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3" vs. 4"

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#1 Victor Kennedy

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Posted 23 October 2003 - 06:37 AM

OK, I know about light-gathering ability being proportional to the square of the diameter, three-color crossing and all that stuff, but am I going to be happy :jump: or disappointed :bawling: if I exchange my decent 80 mm achromat for a 102 mm something-or-other? I keep seeing these ads on Astromart for TV Genesises, Tak 102s, etc., and thinking, "Hmm, maybe I could afford that."

This would not be my only scope. I have an SCT, but I really enjoy the quality of the view through my refractor. The question is, does the increase in aperture from 80 to 100 mm make that much difference?

All input is welcome, of course, but I would be particularly happy to hear from folks who own, or have owned, both.

#2 Ron B[ee]

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Posted 23 October 2003 - 09:22 AM

This would not be my only scope. I have an SCT, but I really enjoy the quality of the view through my refractor. The question is, does the increase in aperture from 80 to 100 mm make that much difference?


Well, Victor which 80mm achromat do you have, Vixen 80mm f/11, ST80, etc? As you can see, it can make a big difference when considering the upgrade. You already have the SCT but like the view of the refractor but you should think about what you would use the 102mm refractor for?

To answer your "difference" question, there is a difference between 80mm and 102mm, but it won't be jaw dropping and more like subtle difference, the ability to to see very small fine details in the 102mm while eluding the 80mm. Here's a comparison I made (with 85mm).
http://members.cox.n..._TW-85_moon.htm
And because you're looking at Tak, TV, etc, you'll see a very noticeable difference in sharper more contrasty view through the APO than through the ST80 for instance.

Ron the 4-inch Tall Evangelist B[ee]


#3 Victor Kennedy

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Posted 23 October 2003 - 10:01 AM

Hi Ron, I was hoping to hear from you. I have the Stellarvue AT1010, f/6, which gives very nice low power, wide field views, but runs out of steam over about 120x. I love the view through a 34 mm, 2" eyepiece, but when I want to zoom in with a higher power, things get dark and grainy. In August, Mars was small, dark, and blobby, while I could see a lot more with the C8. I'm thinking a 4" apo or semi-apo would still give nice wide fields of view in a portable package, but would also give a brighter image with more detail at higher magnifications. Not least in my thinking is your reports of observing with your Light Cup.

#4 jrcrilly

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Posted 23 October 2003 - 10:17 AM

Hi, Victor.

I'll confirm that the 4" APO I had handled magnification much better than the 80mm semi-APO. Less dimming and more detail.

#5 Ron B[ee]

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Posted 23 October 2003 - 05:44 PM

Hi Ron, I was hoping to hear from you. I have the Stellarvue AT1010, f/6, which gives very nice low power, wide field views, but runs out of steam over about 120x. I love the view through a 34 mm, 2" eyepiece, but when I want to zoom in with a higher power, things get dark and grainy. In August, Mars was small, dark, and blobby, while I could see a lot more with the C8. I'm thinking a 4" apo or semi-apo would still give nice wide fields of view in a portable package, but would also give a brighter image with more detail at higher magnifications.


Well Victor, the 4-inch Tall Evangelist is always vigilant in helping anyone looking for a 4-inch telescopes ;).

The AT1010 while great for wide field and medium power, was never designed for high power use. The 4-inch refractor will give you a brighter view and in case of an APO be able to attain much higher magnification than the AT1010 assuming you have good seeing. In terms of brightness, the view will be brighter than the AT1010 but not as bright as through the 8-inch.

In order for the Light Cup to serve you best, first a few more questions :crazy::

o What was the high power you use in the AT1010?

o How do you like the view through your C8? For instance, although I don't have an 8" SCT, the view through my 8-inch Dob is more detailed than my 4" APO. If you like your C8, think about how the upgrade will complement your C8.

o One of the strength of the AT1010 is its portability; the 4" refractor while still portable is just not in the same portability class. So are you ready for a less portable refractor?

On the other hand, you might just want to consider an 80mm/85mm APO and if you like the AT1010, SV has a couple of models to offer(forget about the SV85L :foreheadslap:) perhaps for a quick look and still deploy the 8" SCT for real serious look (SV also have a couple of 102mm APOs as well.) Of course, there are other fine choices such as Tele Vue, TMB, Tak, Vixen and AP. Besides, it's will still be small enough to travel with ease. The 80mm/85mm APO will allow higher magnification with crisp view than the AT1010. But remember, it'll still be aperture limited despite those reports you hear in some group about 300x on the planets through the 80mm/85mm APOs - well let's just say I personally wouldn't like the dim, lack of contrast view at that magnification :bawling:. But ~200x ought to be good enough. With my 4-inch APO, I find 293x is just about the limit of an acceptable view on those rare nights with 220x-240x the more likely scenario. As you know, the 8-incher can keep on marching without lacking contrast and brightness. So if you want to have a minimum aperture for a serious observation of the planet and I mean bare minimum, I follow the recommendation of the ALPO and consider the 4-inch aperture suitable (and boy there were times when I wish for a larger APOs :jump:).
http://www.labbey.co...Telescopes.html

Personally, I find myself still using my Light Cup more than my Dob despite seeing less (if this makes any sense) because the Light Cup can be deployed and gave good view almost immediately whereas my 8-inch Dob needs to cool down. (The latest review of the TEC-140 in S&T magazine went into this point a wee bit.)

If you like to see how my 4-inch Light Cup compared to my 8" Dob, I invite you to have a look at these links.
http://members.cox.n...-TV102-PDHQ.htm
http://members.cox.n...90803-TV102.htm
http://members.cox.n...-TV102-PDHQ.htm
http://members.cox.n...-TV102-PDHQ.htm

Ron the 4-inch Tall Evangelist B[ee]
PS - Just this weekend, I saw the Cassini Division (ie. gap with width!) through my 8-inch Dob that I could never see in my 4-inch APO :( :cool:.


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