is too bulky and takes too long to cool down to the right temperature. What is the difference between a cheap $300 saxon refractor and a high quality apo... visually speaking.
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Old 07-12-2018, 04:44 AM
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With low power (e.g. deep sky wide fields) the difference between a $300 achro and a $1000+ ED or $2000+ APO is negligible.

But with high power for planets, there is a difference.
An ED or APO has less (or an APO almost no) color errors appearing as blue fringes around the planet's sphere.

I have a 110mm ED f/7 Chinascope which performs decently on planets at 180x.
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Old 07-12-2018, 07:44 AM
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mental4astro (Alexander)
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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.

Click image for larger version

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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!
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Click for full-size image (Jewel Box and Coal Sack - CN.JPG)
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Click for full-size image (Omega Centauri wide field - CN.JPG)
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Last edited by mental4astro; 07-12-2018 at 08:31 AM.
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  #4  
Old 07-12-2018, 08:13 AM
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DeWynter (ILYA)
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Quote:
Originally Posted by mental4astro View Post
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!
Off topic:

Wow! What are great sketches!!!!!!
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  #5  
Old 07-12-2018, 11:54 AM
D44bond (Dragos)
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Thanks guys, you have more than answered my questions.
My next project is the corflute insulation... it almost sounds too good to be true.
Achromat 100mm refractor sounds perfect for what i need.
Those are some amazing sketches...
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  #6  
Old 07-12-2018, 12:02 PM
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mental4astro (Alexander)
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You can get the Corflute from Bunnings. They have two thicknesses - for you SCT I'd suggest the thicker one.

EDIT:

There is another option too. Rather than Corflute, you could get your hands on a car windscreen shield, really cheap from $2 shops. These are reflective on both sides, and will offer more insulation for your thin walled SCT. You can cut it to size, and also place that black felt on it.

Below are a few pics of how I prepared the Corflute for my Mak. It rolls up nice and tight and holds its shape nicely. I also cut some slots and holes to accommodate the various bits.

Thanks for the compliments on my sketches too,

Alex.
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Last edited by mental4astro; 07-12-2018 at 12:27 PM.
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  #7  
Old 07-12-2018, 12:04 PM
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Merlin66 (Ken)
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For more than 300 years after the invention of the telescope, the choice for refractors was basically limited to Achromats.
All the discoveries made during this time were made with Achromats.

It's only in the past 60 years that multi element, exotic glass has been available and used to produce the "APO" type telescopes of today.

The long and illustrious tradition of the Achromat should not be forgotten.
Bang per buck they can still do very useful work.
My 2c
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  #8  
Old 07-12-2018, 12:44 PM
D44bond (Dragos)
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Thanks for that. I will stick with the coreflute... it actual looks decent.
They should start making SCT and other telescopes with insulation as standard.

Thanks Merlin thats very true
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  #9  
Old 08-12-2018, 09:06 AM
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mental4astro (Alexander)
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Achros and apois borh have their niche in astro. Andd there are excellent examples of both, and less than excellent too...

As for manufacturers changing their designs, not going to happen. Their first thought will be that the public will think their original design is somehow flawed, and worse still that they have done something wrong. It's like painting a scope & mount black - sure looks sleek, but it cools so far below ambient temperature that black surfaces form dew before everything else, and frost when the ambient temp is just above zero°. White painted scopes and mounts don't behave the same way. And try telling the manufacturers they stuffed up!!! You wouldn't believe how much surface colour affects the thermal behaviour of items. I've seen the black light shroud of big dobs soaked and with dew pouring of the bottom them them, onto mirrors and screwing up mounts and electronics, but any exposed light coloured timber parts bearly wet with dew!

For this reason, DON'T USE BLACK CORFLUTE!! Use only white.
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Old 08-12-2018, 05:24 PM
astro744
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Quote:
Originally Posted by Merlin66 View Post
For more than 300 years after the invention of the telescope, the choice for refractors was basically limited to Achromats.
All the discoveries made during this time were made with Achromats.

It's only in the past 60 years that multi element, exotic glass has been available and used to produce the "APO" type telescopes of today.

The long and illustrious tradition of the Achromat should not be forgotten.
Bang per buck they can still do very useful work.
My 2c
The achromat is perfectly fine if the focal ratio is f15 or greater and that is what it was for most of those 300 years. Short refractors (f5) have become popular in recent years but to truely appreciate what they can offer one needs such a short refractor to be an APO. There are many middle ground instruments between f5 and f15 and some are excellent around f9, e.g. TV-102 or similar quality instrument as these use exotic glasses to help reduce the chromatic aberration.

I have an ED80 (f7.5 with ED glass) but don't use it as much as my little Tele Vue 60 (f6) which provides me far more enjoyment. I bought my son an ST80 (f5) and for wide field low power it's great but for planets it has far too much chromatic aberration to be enjoyable and he prefers the view though the ED80 and TV-60 or Newtonian or SCT that I have.

The very expensive APOs with quadruple elements such as the NP-101 offer the best of both worlds but at a high price. You get a flat widefield for visual and photographic as well as high power views of planets that show no signs of chromatic aberration. Previous generations of the 101 (TV-101, Genesis) can be found on the used market and they can represent great value. There are other brands such as Takahashi which seem to pop up on the used market often.

It all comes down to what you enjoy viewing and if low power wide field then a short achro is great. If you prefer planetary viewing than a longer achro is preferred. If a long achro is too long and heavy for your mount then a shorter achro with exotic glass will be fine. I also have a TV-101 and use it more than my C9.25 but I use my TV-60 more than my TV-101 simply because it is more convenient and provides stunning images of planets in the evening twilight which is when I usually observe them (when they're up of course).

Note though aperture still rules and whilst the smaller refractors provide stunning high contrast images at the drop of a hat, my C9.25 and more so my 10.1" f6.4 Newtonuan with premium mirror trumps them all for planetary detail and on a superb night this is my preferred instrument. However superb nights are few and far between and on most nights (except those with very poor seeing) the refractor usually provides the more enjoyable view and the high contrast they offer due to the lack of central obstruction should not be underestimated.

If you can get to a star party and look through a few different refractors then you will see for yourself and if you find enough enjoyment from a lower cost instrument then that's more you can spend on eyepieces.

Whatever you choose, enjoy!
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Old 09-12-2018, 11:32 PM
D44bond (Dragos)
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Alexander Ive already purchased the materials including the 5mm thick white corflute.... looking forward to testing it out once its finished. I will post up my results. Last time I went out to see Mars the SCT became white with dew after less than an hour so we didnt really have a great experience.

Astro744 you have some nice telescopes. I have never looked through an APO before only through Achro's so I cant compare the performance differences. But like you said, the next star party is probably my best option... if i can wait that long
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Old 10-12-2018, 10:48 AM
Madanie7 (Brendan)
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Have you made a decision on what to buy?

I am following this with great interest as I am in a similar boat.

I am looking for a grab & go scope that doesn't take up too much room in a car. I have a 3 month old that with his gear, his mother, pram, toys and whatever else he apparently needs leaves no space for any of Brendan's toys.

The last trip we did in Nov I put the 6" SCT in the car and had to pull it out as other things took priority.

These trips are generally to my home town where the skies are darker than Brisbane so I want to take something and a smaller, compact wide field refractor seems perfect.

The other thing is I don't get a lot of time at the moment to set up and view with the SCT so I need something quick to get 30 mins or so in when i get the chance.
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Old 10-12-2018, 11:36 AM
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mental4astro (Alexander)
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Brendan, with your 6SE, it might be an option to look at a simple alt-az mount for your 6" SCT, something without the goto. It gives you a set up that takes just 2min to put up and take down. If you do get yourself a small refractor, you'll have an alt-az mount that can accommodate both scopes with no payload issues.

Just a thought on how to better exploit the gear you already have

Alex.
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Old 10-12-2018, 12:28 PM
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Originally Posted by mental4astro View Post
Brendan, with your 6SE, it might be an option to look at a simple alt-az mount for your 6" SCT, something without the goto. It gives you a set up that takes just 2min to put up and take down. If you do get yourself a small refractor, you'll have an alt-az mount that can accommodate both scopes with no payload issues.

Just a thought on how to better exploit the gear you already have

Alex.
Thanks Alex, I don't mind setting the SCT up when I get a decent session.
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Old 10-12-2018, 01:19 PM
Madanie7 (Brendan)
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Alex, What brand is your 100mm f5 achro?
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Old 11-12-2018, 03:35 PM
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Sorry mate, just sae your question now!

Skywatcher.

Alex.
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Old 11-12-2018, 04:14 PM
Madanie7 (Brendan)
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Thanks Alex. I am seriously considering a TV-60.
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Old 11-12-2018, 05:09 PM
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Thanks Alex. I am seriously considering a TV-60.
Beautiful telescope! Be prepared to spend as much again on eyepieces too. Because of the short 360mm focal length you need short focal length eyepieces for planets and double stars. The Tele Vue 6-3mm zoom is a perfect complement. The 24mm Panoptic gives you maximum field if view of 4.3 deg at 68 deg apparent field. Then something in between; I like the 16mm Nagler but for higher power the 9mm Nagler is nice. Note the TV-60 is 1.25" only so no 2" eyepieces. The 60 deg. Everbrite diagonal is great for low altitude objects.

I mainly use mine on planets for a quick look in the evening (or morning). Cool down time is almost instant. It's a great nature 'scope too, birds, bugs. I was watching a Christmas beetle chewing on a leaf from 30-35m and my 6-3mm Nagler zoom with the TV-60 and the image was sharp, bright and contrasty.

Even though it is only 60mm it is one of the best 60mm 'scopes you can get. Enjoy!
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Old 11-12-2018, 06:31 PM
Madanie7 (Brendan)
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Originally Posted by astro744 View Post
Beautiful telescope! Be prepared to spend as much again on eyepieces too. Because of the short 360mm focal length you need short focal length eyepieces for planets and double stars. The Tele Vue 6-3mm zoom is a perfect complement. The 24mm Panoptic gives you maximum field if view of 4.3 deg at 68 deg apparent field. Then something in between; I like the 16mm Nagler but for higher power the 9mm Nagler is nice. Note the TV-60 is 1.25" only so no 2" eyepieces. The 60 deg. Everbrite diagonal is great for low altitude objects.

I mainly use mine on planets for a quick look in the evening (or morning). Cool down time is almost instant. It's a great nature 'scope too, birds, bugs. I was watching a Christmas beetle chewing on a leaf from 30-35m and my 6-3mm Nagler zoom with the TV-60 and the image was sharp, bright and contrasty.

Even though it is only 60mm it is one of the best 60mm 'scopes you can get. Enjoy!
Thankyou.

I have a 24mm Panoptic and 13mm Nagler but 90 deg everbrite diagonal already.
I was already looking at the 9mm for my SCT so that will work and I'd happily buy the zoom to compliment....so I am half way there with the eyepieces.

Last edited by Madanie7; 11-12-2018 at 08:50 PM.
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Old 11-12-2018, 07:22 PM
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Originally Posted by Madanie7 View Post
Thankyou.

I have a 24mm Panoptic and 13mm Nagler but 90 deg everbrite diagonal already.
I was already looking at the 9mm for my SCT so that's will work and I'd happyily buy the zoom to compliment....so I am half way there with the eyepieces.
You're pretty much set then. I also have the 90 deg Everbrite and it is more convenient for higher altitude objects but the 60 deg pretty much lives in my focuser. The 90 is perfectly fine for terrestrial viewing if the 'scope is lower on the tripod.

The 13mm Nagler is a great choice also and probably a better step from the 24mm Panoptic. The 6-3mm Nagler zoom is highly recommended to provide variable power. The 4-2mm is very useful too if you can find one used as they were discontinued. 6-3 is probably more versatile.

Note if you want to mount the TV-60 on your SCT you can get an X-Y mount from Tele Vue. This also includes the parts required to mount to a Tele Vue clamshell which is what I have done when I want to pair it with the TV-101.

See http://www.televue.com/engine/TV3b_page.asp?id=67
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