Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

5" APO verses BEST 7" MAK CASS

  • Please log in to reply
190 replies to this topic

#26 Axunator

Axunator

    Viking 1

  • *****
  • Posts: 613
  • Joined: 23 May 2015
  • Loc: Helsinki, Finland

Posted 20 January 2019 - 04:57 AM

Sure, you're right. I was only speaking to the Gregory MCTs in the test.


Interestingly, some of the mass-produced Gregory Maks on that site do show quite similar intra- and extrafocal patterns. What should we make of that, then? Are they better or worse examples, if they don’t exhibit expected HSA?

Of course, in-focus performance is what counts at the end of the day. But the fact that star test patterns show such a huge specimen-to-specimen variability (even between examples of same models from the same factory) makes one think...
  • Earthbound1 likes this

#27 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 15673
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 20 January 2019 - 05:59 AM

Interestingly, some of the mass-produced Gregory Maks on that site do show quite similar intra- and extrafocal patterns. What should we make of that, then? Are they better or worse examples, if they don’t exhibit expected HSA?

I do not know the answer. I suspect there is some leeway in the HSA observed to remain diffraction limited, so not sure how much is expected.  Unfortunately the site does not provide any information, RMS or Strehl, as to whether the samples are good or not. It seems to be more pronounced closer to focus, say near 4 waves, as longer defocus normalizes into the familiar pattern. 

 

My guess would be to look for a nice clean diffraction pattern with good snap to focus and nice contrast. Since the in focus pattern has a unique obstructed ring structure, you might be able to tell something that way, too. In focus my sample looks somewhere between RMS 0.017 and RMS 0.0375, closer to the high end, in the illustrations below and surely not like the diffraction limited sample at 0.075 RMS. The diffraction artifact is clean with high contrast and a uniform first bright ring and good focus is not questionable. 

 

I did find they were interesting in the way they consistently looked different than a good star test we might be familiar with and they cannot test similar to pure lower order SA as we are used to seeing. Yet, SCTs seem to test more classically with their correction put to the corrector. Suiter's book has some illustrations, and Telescope optics.net does, as well.

 

See figure 189 at the bottom.

 

"As the correction error increases, the intrafocal pattern becomes noticeably dimmer than extrafocal pattern at 4 waves of defocus, with the latter becoming brighter, more contrasty in its inner portion. At the doubled defocus error, the main difference between the extrafocal patterns is that one side shows contrasty concentric pattern, while the other is increasingly dimmer and diffuse in its outer portion, with noticeably larger and darker central hole (note that for the reversed sign of aberration the patterns would switch the sides). At the diffraction limit, the difference in extrafocal patterns is glaring..."

 

https://www.telescop...k_spherical.htm


Edited by Asbytec, 20 January 2019 - 06:29 AM.


#28 Magnetic Field

Magnetic Field

    Viking 1

  • -----
  • Posts: 791
  • Joined: 28 Aug 2017
  • Loc: UK

Posted 20 January 2019 - 06:11 AM

IME exceeding 300X is rare due to the seeing, and 500X is something very few have access to (due to location)

For those types of magnifications a large mirror (10"+) is best as things get dim otherwise (at lest for me).

 

I am always sceptical when people brag about magnifications greater  than 300x.***

 

Why does this bragging claim often come from people who own refractors?

 

 

***It is a fact of live that for us mere mortals atmospheric seeing for most of the time is stacked against us.


Edited by Magnetic Field, 20 January 2019 - 06:15 AM.


#29 Magnetic Field

Magnetic Field

    Viking 1

  • -----
  • Posts: 791
  • Joined: 28 Aug 2017
  • Loc: UK

Posted 20 January 2019 - 06:27 AM

It is interesting to see the actual star and Ronchi tests. One thing I find interesting is the MCT star tests, they look pretty ugly and different of both sides of focus. But, we have to be more careful when evaluating the star test of these complex designs because they do not test like a parabola which can form a more or less spherical wavefront with some classic primary spherical present. MCTs have a large amount of secondary spherical induced by the meniscus as a function of it's design. However, the combination of higher order SA from the meniscus cannot be canceled by lower order SA from the primary. They will not test in the more classic sense of primary spherical aberration alone including the shadow breakout (the preferred 33% obstruction aside). And if done well, the result is a very low RMS value and a high Strehl.

 

The result is not a spherical wavefront, but one that approximates a sphere by smoothly undulating around a reference sphere. The resulting undulating waveform affects the star test and they will not be the same on either side of focus as each zone comes to focus in a slightly different point. If you notice, almost consistently, the MCT has a brighter outer ring outside focus (marginal zone) and some indication of an inner ring inside focus (paraxial zone). It's almost and generally like an undercorrected condition with each zone focusing differently. This also seems to show in the Ronchi tests, but it is not a classic undercorrection of primary spherical alone and the star test should not be interpreted as such. I believe this is what Roland was talking about in his (in)famous article on complex designs. They can be made to "please the star test crowd" using an asphere and touching up some zones, but performance in focus is not really improved. 

 

What the star tests seem to be telling me is the effect of balancing the higher order from the meniscus with opposite amount of lower order from the primary is working as deigned and it suggests no aspheric term was employed (often debated and seen in the Meade). The desired wavefront is being achieved somewhat consistently. Then performance is improved further with balancing with defocus to the point along the caustic of best diffraction focus (at the ~70% zone) where spherical aberration is again minimized.

 

The MCT has no higher order term built into the meniscus as an SCT does, and you may notice the SCTs tend to test more in a classical sense (with the higher order term dealt with) and any residual lower order SA can be easily seen. Not so in the MCT because of it's complex design. This ugly star test does not mean the MCT is a bad optic. In fact, there can be as much as 0.4 PV wave of higher order SA and the MCT will still be diffraction limited. This is because each undulation of the resulting wavefront closely approximates a reference sphere (perfect wavefront) and each zone covers only a small area of the wavefront and the RMS can be very good.

 

Fast APOs also have residual higher order spherical due to their steeply curved surfaces. In this sense, Maks kind of are refractor-like. smile.gif

 

http://www.csun.edu/.../startest2.html

 

Some pertinent quotes from the link above:

 

"When the optical system gets more complex than a simple parabolic mirror, then there are inherent aberrations that affect the star test."

 

"Machines exist now that can lay down a 1/20 wave or better spherical surface on a piece of glass without any human intervention."

 

"In this pure form, the Mak-Cass has left over 5th order aberrations and, depending on design, these can be less than 1/10 wave on the wavefront."

 

"By the way, fast Apo refractors have these same aberrations also. The RMS value will be better than 1/50 RMS and the Strehl ratio will be exceedingly high."

 

"When tested on the night sky, the inside and outside diffraction patterns will be quite different."

 

"I can tell you that it is easy to do some rough compensation with quick local polishing at several zones to get more equal inside and outside star patterns, but the result will almost certainly be a loss of contrast."

 

More info here:

https://www.telescop...n_telescope.htm

 

"An interesting aspect of the commercial Maksutov-Cassegrain is the question of its star test. There is a notion that its optics has special properties, making it sort of exception in that its intra and extra focal pattern are not supposed to be identical, even when it is near perfectly corrected. Or, put somewhat differently, that it doesn't need to have near-perfect star test for near-perfect performance.

 

The answer to this special status is in its higher order spherical aberration. Due to its steeply curved optical surfaces, especially those of the meniscus corrector, Maksutov-Cassegrain systems generate 6th-order spherical aberration that can't be cancelled (w/o aspheric surface terms), only minimized by balancing it with the 4th-order aberration. While roughly as much noticeable in the star test as the lower-order spherical aberration for given P-V wavefront error (FIG. 189), the balanced form is considerably less detrimental to image quality." 

I have long given up upon the intra- and extra- focal (Suiter) star test. It cannot be done (and everyone who thinks he can star test his telescope in the field and derive some numbers  is under an illusion). It is only useful for rough collimation checks or maybe giving a first guess of a possible coma.

 

What counts is the focused star and this can only be tested interferometrically.


  • nicoledoula likes this

#30 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 15673
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 20 January 2019 - 07:28 AM

I am always sceptical when people brag about magnifications greater  than 300x.***

 

Why does this bragging claim often come from people who own refractors?

 

 

***It is a fact of live that for us mere mortals atmospheric seeing for most of the time is stacked against us.

The exit pupil at high powers also works against us. We can certainly go well beyond 300x on some objects, like double stars, some small bright planetary nebula, and the moon. Mars, too. I've taken my scope up to about 100x per inch, but only looking at Arcturus in very good seeing. Not much to see up that high except a huge bright diffraction artifact. But, we can go up that high if we want, just not going to see much. 

 

I ran across this interesting quote from Suiter, not sure what to make of it. 

 

"Angular errors in the instrument are magnified until they are bigger than errors in the eye. Once the separation between the finest possible details have been magnified beyond 5 arcminutes (i.e., 1/6 the diameter of the Moon), aberrations in the telescope begin to dominate aberrations in the eye...Ironically, some people boast about telescopes that can "withstand more than 100 power/inch" (40 per cm). What they don't realize is that they're not bragging about the telescopes. They are inadvertently admitting the poor quality of their own visual acuities. When using extremely high magnifications beyond 100/inch, the diffraction disk appears bigger than two-thirds the angular diameter of the full Moon." Ref: 1st Edition, chapter 13.3.  Medium-Scale Roughness, or Primary Ripple page 241

 

I've certainly been that high, but only on a star. Not viewing generally. But 0.5mm exit pupil (50x/inch) is very common (in my MCT) and not unheard of when seeing permits. Interestingly, in my 8" Newt under seeing fluctuating around Pickering 5, I was able to hold the moon at 300x with blurring in only the worst moments. I was stunned at how sharp the image remained most of the time even though the tiniest detail for the aperture was not resolved due to seeing. I did not have to back off magnification in what I consider mediocre seeing for the tropics. That's cool. 

 

I have long given up upon the intra- and extra- focal (Suiter) star test. It cannot be done (and everyone who thinks he can star test his telescope in the field and derive some numbers  is under an illusion). It is only useful for rough collimation checks or maybe giving a first guess of a possible coma.

 

What counts is the focused star and this can only be tested interferometrically.

You're right we cannot put a number to it with any confidence, especially PV error or Strehl. You need data and computers for that. I would not be too hard on the star test, though, it does take practice, good seeing, and some understanding. You can make some educated guesses as to whether it's good or bad and approximately this or that level of presumably smooth SA (and be dead wrong about the actual PV error). I might agree, sometimes simulations are difficult to read and compare. (The MCT star test confused me for a long time, and still kind of does. It really scared me when I removed the baffle.) But, we can overcome that by understanding what's going on with what we see. But, you're right, it is hard if not impossible to quantify it accurately. At best, it's a wag.

 

For example, I am quite pleased with my Newt in that the obstruction shadows do not differ significantly either side of focus with a 33% obstruction at 10 waves defocus. There is some obvious under correction seen at 8 and 10 waves defocus, but it does not /look/ severe. Especially taking the shadows into account. Best I can tell, no significant roughness (when cooled in reasonably good seeing) or at least not worse than the seeing at the time. Thankfully, no indications of a significant turned edge, even though it may well have a slight one - up or down. I could not discern astigmatism at higher power even though there is likely some amount, and I see no glaring zones even though they likely exist to some degree. There is some coma, but it is a f/6 Newt after all and fairly well collimated. Not concerned about it.

 

I do not need to know the PV wavefront error or Strehl, I just have to see something that is not obviously concerning. The mirrors are certainly not premium, but they /appear/ pretty good. Good enough, best I can tell as a caveat. It approximates something around 1/6 LSA (according to Suiter's LSA approximation of the shadow ratio and the /perceived/ ring brightness) even though SA is certainly is not the only aberration present. The rest, outside of coma, are just not readily visible - to me, anyway. So, I do not concern myself with them. As I said above, the moon looks surprisingly good. I'm happy...so far. That's all I need to know. 

 

Sorry for rambling, just chatting...scope is cooling and the Pup awaits. 


Edited by Asbytec, 20 January 2019 - 07:39 AM.


#31 dr.who

dr.who

    Voyager 1

  • *****
  • Administrators
  • Posts: 14061
  • Joined: 05 Jan 2012

Posted 20 January 2019 - 06:55 PM

Forget everything and get a Dob... Oops! Sorry! Wrong forum. That should be over in Beginners...


Seriously though, if you want the best in a Mak then you will be buying used. They are the TEC’s, Intes, and the like. The Orion is the same scope as the Skywatcher. Synta, the parent company of Skywatcher and Celestron, makes the scopes and mounts for Orion.

A better idea would be a 8” EdgeHD SCT with a Feathertouch focuser and TEMPest fans from Deep Space Products. My most used scopes are my TSA-120 and Mewlon 210 because they compliment each other so well. They are both super portable, and easy to work with. Before I got the 210 it was the 8” EdgeHD in its place.

If you really want the boost for the buck in a lower weight (the Meade is a nice scope but heavy for its size/aperture) the Celestron 11” EdgeHD with the FT and TEMPest is the way to go. On very good nights you can go above 500x with it with no breakdown. One night that sticks out in my head with my 11” was when I was looking at M81/82. Super good seeing, super high powe on them, and it looked just like an AP image. It was awe inspiring.

#32 Kevin Barker

Kevin Barker

    Viking 1

  • -----
  • Posts: 795
  • Joined: 22 Apr 2009
  • Loc: Auckland, NZ

Posted 20 January 2019 - 08:53 PM

An interesting thread. As to what is meant by best probably needs to be quantified. 7 inches is a fair bit more aperture than 5 inches.

 

I have in recent years done a couple of shoot out side by side comparisons.

 

Firstly I spend several nights with good to very good seeing compared my Zeiss triplet APQ130/1000 with an Intes Deluxe 703. Conclusion was the 703 did not perform as well in terms of low contrast planetary detail on Jupiter, saturn and the Moon. But the 703 showed more in some nebulae and DSO's. The sky was not dark. Seeing was perhaps 8/10 at best.  The larger exit pupil of the 703 allowed higher mag on brighter DSO's.

 

I then compared a f-15 SW180 (newer version) with the 703 over a couple of evenings and concluded the two scopes were very similar. The SW180 did show slightly better contrast on DSO's and slightly better low contrast in planets. It also went a tiny bit deeper in terms of faint field stars. All in all pretty close to a draw. but I suspect the SW180 might have better coatings and a bit less CO.

 

On another occasion in close to perfect seeing I was able to see a deluxe Intes 815 (24% CO) was slightly better than the APQ 130/1000 in terms of showing low contrast planetary and lunar detail. The 815 went a fair bit deeper and resolved more detail. Hardly a surprise.


  • Bigdaveinhull likes this

#33 CrazyPanda

CrazyPanda

    Surveyor 1

  • *****
  • Posts: 1926
  • Joined: 30 Sep 2012

Posted 20 January 2019 - 08:59 PM

The f ratio is f 10 so its better on DSO's.

Wut?



#34 De Lorme

De Lorme

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1973
  • Joined: 30 Dec 2008

Posted 20 January 2019 - 11:52 PM

I was hoping the design of the Mak and it's 7" plus the optical quality of either the Sky Watcher or Orion would allow me to go too higher powers with my Binotrons.  I'm hooked  on the Binotrons.  The power switch is so cool and convenient.  

 

I'll just have to wait  until I can afford a better quality mak/cass on the used market.  Really like my 5" FCD100 but  a bigger

refactor would be big $$$$ plus another mount.   I think the 8" edge is probably really good but with large scale manufacturing

to keep the cost down  I'm not going to get the hand figured optics  that will give me the quality views like the  FCD100 lens gives.

 

In your opinions which are the top 3 7" & 10"  Mak/Cass?   Thanks for all the valuable advice.  

 

Clear Skies,  De Lorme


  • glmorri likes this

#35 Uranotopia

Uranotopia

    Explorer 1

  • -----
  • Posts: 72
  • Joined: 29 Sep 2018

Posted 21 January 2019 - 12:05 AM

You might just consider a used C9.25. About the same cost, weight, length, and cool down time as a 7” Mak, 75% more surface aperture for light gathering, and about a third more aperture diameter for angular resolution.  The longer focal length (higher F-stop) produces darker fields and much more Mak-like views than a C8, so very nice on planets, globular clusters and double stars. Sorry, I didn’t mean to side-track things.

This is absolutely correct, IMO. I own a 7" SW MAK, an older C9.25 and a new ES 5" FCD-1 APO.

Although my C9.25 is older than the others and I bought a used one for cheaper price, it shows best performance of them all!

But if I would be asked, I like the APO more than the MAK, as shorter focal length is comfortable in observing deep sky objects and the 5" refractor shows the same details as the MAK when I observe lunar or planetary objects under"real sky conditions"  in the region where I live... Here light pollution and often poor seeing are factors that limit observation most of the time. Whereas the refractor shows very good contrast on Jupiter's faintly colour differences, you can use higher magnification in the MAK (IMO about 300x vs 250x) - so the result in visual observation seems nearly the same. But because of shorter focal length of the APO, the field of view is larger what makes little difference when observing larger deep sky objects.

But my C9.25 shows brighter images and also very good contrast, and you can also see more dim deep sky objects.

Of course this is only my personal opinion when I compare these telescopes under"real world conditions" in my region and for visual observing with my (not so perfect) eyes...


  • terraclarke, Ptkacik and Cpk133 like this

#36 luxo II

luxo II

    Apollo

  • -----
  • Posts: 1082
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 21 January 2019 - 02:02 AM

Agreed a 9” represents quite a noticeable increment in image brightness.

#37 Magnetic Field

Magnetic Field

    Viking 1

  • -----
  • Posts: 791
  • Joined: 28 Aug 2017
  • Loc: UK

Posted 21 January 2019 - 03:11 AM

Wut?

He meant more useful and everyone understood it.

 

Not to say you cannot use an f/15 telescope for observing open clusters, globular clusters, etc... but a f/10 is definitely in general and broadly speaking preferable to the tunnel vision f/15 or larger on deep sky.



#38 Magnetic Field

Magnetic Field

    Viking 1

  • -----
  • Posts: 791
  • Joined: 28 Aug 2017
  • Loc: UK

Posted 21 January 2019 - 03:23 AM

Keep in mind a Mak is not nearly as efficient. Maybe 70-75% of the light reaches the eyepiece compared to maybe 97% with your Apo. Let’s round up to 75% for the Mak and 100% for the Apo. So the Mak has twice the light grasp but the views will only be 50% brighter. An excellent one might get you to 450x. In practice I normally hear about people pushing these 7” Maks to about 350x. Although maybe their skies are not as steady as yours. If you really want 500x I think you are looking at a premium 8” Mak ($$$$$$).

Scott

A Mak has more like >80% if taken everything into account and with modern coatings.

 

And this still gives you an aperture of 150mm for your 180mm Mak: sqrt(170*170*0.8) . 170mm is the effect of obstruction.

 

And your 5" apo has at most 95% light grasp if you are lucky (so 120mm effective).

 

Sure the gap between 120mm and 150mm is not mega large but will be noticeable (not to speak from the better resolution capabilities of the larger instrument if seeing permitting).


Edited by Magnetic Field, 21 January 2019 - 03:24 AM.


#39 luxo II

luxo II

    Apollo

  • -----
  • Posts: 1082
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 21 January 2019 - 03:50 AM

...but a f/10 is definitely in general and broadly speaking preferable to the tunnel vision f/15 or larger on deep sky.

Well I for one don't agree with that. f/10 is jack-of-all trades but not outstanding at anything.

 

If you want wide fields, buy a fast wide-field scope.


Edited by luxo II, 21 January 2019 - 03:51 AM.


#40 Magnetic Field

Magnetic Field

    Viking 1

  • -----
  • Posts: 791
  • Joined: 28 Aug 2017
  • Loc: UK

Posted 21 January 2019 - 03:55 AM

Well I for one don't agree with that. f/10 is jack-of-all trades but not outstanding at anything.

 

If you want wide fields, buy a fast wide-field scope.

 

I still think f/10 is a good compromise for most us. f/15 for deep sky is just a pain.

 

I know there will always be people go both extremes: f/15 for planets and f/5 for deep sky.


  • Kevin Barker and Bomber Bob like this

#41 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 15673
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 21 January 2019 - 04:26 AM

On focal ratio, I find f/13 frames smaller bright NGC galaxies pretty well. However, obviously, it's not conducive for the big stuff. At f/6, at times I find myself starved for a little more magnification, but I can easily get low magnification. There is a lot of deep sky that frames well at 60 degrees AFOV and 1.5mm exit pupil.

 

DSO viewing does not necessarily mean widest possible FOV at a 7mm exit pupil. Rich field means that, but not necessarily deep sky. Depends on what you're viewing. 

 

To the OP, if you are bino viewing, I'd consider the back focus of a MCT. It may be too short causing some vignetting. They are baffled pretty tightly, at least the ones I have worked with (Orion and ETX.) The Orion 150 has undergone some remodeling on the baffles and reintroduced to the market, but I am not sure the Synta 180s have nor how much back focus they offer with tight baffles and a small secondary. I believe SCTs offer a good amount of back focus, and no idea how much back focus the higher class MCTs offer. 

 

Commercial Maks can do a lot of deep sky, but really they seem to be optimized for lunar and planetary at long focal lengths, smaller FOV, and are well baffled for that work. Maybe too well. I suspect they are a more specialized scope than an f/10 SCT. I love my MCT, but for binos I suspect you may need more back focus. Others can chime in on that issue, but I think it's worth exploring.


Edited by Asbytec, 21 January 2019 - 04:30 AM.

  • Magnetic Field likes this

#42 De Lorme

De Lorme

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1973
  • Joined: 30 Dec 2008

Posted 21 January 2019 - 04:16 PM

Asbytec,  When you say back focus do you mean toward the telescope{inward travel}or away from the telescope? 

If towards than I would have a definite problem. But away I have extension tubes. But I will do further research on the this.

Thanks much for bringing this to my attention. I just assumed{mortal sin LOL}that I wouldn't have a problem focusing with the Binotrons.

 

De Lorme



#43 chuckscap

chuckscap

    Apollo

  • *****
  • Posts: 1027
  • Joined: 18 Jul 2009
  • Loc: Colorado Springs, CO USA

Posted 21 January 2019 - 06:20 PM

I don't have a 5" Apo, but I do have a TEC 7  f/15 Mak.   The best telescope i've ever owned.   My Mewlon 250 with in baffle corrector was superb also.    The Apo will cool down much faster and have a much wider field of view, big pluses if you live in very dark skies.    I live in suburban skies and the TEC is superb at planets and double stars.


  • elwaine, Richard Whalen and Tyson M like this

#44 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 15673
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 21 January 2019 - 06:56 PM

I don't think you will have trouble finding focus as small changes in mirror spacing produce large amounts of focal plane shift. However, you might begin to limit the fully illuminated FOV if you have to reach focus (behind the primary) due to the increased(?) light path of the binos and any required accessories. If you visualize it, when you add everything to the visual back, you have to push the focal plane outward from the normal diagonal position to achieve focus with all the stuff required, including the bino itself. The eyepieces are mounted much further behind the normal diagonal and, I believe, any prisms also lengthen the light cone a bit.

 

Anyway, you need outward focus with a moving mirror system because you cannot bring the bino closer to the primary as you can with the fixed focus of a Newt or refractor. In the latter case, focuser in travel makes this possible. With a fixed focuser and moving mirror, you have to move the focal plane back (away) to meet the bino. 

 

There used to be a lot of discussion about this and some charts folks made to show it (looking for some). Here's one thread of an SCT. Click on the animations, especially the second one (which shows the generous back focus of an SCT. I am not sure Maks are so generous, but they may be depending on the make and model. The tight primary baffle seems to be the cause of some vignetting, the smaller secondary may also contribute.

 

https://www.cloudyni...-vignettingetc/

 

I know my Orion is pretty tight. On axis, I can barely see the entire secondary viewing with my eye at the diagonal (which is fine for narrow fields of view for high power lunar and planetary.) I can still fully illuminate enough field of view to capture Jupiter many times over. But, if I moved back a couple of inches with an extension tube, the full secondary is no longer visible. On axis light becomes clipped by the primary baffle causing a reduced effective aperture. (I think, anyway, because the geometry of the light cone and moving mirror might be more complex than I know of.) I do not have a lot of room in the FOV because I can begin to see vignetting if I take a star off axis to some amount. The defocused image becomes clipped by the primary baffle, you can see it cutting the light cone causing a flattened defocus image.That's light loss and reduced effective aperture and resolution. 



#45 De Lorme

De Lorme

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1973
  • Joined: 30 Dec 2008

Posted 21 January 2019 - 06:59 PM

The skies are not great here either.  But not having to deal with the snow any more is just great. Moved to Killeen 2 years ago.

Though it's cold here in January I can still look up  but not in Colorado Springs.

Have you ever tried using binoviewers with your TEC?  If so were you able to come to focus?

 

Clear Skies,

 

De Lorme



#46 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 18426
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 21 January 2019 - 07:15 PM

I am always sceptical when people brag about magnifications greater  than 300x.***

 

Why does this bragging claim often come from people who own refractors?

 

 

***It is a fact of live that for us mere mortals atmospheric seeing for most of the time is stacked against us.

300x to me is just getting warmed up. And i sure do have the seeing that supports crazy powers.  Even with my 3" APO's 300x is no problem on the moon and doubles. Planets start to dim at 300x in 80mm and smaller scopes. But i have used 400x with a FS78 on doubles without breaking a sweat.

 

With my bigger Newts at around 14.5" and 15" i have done over 1000x on Jupiter on my best dead still nites.  I have a 10" Cave with super refigured Cervit blank that gave me the best view of Mars about 10 days ago at 500x.  Super well made optics and super seeing can show some crazy stuff.  It is not like every nite i can set up a scope and use crazy powers, most nites are not that great. But Feb seems to be my best month for steady seeing when we have super warm air and no temp drops at nite and sea fog rolling onshore.  I am right on the gulf in FL.  Summer nites never come close to very warm winter nites for high power work.  My very good SW150 Mak would do 600x on doubles and the moon, the moon was dim but still sharp. 350X on Jupiter was easy. Past 400x and Jupiter gets dim in the Mak. But Venus could take much higher powers since it was much brighter.


Edited by CHASLX200, 21 January 2019 - 07:18 PM.

  • Earthbound1 likes this

#47 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • Posts: 15788
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 21 January 2019 - 07:35 PM

I still think f/10 is a good compromise for most us. f/15 for deep sky is just a pain.

 

I know there will always be people go both extremes: f/15 for planets and f/5 for deep sky.

F5, F10, & F15...

 

ESC4 and AR102 and JM4 - Three Amigos S01.jpg

 

I owned a D&G 5" F10, and found it to be good on every object, and sharp from 25x to 400x.


  • Jim Curry and Paul Morow like this

#48 De Lorme

De Lorme

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1973
  • Joined: 30 Dec 2008

Posted 21 January 2019 - 08:47 PM

Asbytec,  Your probably right about aperture loss in the Mak. Really defeats the purpose of buying the Sky Watcher.   This might be a deal breaker for me.  The Binotrons are just to good and convenient not yo use.    Will have to think about this some more.



#49 De Lorme

De Lorme

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1973
  • Joined: 30 Dec 2008

Posted 21 January 2019 - 08:53 PM

Asbytec,  Would the A37 OCS make the difference in not losing light?   Would start off at a much higher power but that's ok with me. 



#50 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 15673
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 21 January 2019 - 08:55 PM

Asbytec,  Your probably right about aperture loss in the Mak. Really defeats the purpose of buying the Sky Watcher.   This might be a deal breaker for me.  The Binotrons are just to good and convenient not yo use.    Will have to think about this some more.

I have enjoyed my MCT so much, I really do not wish to talk you out of one. It may not be a problem with the Synta 180. I just believe it is with my 150. But, a lot of people are using binos with MCTs and raving about it. I am just not one of them for that reason. My Newt, however, has some generous back focus, so I might give it a try in the coming year or so (add one to my already long wish list, maybe toward the top.) 




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics