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6" f/5 Achro Refractors

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#51 Mitrovarr

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Posted 05 October 2020 - 12:19 PM

I think there is more to making an ED scope than just different glass. It's possible that doing so at a high speed like this would erode factors that make the scope cheap and easily manufactured. Like, maybe it would require steeper curves that couldn't be easily machine-manufactured or more precise spacing that would require a fancier lens cell.

It'd be interesting to hear from a manufacturer on why a scope like this hasn't been produced. I bet there is a good reason.

#52 Marcsabb

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Posted 05 October 2020 - 01:25 PM

Sounds interesting but why not just buy an APM 140? 

Because its dimensions and weight make it unsuitable for field use on a light mount. It's also an f/7 scope.



#53 StarAlert

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Posted 05 October 2020 - 02:05 PM

The difference in length is certainly not insignificant. The difference in weight would be 200mm of six-inch aluminum tubing. What is that? A pound? 

 

I use an AR152 mounted on a DSV3 in the field every month. It's very stable and I have no problems at all. Low-power, wide field views don't need huge mounts to work well. 

 

Marcus at APM has mentioned in posts here on CN that the Chinese manufacturers will build scopes for anyone if they order 100 units. If you really think this is a marketable scope, you should place an order. I might be interested in buying one after I read some reviews.  smile.gif


Edited by StarAlert, 05 October 2020 - 02:05 PM.


#54 Mitrovarr

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Posted 05 October 2020 - 02:14 PM

It might be hard to find 100 buyers. I'm pretty sure even with a less expensive ED glass such a scope would be over $2k.
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#55 daquad

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Posted 05 October 2020 - 03:20 PM

As a 'rule of the thumb' a doublet with FPL51 reduces longitudinal CA by a factor of ~3.5; depending on the choice of the mating element it could be a bit more or a bit less. For example, the APM 6" F/8 is said to have CA comparable to a F/30 achro. That's a 3.75x improvement.

 

If I were to design a large, but portable and compact rich field refractor with ED glass I would probably choose a diameter of 140mm. There is already a wide choice of 140mm refractors, but all expensive triplets and all quite heavy and long, with f-ratios of 6.5-7. A larger diameter, like a 6", would be preferable for faint DSOs, but the resulting telescope would be too much front heavy (a 6" doublet lens cell weights 2.5-3kg, which would be 50% of the weight of the whole scope and all concentrated on one end of the tube)

 

As for F ratio, F/5 surely looks doable: the resulting scope would have FL= 700mm and would be also around 700mm long, lightshield included. Sidgwick ratio for such a scope would be around 3.2 if we use the mentioned 3.5x factor of improvement for CA over a standard achromat lens. However I would relax a bit the F-ratio increasing in to 5.5-5.6 to reduce CA a bit further and also to make lens design less critical.

 

The resulting scope, with a FL of ~ 780mm (F/5.6) would be still very compact and lightweight, being able to be mounted on any mount that can take a 100mm F/9 such as the 100ED or the slightly larger 120ED. The extra aperture would make a difference with faint DSOs and the decent CA, coupled with a generous 140mm aperture, could make the scope useful also for some occasional moon/planet gazing.

I guess I was being conservative in my estimate, assuming a reduction in longitudinal CA by a factor of 3.

 

To meet the Sidgwick standard, a 6" needs to be f/18, which results in a color blur (CB) of 3 times the Airy disc, measured to the first minimum. At f/30 the Conrady standard is met and the improvement would be 30/18 = 1.67, resulting in a CB of 1.8 times the Airy disc.

 

Using Rutten, et al's formula for the color blur of our hypothetical 6" f/6 ED RFT using FPL51 I get:

 

CB = (152*735)/(6*6000) = 3.1, which is closer to the Sidgwick standard than the Conrady.  I used 1/6000 for the dispersion factor of of FPL51.  

 

Using the APM equivalent example, (6" f/30) the CB is claimed to meet the Conrady standard, and hence is 1.8 times the Airy disc.  The APM 6" f/7.9 ED uses a FPL51/La cocktail.

 

Working backwards, if the APM claim is correct, then the required dispersion number for FPL51 in combination with the Lanthanum element is:

 

dispFPL51/La = (152*735)/(7.9*1.8) = 7856.  Actually, its reciprocal, 1/7856

 

If we use 1/2000, the dispersion value for the standard achromat used by Rutten, the improvement claimed by APM is 7856/2000 = 3.93.  So fairly close to your estimate of the longitudinal CA reduction factor.  

 

If we use 1/7856 for the hypothetical 6" f/6 ED with FPL51/La then the CB is calculated to be:

 

CB = (735*152)/(6*7856) = 2.38.  That is about what I calculate for my Jaegers 4" f/15.5 achromat, which is color free on just about every object, except Venus and the brightest 1st magnitude stars.  I see no false color on the moon.

 

I have the Altair Starwave 6" f/5.9 RFT and it is very good for its intended purpose, but if it were an ED it would be the only refractor I would need.  It would be the same weight and dimensions as the Altair Starwave and probably the same cost as the APM 6" f/7.9 ED.  Both instruments (Starwave and APM) are made by Kunming.  

 

Dom Q.


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#56 Marcsabb

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Posted 05 October 2020 - 04:10 PM

The difference in length is certainly not insignificant. The difference in weight would be 200mm of six-inch aluminum tubing. What is that? A pound? 

 

I use an AR152 mounted on a DSV3 in the field every month. It's very stable and I have no problems at all. Low-power, wide field views don't need huge mounts to work well. 

 

Marcus at APM has mentioned in posts here on CN that the Chinese manufacturers will build scopes for anyone if they order 100 units. If you really think this is a marketable scope, you should place an order. I might be interested in buying one after I read some reviews.  smile.gif

It's more a problem of balance than weight itself and also oscillations. You have basically 40% of the weight concentrated in one end of a long tube. One of the reasons sct became so popular is that you don't have to constantly worry about rebalancing, especially with the smaller, lightweight mounts. With a SCT, once you tighten the clamp of your mount, you can forget about it and concentrate only on your targets for the night.

 

It is true that we're seeing a proliferation of scopes in recent years. But many of them look all too similar. If I visit an online shop, any online shop, I can find two dozens, or maybe more, of 80mm refractors between f5 and f8 and a dozen of 100-110mm scopes between f/6.5 and f/7. People on the forum often discuss about the very same scopes, from the same chinese manufacturer, with a different coat of paint and a different name on the hood. How boring is that? I'm done buying the same scope over and over again. I want to try something new flowerred.gif

 

Since we're talking about discretionary goods, part of the success of a business like selling telescopes has to be creating a demand or convincing your would be customers that they really need a scope like the one you're selling and that wasn't available before. The guys designing catadioptrics seems to be more creative in this regard. In the last 15 years, they pushed on the market affordable aplanatic sct, ritchey chretyens and more recently "true" cassegrains. On the other side, it's hard to be excited about something when you take a peek into refractor land nowadays...tongue2.gif

 

To conclude this long rant (bad weather for one week is taking a toll on my mood!), my pitch for a large but portable rch field ED refractor is to offer something new to existing rft owners (120-150mm f/5) and that, to my knowledge, hasn't been tried before (there was a Televue 140 f/5 but that's another story). 


Edited by Marcsabb, 05 October 2020 - 04:19 PM.

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#57 Marcsabb

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Posted 06 October 2020 - 01:27 AM

(...)

 

If we use 1/2000, the dispersion value for the standard achromat used by Rutten, the improvement claimed by APM is 7856/2000 = 3.93.  So fairly close to your estimate of the longitudinal CA reduction factor.  

 

If we use 1/7856 for the hypothetical 6" f/6 ED with FPL51/La then the CB is calculated to be:

 

CB = (735*152)/(6*7856) = 2.38.  That is about what I calculate for my Jaegers 4" f/15.5 achromat, which is color free on just about every object, except Venus and the brightest 1st magnitude stars.  I see no false color on the moon.

 

I have the Altair Starwave 6" f/5.9 RFT and it is very good for its intended purpose, but if it were an ED it would be the only refractor I would need.  It would be the same weight and dimensions as the Altair Starwave and probably the same cost as the APM 6" f/7.9 ED.  Both instruments (Starwave and APM) are made by Kunming.  

 

Dom Q.

Exactly. I had the TS version of that scope (152/900) and it was a good performer, with nicely figured lenses, but CA is hard to ignore. I also owned a Skywatcher 150/750 that I liked a lot because it was so short and compact that could ride atop any mount, not a small feat for a 6" refractor.

 

I also played with CB formulas using similar coefficients to yours: 6670 for a FPL51/FK61 + cheap flint but a more conservative 7000 for FPL-51 + a lanthanum flint like H-LAF53

 

As for the lens design, I reckon that a fast F ratio scope will have other problems even if you address the longitudinal CA. An interesting design study is the Takahashi Sky 90, with its widely spaced doublet. Excellent color correction for a F/5.5 doublet but it's lens design was as radical as impractical. 

 

You can try the same route of the Vixen ED114/ED130SS: a front doublet with a more relaxed F-ratio followed by a rear corrector that also act as focal reducer. Good but you have to sacrifice compactness: you would end up with a scope as big and heavy as a APM 140, so you could as well buy one of these and add a 0.8x reducer. Same if you try the Petzval route, since the first rear element sits at about 2/3 of the focal length of the front doublet. Vixen also tried that with the NA140 and it was over 1 meter long despite a fast F/5.7 ratio. 

 

So we're back again to a simple doublet that inevitably will have some design compromises but I'm convinced that, if carefully planned, it could be a versatile design fit for people who want to stay on the light side and still have a capable refractor. 


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#58 Chris K

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Posted 07 October 2020 - 10:19 PM

I have the Altair Astro 152 (in red) mentioned above.  There is CA on very bright stars and planets. However for clusters and deep sky—the views are exquisite.

 

I've not bought or tried any filters yet but I've read they help with CA.

 

It took some getting used to as far as the weight is concerned but I hardly notice it now.


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#59 Chucke

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Posted 09 October 2020 - 06:03 PM

I have a 6" f/5 Jaegers.  For low power views of the Milky Way etc. it is great although there is a strong violet glow around brighter stars such as in the Pleiades.  I have tried it on Jupiter and it was pretty bad.  On the Moon it is awful.  Extremely colorful and not particularly sharp.


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#60 oldtimer

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Posted 12 November 2020 - 01:53 PM

I am anxiously awaiting delivery of a  Celestron Omni 150mm F5 I found here in the classified.  I will be mounting it up on my Richard Berry alt-az mount equiped with Sky Commander digital setting circles.  It will replace my 100mm F4.5 RFT and hopefully allow me to see those fainter open clusters that have been evading my 100mm.

 

Gary (oldtimer)


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#61 Sarkikos

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Posted 13 November 2020 - 12:57 PM

I have the SW 120 F5 and SW 150 F5.

Both are excellent at what they were made for (Wide Field).

I rare (If ever) point either at bright objects.

Only if they are bright nebulae!  wink.gif

 

Mike



#62 beanerds

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Posted 17 November 2020 - 06:37 AM

I had a Celestron CR6, the whole package with the GoTo equatorial mount. It was a very nice scope. I even added a 12” pier to get it higher for viewing. I was able to find quite a few DSOs with that. The only problem I had with it was the fact that I always had to set up and tear down after viewing. It is the type of scope to set up in one area and leave up, preferably in an observatory or a runoff shed. I always had 4 or 5 trips in and out to set up and view. After awhile it got very tiring.

I just grabbed one of the William Optics HD hardwood tripod's when they sold them a while back and it hold's my 150mm F8 achro on HEQ5 extremely well , high enough to be comfortable and solid .

 

I still have this scope even tho it's in another country , my Nephew is looking after it and using it .

 

Beanerds 

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Edited by beanerds, 17 November 2020 - 06:38 AM.

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#63 Sarkikos

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Posted 17 November 2020 - 08:17 AM

I mount my 150 Omni XLT f/5 on a DM6 and T-Pod 110.  I switched out the focuser for a Crayford.  It is a great medium-to-low power, wide-field, deep-sky scope.  Viewing the bright planets or the Moon is disappointing because of the CA, so I don't view bright planets or the Moon with the 150 Omni XLT.  

 

I'm surprised only one other post has mentioned the field curvature.  To my eyes, the FC is pretty obvious in wide field eyepieces.  But a TSFLAT2 field flattener flattens the field very well, even in an Ethos 21, an ES 25 100 or ES 9 120. 

 

I could see a little fuzziness to stars toward edge of field in a 31 Nagler, though it looked more like residual astigmatism from my eyes rather than FC from the telescope.  

 

Mike


Edited by Sarkikos, 17 November 2020 - 08:21 AM.

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#64 oldtimer

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Posted 21 November 2020 - 06:43 AM

OK i just received my Celestron 6" F5 scope the other day. My objective for this scope is to be able to use a Celestron 23mm Luminos eyepiece (82 degree FOV) without ANY vignetting. I took the scope apart and did a lot of measurements and manual (drawing) ray tracing and found that in the scopes present form with original focuser  I  could NOT fully illuminate the image plane required to aviod any vignetting. First my ray tracing revealed the two internal tube baffles needed to be moved to the rear about 1" which  did.  I remeasured again using a GSO  10 to 1 focuser which had a shorter draw tube but the results were no better. Secondly the 2" diameter draw tube was still the real problem and could not be further physically shortened. In the final analysis and as posted by someone previously a focuser with at least a drawtube of 2.5" is required to meet my goals.  I have just ordered from another CNer a AP 2.7" focuser. My next challenge is to get it mounted up.. Even without having the AP focuser in front of me to measure  I'm pretty sure this is as close as I can get get to meet my objective.

 

Gary (oldtimer)


Edited by oldtimer, 21 November 2020 - 06:45 AM.

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#65 j.gardavsky

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Posted 21 November 2020 - 07:11 AM

The 2" focuser is the first thing to get replaced on the 6" F/5 achro.

 

Best,

JG


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#66 oldtimer

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Posted 21 November 2020 - 09:58 AM

JG

 

What was your replacement  focuser?

 

Gary



#67 StarAlert

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Posted 21 November 2020 - 10:23 AM

OK i just received my Celestron 6" F5 scope the other day. My objective for this scope is to be able to use a Celestron 23mm Luminos eyepiece (82 degree FOV) without ANY vignetting. I took the scope apart and did a lot of measurements and manual (drawing) ray tracing and found that in the scopes present form with original focuser  I  could NOT fully illuminate the image plane required to aviod any vignetting. First my ray tracing revealed the two internal tube baffles needed to be moved to the rear about 1" which  did.  I remeasured again using a GSO  10 to 1 focuser which had a shorter draw tube but the results were no better. Secondly the 2" diameter draw tube was still the real problem and could not be further physically shortened. In the final analysis and as posted by someone previously a focuser with at least a drawtube of 2.5" is required to meet my goals.  I have just ordered from another CNer a AP 2.7" focuser. My next challenge is to get it mounted up.. Even without having the AP focuser in front of me to measure  I'm pretty sure this is as close as I can get get to meet my objective.

 

Gary (oldtimer)

I replaced the 2” focuser on my 6” f6.5 (AR152) with a 3” FT. I’m Very happy with the performance.
https://www.cloudyni...or-binoviewing/


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#68 j.gardavsky

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Posted 21 November 2020 - 10:58 AM

JG

 

What was your replacement  focuser?

 

Gary

Gary,

 

it must have been this one 3" : https://www.teleskop...connection.html

or its predecessor.

TS makes an adapter for the focuser flanch to the tube diameter of the OTA.

 

Monorail is eventually not the best choice, but it has been on budget,

JG



#69 glend

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Posted 21 November 2020 - 04:15 PM

The Bresser 150 f5 Achro is not a bad scope, I had one for several years and used it for narrowband imaging (which it did surprisingly well). Available through TS, but out of stock till mid February I believe. Focuser was good enough to support an ASI1600MM-C and filter wheel. Very cheap for its size which may explain why it sells out so fast.

 

https://www.teleskop...60-mm--OTA.html


Edited by glend, 21 November 2020 - 04:18 PM.

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#70 j.gardavsky

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Posted 21 November 2020 - 04:26 PM

The Bresser 150 f5 Achro is not a bad scope, I had one for several years and used it for narrowband imaging (which it did surprisingly well). Available through TS, but out of stock till mid February I believe. Focuser was good enough to support an ASI1600MM-C and filter wheel. Very cheap for its size which may explain why it sells out so fast.

 

https://www.teleskop...60-mm--OTA.html

This Bresser achro is actually a Petzval,

not just an achromatic doublet refractor.

And as being a Petzval, it is better corrected across the focus plane than an achromatic doublet.

 

Certainly better suited for the narrowband imaging,

JG


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#71 oldtimer

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Posted 22 November 2020 - 09:05 AM

I once owned a Vixen 140mm Petzval that was an 800mm FL (F5.6) Someone actually measured the actual focal length of the front elements and it came out to be F8. I imagine the Bresser 150 F5 Petzvl  has a true front element FL of about F8 which would partially explain better control of CA and field curvature.

 

Gary (oldtimer)




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