A Viewing Question

[UK1 (Rob)]

A Viewing Question
I went to a friends house, he was showing me how to set up a scope in case I was doing it wrong as I have trouble spotting anything but set up seemed to be ok. ( thats for another day )
The viewing question is………. We looked through his ED80 +2x barlow, 15mm eyepiece, we saw the moons of Jupiter very sharp and a very good view of the bands, also just saw red spot.
When looking through my 8" + 2x barlow, 15mm eyepiece, I also see the moons very sharp, but, cant see the bands, Jupiter is very bright and large, even with a filter...strange…… any ideas are welcome ………..I know the scope seems to be collimated ok otherwise not see moons sharp or other stars.

[Waxing_Gibbous (Peter)]

Too much of a good thing maybe. Asssuming all else was the same, you may be getting too much light and its overwhelming the ability of the eyepiece to distinguish detail. Try a little less power. Around a 10-16mm EP should be more than adequate for the longer focal length.
Also, and since I've never owned an SCT I could be wrong, but the central obstruction tends to cause a minor loss of detail compared to refractor.
G'luck.

[UK1 (Rob)]

thanks for the reply 'll give it a go

[shane.mcneil]

Hi Rob

Sorry can't help, but I think I have the same issue. I just bought a 10" dob and I can get the moons sharp but Jupiter itself is not as sharp as I was expecting. Though I did see a moon shadow on it. I also get a cross of light extending vertically and horizontally with Jupiter centered in it. I assume I still haven't got my collimation right?

Perhaps focusing on a point of light is different to seeing planet detail. I'm waiting for the moon to come back so I can see how sharp it appears.

Regards

Shane

[mental4astro (Alexander)]

Hi UK1 & Shane,

is it an SCT or a newtonian you have? Reason being the longer focal length, hence a larger f/ number, would mean easier planetary viewing.

Newtonians, especially dob mounted ones, have fast focal ratios, making the images brighter for a given EP which can wash out detail in planets.

One trick used by the giant dob users when viewing the planets is to 'stop down' their scopes by placing a cardboard with a hole cut-out, with a diameter smaller than the mirror of thier scope. This increases the f/ratio and improves contrast in fast scopes. Much like closing the diaphram in a camera does.

You might like to give that trick a go. Cheap as chips a solution. Tried it on my 10" f/4.9 dob & it worked.

Another thing to keep in mind is the prevailing atmospheric conditions. Higher powers may not be viable, regardless of the scope you have. This last Saturday night, though nice for wide field viewing it was lousy for high power. Could bearly make out the two main bands on Jupiter beyond a dirty smudge. I was using my 17.5" dob, which is fast, f/4.5, but I rarely use a mask for high power, using a 5.5mm EP (363X). I'm more of a casual planetary observer. More serious observers would use one.

The month before the same issue & I was able to view through my dob & a 7" maksutov (f/12). Poor atmospheric conditions too, though not as bad.

I've only been lucky enough twice to get good seeing conditions to see good detail on Jupiter so far this year. In other postings here in IIS some folk have struggled most of the year as well for good conditions. This year seems to have been a rough one for good, stable atmospheric conditions.

[astro744]

The cross you are referring to is caused by the spider of the secondary mirror holder. If you have a four vane spider you get 8 lines with pairs overlapping each other resulting in 4 very bright lines.

If you have a three vane spider you get 6 sines but not as bright since the diffraction energy is dispersed with no overlapping.

If you have a curved spider you get no lines since the difraction energy is dispersed radially, (also less contrast).

If you broaden your spider with removeable self adhesive black felt you will get much fainter lines but will get a halo around planets. It'll all come down to what you dislike the most. The effect on Jupiter is particularily bad with a four vane spider.

A refractor with no spider vanes shows no diffraction spikes. A SCT telescope with no spider vanes shows no diffractions spikes but still shows diffraction as the light energy is spread outward from the centre and the most noticeable effect is less contrast.

In response to the original post, you did not state focal length of the 8" so I'm not sure what magnification you are getting. I doubt you have a 600mm f.l. 8" (would be f3) so the effect is purely a difference in magnification. Try eyepiece combinations that give the same magnification on both telescopes and then compare.

[Allan_L (Allan)]

Seeing detail in planetary objects is so much a cocktail of variables, don't necessarily assume you have something wrong.

depending on type of scope, the magnification from the same eyepiece may be too much for your scope (depends on focal length, and to a smaller extent on aperture)

Also, viewing before allowing the scope to cool to ambient temparature can majorly deteriorate viewing experience. Open Newtonians are generaly quicker to cool here than a closed system. And the bigger the set up the longer it takes to cool.

Atmosphere problems have been plentiful as well lately. If close to the horizon, this is amplified. Locally, we have had a lot of crap in the atmosphere from fires and such. And there has been persistent "whispy" clouds that are not apparent visually, but can fuzz up the seeing something bad.

So allow time to cool, view near Zenith, and hope for clear skies.

The tip about closing off some of your (expensive) aperture is also a good one some times. One of my better views on an early Dob was with the end cap on (with the smaller (solar viewing?)opening open).
But make sure your opening avoids the secondary obstruction!)

a simple addition of a Nuetral density (moon) filter can also have a similar positive effect on views of a bright Jupiter.

I'm newish too, and only know enough to be dangerous, but these all conspire to fuz up my Jupiter regularly.

Hope to have helped.

[UK1 (Rob)]

thanks for all the tips they are makinging things a bit easier to understand please keep them coming

btw, 4 got to say it is a8' newtonian