The Short Tube 80

[Richard N]

At our recent Star Party I greatly enjoyed using my little Meade Adventure Scope (ST80). A test that I did was interesting. I have never managed to split the double double with it. As @cajen2 was willing to let me try his Morpheus eyepieces I was quick to try again. Even a 7mm + a 2.5 Barlow didn’t split them. Possibly some ovalness was present. I’m wondering if the basic optics are just not quite up to the job? I accept that it could be down to the seeing. Any thoughts?

It’s a lovely scope for everything else. At home it splits down to about 5 arc seconds 

Edited November 13, 2023 by Richard N

[John]

Epsilon Lyrae has separations of 2.2 and 2.8 arc seconds between the two pairs. Theoretically an 80mm scope should be able to resolve to around 1.5 arc seconds but this does assume perfect optics and conditions. In reality neither are encountered that often !

I'm sure that I have read of folks splitting the double double with ST80's but the optical quality of these fast F/5 achromats can vary. 

Were other folks with similar apertures getting the split at that time ? Seeing conditions can make things tough and scrub a lot off the theoretical optical performance.

 

 

Edited November 13, 2023 by John

[Louis D]

Try using a #56 green filter to see if you can eliminate some of the inherent chromatic aberrations at the violet and red ends of the spectrum.  That, and minimize the spherical aberration that can vary across the spectrum, generally being lowest in the green part.  I have an ST80, but I consider the views too mushy without heavy filtration to be very usable.   My 6" f/5.9 refractor is even worse with respect to chromatic aberrations, but a bit better wrt spherical aberrations.  Both scopes are best used on large, dim objects at lower powers.

[Richard N]

1 hour ago, Louis D said:

Try using a #56 green filter to see if you can eliminate some of the inherent chromatic aberrations at the violet and red ends of the spectrum.  That, and minimize the spherical aberration that can vary across the spectrum, generally being lowest in the green part.  I have an ST80, but I consider the views too mushy without heavy filtration to be very usable.   My 6" f/5.9 refractor is even worse with respect to chromatic aberrations, but a bit better wrt spherical aberrations.  Both scopes are best used on large, dim objects at lower powers.

Thanks. I think basically it’s a cheap OTA with relatively lax manufacturing tolerances. It actually does a very good job overall but the weaknesses show at the performance limits. We did intend doing a comparison between the ST80 and Clive’s Starfield 102 but that didn’t come off. Of course every aspect of the Starfield will probably be superior to the ST80. 

[Stu]

I have one so can give this a go, when it is next clear….. 🙄

One comparison I want to do is ST80 vs LT80AZ to see the difference between short and long focal length entry 80mm scopes.

Accessories make a huge difference, were you using a decent diagonal or the standard 45 degree prism?

[Richard N]

3 minutes ago, Stu said:

I have one so can give this a go, when it is next clear….. 🙄

One comparison I want to do is ST80 vs LT80AZ to see the difference between short and long focal length entry 80mm scopes.

Accessories make a huge difference, were you using a decent diagonal or the standard 45 degree prism?

I use a Celestron diagonal. 

[Gonariu]

A few years ago I solved the double-double ε Lyrae with an old 80/400 (the Konus Vista), at difficulty; putting a Barlow X2 in it also split it without problems. I had used an old mirror diagonal supplied with the Konusuper 120.

[cajen2]

There's a thought, Richard. I should have offered you my Tak prism diagonal, just to see if it made any difference. I'd have been very interested to see. It never occurred to me, sorry.

[cajen2]

I also think using such a small aperture and focal length scope with any filter is counterproductive. When we tried an Astronomik UHC, it did nothing for the view except make it darker.

I have to say, though, that Richard's Meade was an extremely competent all-round scope when you consider its cost 

[bosun21]

12 minutes ago, cajen2 said:

When we tried an Astronomik UHC, it did nothing for the view except make it darker.

I’ve never used a UHC filter in a 80mm scope, but it’s certainly beneficial in my 102 and larger scopes. It does darken the background but brings out the details on emission nebulae. My best ever view of M42 was with a UHC filter.

[Stu]

27 minutes ago, cajen2 said:

I also think using such a small aperture and focal length scope with any filter is counterproductive. When we tried an Astronomik UHC, it did nothing for the view except make it darker.

I have to say, though, that Richard's Meade was an extremely competent all-round scope when you consider its cost 

A lot of this relates to exit pupil. Keep the mag low and therefore the exit pupil large (5mm or more) and filtered views can be excellent even in small scopes. I’ve used a Lumicon OIII filter with a TS 72mm and long focal length eyepiece very successfully on the Veil or NAN for instance. Not so good for higher power views of PNs for example as it gets very dim as you say.

[cajen2]

Sure but Richard was using a high- mag EP (Morpheus 6.5mm iirc).

[Stu]

Some info in this ‘unfair fight’ comparison I did….

….and a bit more here, with some quick examples of the benefits or using a better diagonal and eyepieces. The fundamental optics are fine, albeit always limited to an extent by being a fast achro, but it is a small aperture so things are not as bad as with a 150mm f5 for example.

 

[Stu]

1 minute ago, cajen2 said:

Sure but Richard was using a high- mag EP (Morpheus 6.5mm iirc).

I was referring to your findings on filters with small scopes. Did you use them at low power with a large exit pupil? Dark adaptation also helps a lot with filtered small scopes; the image is dimmer, but the contrast better so visibility is improved so long as you are dark adapted and keep the exit pupil large. Hope that makes some sense 😄

[cajen2]

No, as I said, R was using a Morph 6.5: relatively high mag so I think that was the limiting factor. We were both well dark adapted.

[Stu]

1 hour ago, cajen2 said:

No, as I said, R was using a Morph 6.5: relatively high mag so I think that was the limiting factor. We were both well dark adapted.

Ah, ok. I read it as that you had tried this before but that’s not what you wrote 🤪🤣. If you try again, use a low power and see what happens 👍

[cajen2]

'twasn't me, it was Richard. I was busy drooling over the Pleiades with the Starfield at the time....

[Stu]

32 minutes ago, cajen2 said:

'twasn't me, it was Richard. I was busy drooling over the Pleiades with the Starfield at the time....

Ah, well now you are definitely playing with me as you said when ‘we’ tried it! Give an old guy a chance will you 😉

[Richard N]

Thanks for the responses. I conclude that the ST80 is what it is; a good value OTA. It’s no astrograph but it probably does most things acceptably well. It remains a lot of fun to use!