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quick question about 70mm vs 80mm aperture

refractor equipment
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#51 ron scarboro

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Posted 19 June 2019 - 05:09 AM

Has Borg ever produced or planned a 80mm fluorite lens? I read that 72 is in the making and the current spread was 71-90-107.

I doubt it since they are limited to Canon/Optron sizes.  The 90FL only weighs 1.8KG, so less than many 80s.

 

The 71FL is 2lb, the 90FL is 4lb, so they probably have it covered w/o the 80mm.

 

A little comparison work I did with my 71FL, and a FS60

 

https://www.cloudyni...ortions-coming/

 


Edited by ron scarboro, 19 June 2019 - 05:14 AM.

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#52 Tony Flanders

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Posted 19 June 2019 - 05:41 AM

A third drawback of a spotting scope is the 1.25 inch focuser. For low power wide field viewing, a 2 inch focuser can nearly double the field of view.


Well, yes and no. As far as I'm concerned, 2-inch eyepieces and serious portability is a contradiction in terms. There are individual 2-inch eyepieces that weigh as much as an entire 60-mm spotting scope -- and don't bulk much less, either.

Moreover, field curvature limits the useful field of view of any small, light refractor. If I notice dramatic field curvature in my 80-mm f/4.4 scope using a 24-mm Panoptic, just imagine how bad it would be with a 40-mm Panoptic! To say nothing of the 9-mm exit pupil.

For my money, a refractor in the f/5 range with a 1.25-inch focuser is the best possible compromise between wide field and portability.

Incidentally, if the OP wants something really tiny and cute, consider the Stellarvue 50-mm f/4 finder. It fits easily in a coat pocket, yet splits Castor handily with a 3-mm eyepiece. Not a lot of light-gathering capability, though.


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#53 Bowlerhat

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Posted 19 June 2019 - 06:03 AM

tony, going 50mm is too far! at that point that's not a telescope anymore, that's a finderscope grin.gif 

 

However skywatcher has evoguide 50ED doublet, so someone down the line obviously has the same idea.

But I'm not going that way, probably a 60/65mm is my limit.



#54 Tony Flanders

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Posted 19 June 2019 - 06:54 AM

Tony, going 50mm is too far! at that point that's not a telescope anymore, that's a finderscope grin.gif

Not really. How many finderscopes do you know that can split Castor? It's really the low magnification more than the aperture that distinguishes a finderscope from a "real" telescope.
 
Remember that Jay Reynolds Freeman observed the entire Herschel 400 through a 55-mm refractor.

Having said that, I did borrow an SV50 with an eye toward taking it to South America, and decided that I really wanted more aperture.


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#55 Chucky

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Posted 19 June 2019 - 07:35 AM

<<  Jon's post # 5 (For all around observing, the planets, doubles, deep from both a light polluted backyard and dark skies, I find that there is a threshold, 80 mm is quite satisfying, 70mm is never quite enough.)  >>

 

I see it the same way.  I have two Stellarvue ED's....a 70 and 80.  Same lens designs.  I'm always amazed how much easier ALL types of objects are to observe in the 80.  


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#56 Bowlerhat

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Posted 19 June 2019 - 07:58 AM

Hence why. Although it's possible, it would be constrained too much. In the end, although it's portable it just sits there on the edge because it has not enough aperture. I'd say that route is better towards monoculars?

 

To be honest, I really do enjoy viewing trough 9x50 finder, so I imagine 50mm ED would work a treat. The idea of a pocketable telescope is great, but rare I think. I wouldn't mind owning one as well!

 

I never heard of stellarvue 50, I thought the skywatcher evoguide 50ED was the first one of 50mm.

It would work great for imaging, but for visual I think it's not enough.



#57 Jon Isaacs

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Posted 19 June 2019 - 08:15 AM

Well, yes and no. As far as I'm concerned, 2-inch eyepieces and serious portability is a contradiction in terms. There are individual 2-inch eyepieces that weigh as much as an entire 60-mm spotting scope -- and don't bulk much less, either.

Moreover, field curvature limits the useful field of view of any small, light refractor. If I notice dramatic field curvature in my 80-mm f/4.4 scope using a 24-mm Panoptic, just imagine how bad it would be with a 40-mm Panoptic! To say nothing of the 9-mm exit pupil.

For my money, a refractor in the f/5 range with a 1.25-inch focuser is the best possible compromise between wide field and portability.

Incidentally, if the OP wants something really tiny and cute, consider the Stellarvue 50-mm f/4 finder. It fits easily in a coat pocket, yet splits Castor handily with a 3-mm eyepiece. Not a lot of light-gathering capability, though.

 

Tony:

 

For me, this gets to the heart of the matter.  Is the primary reason to choose a 70mm or 80mm scope it's compact, lightweight nature or is because of the unique optical capabilities it offers?  Initially, the question seemed to be about the difference in capability between the a 70mm and an 80mm, what would be seen.  It wasn't until the second page that Bowlerhat mentioned traveling by train.  For me, portability is not the primary concern, it's what the scope can show me. 

 

In terms of field curvature, I do have quite a bit of experience and understanding, I've done a lot of calculations.   ST-80's with 2 inch focusers have become quite popular, I have one though it's loaned out right now.  I also have an 80mm F/6 and an 80mm F/7, both ED/apo's.  I don't have to imagine the field curvature visible with an 80mm F/5 with the 41mm Panoptic, it's a combination I often use to get the widest, brightest view, that's 6.6 degrees at about 10x with a 8.2mm exit pupil.  I used to be afraid of exit pupils greater than 7mm and probably most people should be but that was before I discovered that my dark adapted pupil is closer to 8mm than to 7mm.  There's field curvature visible, quite a bit to be sure but the views across the field are cleaner than my 10x50 Ultraview's. 

 

I generally prefer the ST-80 with the 31mm Nagler which offers a 6.0 degree field at 13x with a 6.2 mm exit pupil.  With well corrected eyepieces like the Panoptics and the Naglers, the field curvature is visible but I do not find it particularly bothersome.  In part, this might be because most of my wide field, low power viewing is done with a 4 inch F/5.4 TV apo that is corrected for field curvature, it'll do a 4.9 degree TFoV with stars that are sharp-sharp right to the edge.  When I am using an 80mm, I am looking for a wider field than that and I am willing to accept that field curvature as the price to pay.  

 

My most recent good-high quality apo/ED was the AT-72ED, it has a 430mm focal length so it offers a wide field but somewhat narrow field, somewhat dimmer view than the ST-80 with the 2 inch focuser.  It is a rather heavy scope for a 72mm, not much less than either the 80mm F/6 or 80mm F/7.  It is a nice scope for someone who doesn't have multiple choices at 80mm but for me, it gives up too much at the low mags and the high mags.  It is not enough smaller that a lighter mount can be used. 

 

We all have different situations and different priorities, we all have different alternatives.  I don't travel much by plane and when I do, I take binoculars.  I do travel to the Boston area to assist colleague with his research.   I recent arranged for him to purchase a Zhumell 8 inch Dob so I have something to observe with while I am there.  Most of my dark sky observing is done from our second home in this high desert with the remainder being done traveling about the southwest in our motor home.  

 

I do like 80mm scopes, my primary interest is the optical capabilities they bring to the table with portability being secondary.  

 

As far as the StellarVue SV-50 F/4 finder. I have one, I like it and like you, I can split Castor with a 3.5 mm. The clear aperture of the diagonal does not allow the use of the 27mm Panoptic without vignetting but it still provides a 6.8 degree TFoV at 8.3x with the 24mm TV Widefleld.  With it's 200mm focal length, there's definitely quite a bit of field curvature but the views are still enjoyable.  The 16mm T5 Nagler provides a 6.3 degree TFoV at 12.5x with a 4mm exit pupil.  That's a surprisingly good view and I often use a O-III or UHC filter with that combo.   Even though it's primary purpose is as a finder, it is a very capable RFT.  CN member "Northofsixty" has an SV-50, also called the Little Rascal, that he uses as his primary scope.  He's lives in the Yukon.

 

In terms of small RFT's, Sheldon Farowski sells an air spaced 70mm F/4.5 objective, it's new-old stock, made in Japan, $26.  I modified a Orion 70mm finder that had poor optics with one.  I split Porrima with it last month, it provides a 4.9 degree TFoV, weighs under 2 pounds with a diagonal. 

 

Carton Orion 1.jpg

 

Anyway, enough rambling.  I like small scopes in this 50mm-80mm and even 100mm range and do quite a bit of observing with them.  For me, my primary interest is not their compact size, it's what they show me at the eyepiece.

 

Jon


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#58 Jon Isaacs

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Posted 19 June 2019 - 08:23 AM

Hence why. Although it's possible, it would be constrained too much. In the end, although it's portable it just sits there on the edge because it has not enough aperture. I'd say that route is better towards monoculars?

 

To be honest, I really do enjoy viewing trough 9x50 finder, so I imagine 50mm ED would work a treat. The idea of a pocketable telescope is great, but rare I think. I wouldn't mind owning one as well!

 

I never heard of stellarvue 50, I thought the skywatcher evoguide 50ED was the first one of 50mm.

It would work great for imaging, but for visual I think it's not enough.

 

There's also the Astro-Tech AT-60ED, a 60mm F/6 FPL-53 doublet.  These weigh 3.25 pounds, probably 4.25lbs with a 2 inch diagonal.  

 

https://www.astronom...ed-doublet.html

 

Jon



#59 Bowlerhat

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Posted 19 June 2019 - 10:53 AM

I didn't really expect 60mm, although I can go that light, I'm not sure that I want it. I was looking at 72mm ED at first as well and wondered how much is the difference with 80mm. Size wise they're still a bit light but have sufficient aperture. Moreover, there are more choices starting from 70mm.

 

I'm not looking for 90mm because I think from that size upwards, I'd rather have it as a stationary scope. No binos as well, I'm using glasses so it's not really comfortable.

 

A bit lighter is alright, the space saved by the tube design will help me a lot compared to the SCT anyway. But I don't think I'll go under 70mm. 

So bits of compromising here and there.



#60 davidparks

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Posted 19 June 2019 - 02:44 PM

I know some folks have gone with 80mm and even slightly larger on the AZ-GTi.  I put my WO Z73 fully loaded with flattener, focus motor, filterwheel, guidescope, guidecam, maincam, and dew strips,  and wouldn't go larger.  The 73mm is perfect for that mount with all the imaging accessories, I would not feel comfortable with anything bigger/heavier on that mount.  Not to mention fits perfectly in my bag with all my gear, with exception of tripod... anything bigger would not fit.

 

I'm sure the 70's, 72's would find the same fit and function.  You'd have to survey the folks with 80mm on the GTi to find out how it handles and how they are using it (visual or imaging), assuming you plan to use the GTi...  I know the C5 works great on it also (C6 is too big for the GTi)

 

Consider your whole kit...  while strictly speaking an 80mm vs 70mm will yield certain scores in areas of interest, such as light gathering, resolution, weight, size, this only tells you how they might compare to each other, you also need to consider how well they will work with your other equipment... particularly the mount, but everything else as well.


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#61 gwlee

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Posted 19 June 2019 - 07:48 PM

wouldn't 60mm too small though? I was thinking of either around 70/80mm

Here’s how i see it. Whether 60, 70, 80, or 100, all of which I have owned at one time or another, it comes down to a  tradeoff between optical performance and portability. Smallest offers the most portability. Largest offers the most optical performance. Which of these two desirable, but mutually exclusive features is most important to you given your observing interests, observing site, and other telescopes that you own and intend to keep? 

 

I usually choose the largest scope that meets all of my portability requirements, so I start my decision making process by carefully defining these requirements. To meet them, the scope must MUST be portable enough that it’s EASY for me to use at the observing site where I plan to use it, not just possible for me to use it.

 

I have been using a 72mm scope at my home observing site for several years, and I am now reasonably sure that an 80mm would be sufficiently portable here and offer more performance. For me, either would be fine for automobile travel to a remote observing site. I wouldn’t want to fly with either of them along with all their kit, but many people find an 80mm fine for air travel.

 

For air travel, I strongly prefer a handheld binocular because it’s the largest instrument that is sufficiently portable to suit me. If I intended to fly with a scope, I would choose one of the smaller aperture scopes because the mount and tripod can be lighter and more compact and still offer sufficient stability. Using a 1.25”  diagonal and just two eyepieces would further increase the scopes portability. For this application, I think something like a TV60 with it’s helical focuser would be hard to beat, but it offers noticeably less optical performance than a 70mm scope. If you own 5” reflector now and intend to keep it, a smaller aperture refractor would be a good complement. 

 

If you aren’t sure you whether you will want to travel regularly with your new scope, I think an 80mm might be a better all around choice, a safer choice, given it’s superior optical performance if you are sure that it will be EASY enough for you to handle around home or wherever you plan to use it most often. 


Edited by gwlee, 19 June 2019 - 08:34 PM.

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#62 gwlee

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Posted 19 June 2019 - 07:51 PM

I didn't really expect 60mm, although I can go that light, I'm not sure that I want it. I was looking at 72mm ED at first as well and wondered how much is the difference with 80mm. Size wise they're still a bit light but have sufficient aperture. Moreover, there are more choices starting from 70mm.

 

I'm not looking for 90mm because I think from that size upwards, I'd rather have it as a stationary scope. No binos as well, I'm using glasses so it's not really comfortable.

 

A bit lighter is alright, the space saved by the tube design will help me a lot compared to the SCT anyway. But I don't think I'll go under 70mm. 

So bits of compromising here and there.

Makes perfect sense to me.


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#63 Jon Isaacs

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Posted 20 June 2019 - 05:22 AM

 

 

If you aren’t sure you whether you will want to travel regularly with your new scope, I think an 80mm might be a better all around choice, a safer choice, given it’s superior optical performance if you are sure that it will be EASY enough for you to handle around home or wherever you plan to use it most often.

 

 

 

One issue that has not been directly addressed is achro versus ed/apo. 

 

For travel, an achromat like the ST-80 or the 80mm GoScope that Tony Flanders is taking to South America have some real advantages.  They are typically much lighter than an apo/ed, an ST-80 with a metal focuser weighs about 3 pounds, with a plastic focuser, about 2 pounds. With their short focal lengths, they can provide 4 degree TFoV with 1.25 inch eyepieces.  They're rugged and inexpensive, loss or damage doesn't represent a great loss.  As Tony said, he just may leave the GoScope with a willing taker, scopes are hard to come by in many parts of the world.  And as a companion to a larger scope, an achromat can handle the low power stuff..

 

The ed/apo advantage is at the higher magnifications, if double stars and the planets are of interest, that's when the better color correction comes into it's own.  An 80mm ED doublet is a more versatile scope than an short focal length 80mm achromat.

 

My story.  

 

When I get a good scope, I keep it.  I like 80mm scopes, they are small but not too small.  I have several. I think in terms of their capabilities. 

 

The ST-80 with a 2 inch focuser is very capable low power, wide field scope, ideal for large nebulae like Barnard's loop.  The AT-80 LE is an 80mm F/6 FPL-53 doublet. It offers nearly the wide field performance of the ST-80 but with very good images at high mags. This makes it an excellent combination for dark sky trips where birding is included.  The WO Megrez II FD is an 80mm F/7 FPL-53 Doublet. It is better at high mags than the F/6 but not quite the wide field scope.  

 

And then there is the Meade 310.  These are Mizar's made in Japan in the 1980s, they're 80mm F/11.3 achromats and I find them to be the best of the many 80mm F/11's I have owned. This one is my third one, the other two ended in as part of a laboratory experiment.  80mm F/11's offer a nice balance, they're not overly long and yet their color correction is acceptable so they make a general purpose scope. 

 

When choosing a larger aperture scope, very often the advice is to choose the biggest scope that it not too big.  When choosing small scopes, very often that thinking gets mixed in.. I want small but a 4 inch is more capable than a three inch.  But then a 5 inch is more capable than a 4 inch.. I think one needs to identify the virtues one wants in a small scope and then choose the smallest scope that is not too small.  For me, that turns out to be an 80mm, mostly because of the high magnification requirements, a 70mm is just too small for my old eyes.  

 

I do own a 4 inch, the TV NP-101.. In many ways, it's a supercharged 80mm.  At F/5.4, it's 540mm focal length is right there between the 80mm F/6 and F/7. So it offers the wide field views of an 80mm apo but has the added zoot and the flat field too boot.  

 

Bowlerhat needs to decide what is important and then decide.  If I were choosing one 80mm, it would probably be the 80mm F/7, with a 41mm Panoptic, it provides a generous 4.75 degree field at 13.5x with a 6mm exit pupil.  It's still compact and performs very nicely at high magnifications. 

 

Jon


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#64 aa6ww

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Posted 20 June 2019 - 10:18 AM

I found that my Ivory 70mm Ranger is a perfect finder scope for me because of its light weight, simplicity and optical quality. I've tried ST-80's and even my TV-85 but a finder should be simple and easy and the perfect helical focuser on the TV Ranger just works for me with a 1.25" Everbrite diagonal and my entire collection of ES 68 and 82 deg eyepieces.

I use it as my finder on my C11 and tried it on my 152ED but the 6" doesn't really need a finder, but I always use one.

I keep bouncing around with different finder scopes on my 6" refractor.

Rangers are very inexpensive used and are always in excellent condition, and weight less then a ST-80 and much higher quality then the Stellarvue 80's which are very expensive. The Ranger takes high power well also because when the optical are very good on a finder, it also becomes a second usable telescope. I use 90mm Finder rings from ADM to make it all work.

 

Optically, I prefer the higher quality image in the ranger then in small 80mm Achromats, since it also makes a nice wide field telescope on my C11.

 

...Ralph


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#65 Mark9473

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Posted 21 June 2019 - 06:07 PM

How significant is the difference?
 

with only 1 cm difference in aperture does it really improve the view a lot? Is it worth it?

It depends a lot on your observing conditions, what you like to look at, and your eyesight.

 

For me, my personal combination of those parameters means that with an 80 mm on many nights I can see e.g. weather on Jupiter (cloud detail beyond the two main belts), rilles on the Moon, and good partial resolution of globular clusters.

A 70 mm doesn't do that for me on those targets. It's not just that a smaller scope shows proportionally less than a larger one as dictated by their size difference; it's about first getting above a threshold of detection of whatever you want to see on a sufficient number of viewing occasions.

 

If my interest was more towards galaxies and diffuse nebulosity then it truly wouldn't matter if I chose 70 mm or 80 mm - they'd both be equally bad at it in my conditions.

On the other hand on open clusters, which I like a lot, they'd both be satisfying on a large number of brighter targets and the difference would only become critical on the marginally visible ones.


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#66 MartinPond

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Posted 22 June 2019 - 12:47 AM

How significant is the difference?
let's say we're comparing the two with similar focal length and focal ratio and mechanism.

 

I'm asking because I understand jumping from 80mm to 102mm, for example, would improve the seeing significantly. However with only 1 cm difference in aperture does it really improve the view a lot? Is it worth it?

So far the answers seem a bit of mixed bag, but the 80mm favoritism is always strong.

Has anyone tried both, how's the comparison?

 

The brightness goes up as the square of the aperture...so the 80mm gives 30% more brightness.

That doesn't matter much for the Moon or Saturn, but for something like the Eagle Nebula,

  it helps a lot...  You could keep pushing the aperture, but the length goes up with the aperture,

  and the moment intertia goes up at least squared, so you have a hard time getting high power

  from a 102mm without a much more massive tripod.  80mm seems like sort of a 

  "sweet spot" in terms of seeing more things but still being "hikable".

 

In the daytime or with low power at night, a 70mm can  be fun, but the 80mm

   does (just) get you into some objects that would be faint at 70mm.  Or...gives you

  a tad more power.

 

I like to carry an 80mm/F5 and an 80mm/F9+minus-violet filter. (achros)

Wide field /lo-pwr and sharp / hi  are covered, the load is balanced in 2 hands, and I can share.


Edited by MartinPond, 22 June 2019 - 12:53 AM.

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#67 SloMoe

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Posted 22 June 2019 - 09:41 AM

Morning Bowlerhat, one thing that hasn't been mentioned so far is the CA factor, 

 

I found the least amount of CA so far to be in the WO Zenithstar 66 Petzval ED, and I have a small collection of Short Tube's now.

At night when viewing anything at higher mags the CA in the non ED refract's I own is there, and you're using a C5 now so you'll most likely see the CA as soon as you pick up the mag past 50X.

 

There is an older William's optics 90mm Megretz in the CN's at a fair price, they have some excellent reviews, and after comparing the performance of the two type's, ED and non-ED, I would only recommend an ED view to someone who is already using a good SCT like yours.

 

What I don't understand is why you want one in the first place, the C5 is just as easy to move around, takes a few min's to align, I think if anything I'd be recommending an 8" sct or a dob for a change in views.

 

EDIT: I guess if you're just wanting low mag wide field views then follow Jon's advice, get an 80mm,


Edited by Mike W., 22 June 2019 - 09:43 AM.

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#68 Bowlerhat

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Posted 22 June 2019 - 08:36 PM

Hi Mike,

 

I think regarding CA isn't it possible to use a filter? I haven't tried it yet though.

 

before I got SCT I do have a small refractor and have enjoyed star hopping in it. What funny is since it was so small I don't really notice the CA so much. When I finally had a chance to see through a refractor again I'm surprised at just how much CA it has compared to SCT. So I think I think I'll just use it for low power scope.

 

I just don't see how I'm going to use a dob. With glasses, it's hard to adjust eye relief. So sometimes I still knocked the scope while trying to get a view, even more, when I'm sitting down. I also feel the movement is rather awkward and the overall size is just way too big. It was also part of the reason why I got a C5 and not C6.



#69 gwlee

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Posted 22 June 2019 - 10:40 PM

I just don't see how I'm going to use a dob. With glasses, it's hard to adjust eye relief. So sometimes I still knocked the scope while trying to get a view, even more, when I'm sitting down. I also feel the movement is rather awkward and the overall size is just way too big. It was also part of the reason why I got a C5 and not C6.

There are telescope eyepieces and binoculars with long enough eye relief that they work well with most eyeglass prescriptions. For that reason, all of my eye pieces have at least 20mm of ER. 


Edited by gwlee, 22 June 2019 - 10:42 PM.


#70 SloMoe

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Posted 22 June 2019 - 11:00 PM

Evening Bowlerhat, filter wise, no there's nothing that is perfect, it's the nature of the optic design, that's why they created APO triplets.

 

I've tried a few so far, the polarize filter and a #8 yellow stacked is ok, inexpensive cover for it, then there's the reduced aperture cap that some use to increase the focal length of the light cone, center cap on most ota dust covers.

 

I've tried the Baader Contrast Booster, makes all the colors very intensified and an almost non resistant CA but kind of a red overtone on everything in the fov.

 

I tired the Baader IR-UV Cut filter last weekend terrestrial in the WO Zenithstar 66  Petzval HD ,color change was neutral and in that scope the fringe was gone, but in the Orion ST-90 I only was using my ND] 25%  and a polarize filter stacked to cut the brightness of the moon & Jupiter, still noticed some CA.

 

I have a Baader Fringe Killer coming for the 90 this week, I've already tried most of the other combinations with the 90 a few weeks ago, not the IR-UV tho, just the polarize and #8 yellow, I guess the reason I'm trying them all is that in almost every thread I've read about CA submission it's been said more than once that they all react differently for each individual.



#71 jag767

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  • Loc: Massapequa, NY

Posted 23 June 2019 - 01:42 AM

For me, I guess it's all relative. I'm close enough to Manhattan that so much is washed out, even my 4" can be less than satisfying. At present, its not feasible to keep my 106 set up and ready to walk out the door with, so i picked up a 72mm apo which i can. The beauty of it is i use a very compact mount, on a photo tripod, and the ota itself is even very svelte, with a dual speed helical focuser. I keep a tiny tray with a barlow and 2 more eyepieces attached to the tripod, and when the insomnia kicks in at 2am, i can give myself a pleasant distraction.

Could i do the same thing with an 80mm? Probably, but it'd be pushing it on the tiny tripod (length not weight becomes troublesome), and my next step up makes it difficult to one hand it out the door with 3 cats who would love to emancipate themselves at any given second. The 72mm does the job. Having just used it, i went out for 25 mins, took a look at Jupiter, Saturn, Amdromeda, and went back in. Conditions weren't that great, and scope prolly wasn't fully cooled down til i was about ready to go back in, but it was enjoyable. Andromeda looks like little more than a faint blemish from here, but the planetary views were fair, and fantastic compared to not going out at all.

The other benefit to this setup is it breaks down into such a small footprint (comfortablebackpack load), i could bring it with me anywhere i go. While an 80 still breaks down small, its not quite as portable if i were to split hairs. 432mm f6 vs 600mm f7.5 for the same budget or thereabouts? (or 480mm F6 if we are not keeping budget in mind)
  • CollinofAlabama and BGazing like this


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