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6" f/5 Mak Newt vs 102mm f/7 Refractor. Which One?

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#1 Patrick

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Posted 28 February 2013 - 11:24 AM

I'm trying to decide if I want to keep my 6" f/4.8 Mak Newt or sell it. I'm currently using it with a Canon DSLR, but plan to upgrade to a mono CCD soon. I'm not using the scope for visual as I've got better scopes available for that.

For imaging would a 102mm f/7 refractor be easier to use? What are the pluses and minuses, not just from a theoretical viewpoint, but from a practical hands on point of view. I'm not finding the Mak Newt very user friendly, but I like the wide flat views and fast speed of the Mak Newt.

Should I keep it or sell it?

Thanks,

Patrick

#2 mmalik

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Posted 28 February 2013 - 11:38 AM

For imaging would a 102mm f/7 refractor be easier to use?


For wide-field/faster imaging 102mm APO will be better; here... is a good alternative (there are cheaper options too). So yes, I would sell the Mak. Thx


On a side note, a CCD discussion here....

#3 Patrick

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Posted 28 February 2013 - 02:04 PM

For wide-field/faster imaging 102mm APO will be better



Hi Mike,

Thanks for your response.

It what way would the 102 APO be better? Anything besides the ergonomics?

I'm thinking of the Explore Scientific 102mm f/7 triplet, btw, coupled with an 8300 chip or the new Sony ICX694 chip. Perhaps the wider field of view of the ES 6" MN would not really matter with the smaller chip in the Sony CCD? Then again, maybe the 8300 chip would be able to take advantage of it?

Patrick

#4 jgraham

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Posted 28 February 2013 - 02:56 PM

I've used a couple of different achromats for imaging, the largest being a 6" f/8, and they are very nice. Two things struck me right away using a refractor; no vignetting and no dust donuts, they're dust dots! The field of a refractor is just soooo clean with nothing in the light path. The only problem I had with my refractors is the fact they were achromats. If I could afford an apo I'd pick one up in a heartbeat. Since the focal length of the 102mm f/7 is a tad shorter that the 152mm f/4.8 there's probably not much point in keeping the MN6 if you decide to go that way.

#5 Alph

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Posted 28 February 2013 - 03:39 PM

Both telescopes will give you the same FOV and image scale. The Newt will be quite faster though.

#6 mmalik

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Posted 28 February 2013 - 03:53 PM

In what way would the 102 APO be better?


Mainly the corrected optics of an APO for AP.

ES ED102 is 714mm (correction welcome) while FSQ-106EDX is 530mm FL; I think former should be fine with smaller chip while later definitely would benefit from larger chip. On the DSLR side, FSQ-106EDX is commonly mated with a full-frame sensor. I think you can draw the similar parallel on CCD side. Hope this helps. Regards


On a side note, AT65EDQ 65mm f/6.5 ED Quadruplet is only 420mm FL, quite wide FOV.

#7 Rick J

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Posted 28 February 2013 - 04:13 PM

The 6" collects about twice the photons so expect twice the total exposure time but you lose the spikes. The loss of contrast from the secondary is easily compensated for in processing. Short tube is also an advantage with less wind load and inertia/momentum when guiding. Puts less pressure on the mount. Corrector can create reflection issues not in the refractor but far less than a SCT corrector since it is highly curved. I prefer photons myself but that's a personal decision.

Rick

#8 Inverted

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Posted 28 February 2013 - 05:26 PM

I assume it has a corrector plate? Do you get any coma or curvature? I don't think you will find an APO with that kind of light gathering capacity. APOs also arn't necessarily that we'll corrected either, APOs can still have chromatic aberration and usually need a flattener. A Newt won't have chromatic aberration and if well corrected coma and curvature shouldn't be an issue. So, for image quality and photon collection, as long as it is welm corrected I'd give the edge to the newt, probably not by a small margin. For ease of use though, balancing, colimination, transport I'd give the edge to the APO probably also not by a small margin.

#9 RStar

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Posted 28 February 2013 - 06:52 PM

Hi Patrick,

I've recently made the move from imaging with a DSLR to a mono CCD. My previous imaging scopes were a NP101, NP101is and a 190mm Mak-Newtonian. I found the Mak-Newt to be best for my imaging needs. It's equally a fast, has at least as flat a field, produced smaller star images and doesn't require focus offsets for my filters. Mak-Newts when properly collimated are incredible imaging scopes. The only real drawback is its size and weight. If you can put up with handling the larger OTA I'd stick with the M-N.

Just my :penny: :penny: :)

Bob

#10 microstar

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Posted 28 February 2013 - 07:09 PM

+1 for the 190 Mak-Newt (I have the SkyWatcher version). To get the same performance in a refractor you need a well-matched flattener.
...Keith

#11 Patrick

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Posted 01 March 2013 - 02:01 AM

Thanks for all the great input, guys!

The 6" collects about twice the photons so expect twice the total exposure time but you lose the spikes.



Yeah, that's what I'm thinking too about the total exposure time. It's a whole f/stop difference making the 4.8 about 2x as fast as the APO. I'm not sure what you mean about losing the spikes though. The Mak Newt has a corrector plate with no secondary spider. Is that what you meant?

I prefer photons myself but that's a personal decision.



Yes, and the MN is a lot less expensive than an APO.

Do you get any coma or curvature?

It think it has a very nice flat field. Here's a Picture of the Moon taken with it. Haven't done a lot of processing on the image yet. I think it was cropped a little.

For ease of use though, balancing, colimination, transport I'd give the edge to the APO probably also not by a small margin.



That's the one huge drawback. The scope and case weigh close to what my CPC1100 weighs. The CGEM mount seems to handle it pretty well though. If the weather ever clears I'll be able to get some more scope time.

I found the Mak-Newt to be best for my imaging needs. It's equally a fast, has at least as flat a field, produced smaller star images and doesn't require focus offsets for my filters. Mak-Newts when properly collimated are incredible imaging scopes.




Wow...the MN is pulling a head here! Okay~

The only real drawback is its size and weight. If you can put up with handling the larger OTA I'd stick with the M-N.



That's why the MN is becoming my 'stay at home' scope. Well, so is my CGEM mount which now sits on my pier on my deck. Too much work to haul that around and setup every time I want to use it.

I think I'll spend some more time with the MN on some deep sky objects this spring to see what I can get out of it.

Patrick

#12 jgraham

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Posted 01 March 2013 - 12:42 PM

Of course the nice thing anout your MN6 is that it is paid for. :)

I'd take a hard look at the star field and vignetting before considering a move to an refractor. The vignetting will go away, but I'd be a little concerned about the field curvature. The f/7 doesn't bother me at all, I do a lot of imaging at f/10 these days. Also, the field of my AR-6 is really flat (152 f/8), but I'm not so sure about a 102 at f/7.

If we could just get some darned clear weather!

#13 dr.who

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Posted 01 March 2013 - 01:11 PM

Patrick,

If I may ask what do you see as the problem with it other than the weight? I am looking in the opposite direction as in going to a mak from an APO because of the speed of the scope and specifically this one... If you do decide to sell it by the way I would be interested...

#14 Patrick

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Posted 01 March 2013 - 03:05 PM

Of course the nice thing anout your MN6 is that it is paid for.



Yep...very true!

If we could just get some darned clear weather!



This weather has been awful lately...but cheer up! After we get snow today, we're supposed to have clear skies on Sunday from 9:00am 'til dark! :grin: :smirk:

If I may ask what do you see as the problem with it other than the weight?



It's pretty much the weight and ergonomics of using a Newt on an EQ mount. It's not the best traveling scope out there either. When I took my trip to WSP, I couldn't find room in the car for it.

I was interested in hearing other people's opinions on the scope. It's been so dreary all winter I haven't had a good chance to really put it through it's paces.

Patrick

#15 Rick J

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Posted 01 March 2013 - 03:13 PM

Sorry for the mistake on the spider. The only Mak Newt I've used had a spider. It was home made and when he first put it on the corrector it was too heavy and the torque damaged the figure of his corrector. Apparently that has been solved with commercial units.

Rick

#16 dr.who

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Posted 01 March 2013 - 09:19 PM

It's pretty much the weight and ergonomics of using a Newt on an EQ mount. It's not the best traveling scope out there either. When I took my trip to WSP, I couldn't find room in the car for it.

I was interested in hearing other people's opinions on the scope. It's been so dreary all winter I haven't had a good chance to really put it through it's paces.

Patrick


Ah. I see... Interesting. Any thoughts on it as an imager? I am shooting from Red/White LP skies and the thought of getting a faster imaging scope is appealing as it will cut down on exposure time and corresponding bleed in of LP... At least in my mind that is what it will do... Or am I whistling past the proverbial grave yard...?

#17 Patrick

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Posted 02 March 2013 - 12:17 AM

Aside from the ergonomics issue, I think the scope has a lot going for it for imaging. I'm looking forward to actually using it for that purpose this coming year. My wife passed away in November after a two year bout with cancer, so I haven't had much chance to use the scope even though I've had it for over a year.

The scope has a nice flat field and pretty fast optics, plus at $1000 it's a steal compared to an APO. F/4.8 is a little over 1 f-stops faster than f/7. So, a 2 minute exposure at f/7 would only be 1 minute at f/4.8. That certainly adds up over the course of a night.

Patrick

#18 microstar

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Posted 02 March 2013 - 01:35 AM

Sorry about your wife Patrick. Re the Mak-Newt, I really think these are under-appeciated imaging scopes. My 190 M-N is my main imaging scope. Lots of aperture yet with a fairly flat field and negligible coma and no spikes (although some like those). I like having the corrector plate on the front - don't have to fiddle with spacing and reflections of a coma-corrector as you would with a Newt. I orient mine for imaging with the focuser and camera over the mount axis and it balances quite nicely in that position. My 90mm f/7 refractor piggy-backs nicely on the tube rings, but it is definitely my secondary imaging scope to the Mak-Newt.
...Keith

#19 dr.who

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Posted 02 March 2013 - 03:03 PM

I am sorry for your loss Patrick. Thank you for the answers. If I may ask how long is the cool down time? That was a big factor for me the last time I looked at it as I heard upwards of 45 minutes before start of imaging which is a real negative...

#20 olivdeso

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Posted 02 March 2013 - 05:45 PM

Hi Patrick

Sorry for the big loss. I hope the astro imaging will help you in this difficult moment...

APO 102:
pros
- ready to use out of the box
- easier for visual use
- larger corrected field using proper field flatener (interesting for full frame sensors)

cons
- 2 times longer single exposures -> some objets require 2 or 3 night, especially with narrow band imaging, where the mak newt might do it in one night
- lower resolution for casual planetary viwieng/imaging

Mak newt
pros
- well corrected for both visual use and imaging use
- small obstruction
-> these 2 assets together give a very good contrast in visual use, apo like view.

cons
- requires a precise colimation to get a good correction over the full sensor
- focusing is more chalenging,
-> require a really good focuser to achieve a proper alignment and focusing.
- less friendly for visual use due to the focuser location

So I would say that the Mak Newton is more demanding, but once it is properly tuned, it will also give you more.

The 102 apo remains interessting if you don't want to bother with fine optical allignment and want a kind of grab and go scope.
So it depends also on what time you are ready to spend on it : I mean if you already have some other scopes that require some tuning, might be you may like a simplier one.

The Apo is also better suited if you choose a large sensor, larger than the APS-C. But if you stay with the 8300 or the ICX694, the mak newton is sufficient and even better suited, because of the shorter F/L ratio which gives shorter exposures and also an smaller Airy disks.

The ICX694 may be a very good fit for the mak newton, especially if you want to do some narrow band imaging.

Olivier

#21 Patrick

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Posted 03 March 2013 - 01:11 PM

If I may ask how long is the cool down time? That was a big factor for me the last time I looked at it as I heard upwards of 45 minutes before start of imaging which is a real negative...



I don't know how long it takes to cool down the MN because I haven't had enough clear weather in the last 3 months to know! Since the tube is closed and because it's a carbon fiber tube, it may be longer rather than shorter. However, with some form of active cooling, it may not be too bad. 45 minutes for cooling a 6" mirror is not too bad really. I'm not sure what kind of active cooling can be applied to a Mak Newt though. On mine, the primary mirror is closed off from the back and of course the other end if closed off too.

Any suggestions for active cooling of a Mak Newt out there?

Patrick

#22 Patrick

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Posted 03 March 2013 - 01:28 PM

Hi Olivier,

Thanks for your thoughtful response to the original question. Excellent work!

APO 102:
pros
- ready to use out of the box
- easier for visual use
- larger corrected field using proper field flatener (interesting for full frame sensors)

cons
- 2 times longer single exposures -> some objets require 2 or 3 night, especially with narrow band imaging, where the mak newt might do it in one night
- lower resolution for casual planetary viwieng/imaging



The largest sensor I'll probably use is the APS-C sensor in my Canon 60D or the 8300M sensor, so I'm thinking I should be good with the corrected field in the Mak Newt.

As far as using the MN for visual use, I have other scopes I like better for that, so my intention for this instrument is to use it for imaging only.

Mak newt
pros
- well corrected for both visual use and imaging use
- small obstruction
-> these 2 assets together give a very good contrast in visual use, apo like view.

cons
- requires a precise colimation to get a good correction over the full sensor
- focusing is more chalenging,
-> require a really good focuser to achieve a proper alignment and focusing.
- less friendly for visual use due to the focuser location



- small obstruction...on the Comet Hunter version 6" f/4.8, unfortunately the secondary obstruction is not so small at 49mm (32%). The trade off there is getting a larger fully illuminated field...once again showing that this scope is really designed more for imaging.

The ES Comet Hunter has a nice focuser on it.

The ICX694 may be a very good fit for the mak newton, especially if you want to do some narrow band imaging.



I just wish the ICX694 wasn't so expensive. I'd go for it in a second if it wasn't nearly $3000. :smirk:

Patrick






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