Thought on refractor upgrade

Hello:

I have 2 refractors, an 60 mm and an 85 mm. It is clear that a 25 mm differential in aperture is worth the upgrade. The gain in light gathering capacity and resolution is obvious even to the casual observer. To move up from an 85 mm, what would be the minimum aperture to consider to make it worthwhile? Moving up to 90 mm is probably not worth the trouble, but what about 100mm, or would it have to be 110?

Rey

Hi, I have a TV-85 and a Celestron/Vixen Fluorite 90. The major difference is the focal length, f/7 vs f/9. Doesn't sound like much, but translates to not needing a barlow for high powers.

Having looked through several 4" refractors, I would say you are right: minimal difference going from 85mm to 90mm, but a little more visible going up to 100mm. And, comparing a TV-101 to a Vixen ED 114mm (both f/5.3) the larger scope definitely is brighter.

I would say any 10mm increase in aperature would be visible. Going from your 85mm to 100mm would a jump in performance. A jump to 110mm (or more) would be a bigger jump. Either would be worth it.

Don't forget you will need an appropriate mount.

Steven

When I looked at the 85mm TV and the 127mm Meade ED side by side, the difference was pretty obvious. There is a lot more light grasp with the 127mm and the resolution increase was also apparent.

Reynaldo:

Others have made some good comments. My questions:

What other scopes do you have and what do you want to do with this one?

If this is to replace the TV-85 as a portable "grab and go" scope, then the OTA size and focal length become important to consider so 102 or 110 mm is probably about as large as you want to go.. (I noticed an AP Traveler in the classifieds)

Otherwise, you may want to consider something considerably considerably larger so that you gain different capabilities. As Floyd said, there is quite a difference between a 127mm and the 85mm. Of course that difference includes the 2.3x increase in light gathering and 1.5x increase in resolution as well as the similar increases in weight and length. Which means... Bigger mount, less portable, smaller maximum TFoV.

Personally I find the 80mm of the ED-80 enough and it is still quite portable. I don't find the added aperture going to a 100mm scope a worthwhile trade if I am losing portability. But a good part of that is that I have a variety of scopes so that if I want the added resolution and light gathering over the 80mm I will probably choose a larger reflector or cat.

So, it really depends on what you want to do and how this new scope fits in with your others.

jon

Relatively speaking.... You will find that experienced observers will notice just a few MM in diference with everything else being equal (this goes for eyepieces as well). There is no magic number that says you have to go from A to B before you will see a difference. And yes, to most experienced observers these slight differences may be worth it.

>>>At the size instrument you are speaking of, 5 or 10 mm makes quite a difference in seeing.
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As compared to a TV-85, an equal quality 90mm with gain about 0.12 magnitudes, an equality 95mm woud gain about 0.24 magnitudes.

In the small picture, these are probably noticeable but my thinking is that the differences in conditions from night to night are normally at least this great. I don't see such small increases as being worthy another scope, it just "ain't that much."

In the big picture, going from an 85mm to say a 150mm provides an increase of about 1.2 magnitiudes, in my view, this is certainly worth it.

The advice I see most often is to double your light gathering when stepping up. From the TV-85, this would be 120mm scope and this would go about three quarters of a magnitude deeper.

Jon

I want to thank all for taking the time to post your opinions. You did give me a lot of useful information. It’s great to pick other people’s brains and learn about their experience in this hobby.

By the way, I do not plan to get rid of the TV 85 for the reasons others already pointed out, even though I have larger scopes. I don’t think you can do much better in terms of performance at this aperture, and yes it is very portable. It is the scope I take with me during family trips. If anything, I may have to part with the 60mm. On this one I agree with my wife, at this rate, I am going to end up with too many scopes.

What prompted my query is that, during the last Mars opposition, I was not able to discern much detail on its surface with my refractors. I specifically purchased the TV 85 at that time for this purpose, but it fell a bit short on the resolution end of things. A Celestron C5 did not do much better. Thankfully the C9.25 did give me somewhat better views, but it is a choir to set up. I convinced myself that I would have done better with a bigger refractor, although in all honestly, I think the seeing was terrible from my location during those nights. I did not have an opportunity to drive to a better viewing site.

After reading over these posts, it seems that the next telescope purchase is one that I am going to mull for a while. First I have to sell some of the ones I already have.

Clear Skies

Rey