Not getting quality images in telescope

Question

I have a Newtonian telescope (Celestron 130 eq) with 5.2 inch (~132 mm) aperture with 650 mm focal length, Spherical mirror. And having 4mm, 10mm, 12.5mm, 20mm,20mm (erecting eyepiece), a 3x and 1.5x Barlow. All eyepieces are non-plossl.

I am not getting crisp images when looking at planets like Jupiter and Saturn even when the magnification I used a setup of about 244x, (4mm + 1.5x barlow), not exceeding the 307x limit. When using 4mm (without and with 1.5x barlow) I am getting blur and highly chromatic images.

I am pretty sure that the telescope is collimated, not absolutely sure though, I don't know how to find whether its collimated or not. And the 4mm, 12.5mm, 20mm (non erecting) are all 1 inch eyepieces where the telescope is of 1.25 inches, using an adapter to fit in. However 12.5mm, 10mm gives somewhat ok ok images.

I am living in a Megacity with light pollution of Class 7. I am looking forward to have some tips and guidance on this. Cheers !!

Welcome to Astronomy SE! I'll leave some quick comments but I am sure someone will add a helpful answer in a day or two. If the aperture is 5.2 inches or about 132 mm and the focal length is 650 mm, then this is roughly an f/5 mirror. If it's spherical rather than parabolic you'll have some spherical aberration that will get rapidly worse off-axis. If your mirror isn't collimated then you are always off-axis, which is a big problem. The chromatic aberration can come from your diagonal if it's a prism and misaligned, or from your Barlow or eyepieces especially if they are not high quality. — uhoh, Aug 10 '21 at 10:57
Please add some more specific information on the model of your telescope and the Barlow and eyepieces. Thanks! — uhoh, Aug 10 '21 at 10:58
This may seem like a no-brainer but do you let your telescope come to the same temperature as the outside for a while before taking images? When I was new to observing I had the same issue and it was because there was condensation from the temperature change. One way to test is to see if you can see clearly inside (if possible ie. From a window). — Astroturf, Aug 10 '21 at 13:50
Yes @Astroturf I always let the telescope cool for an hour before stargazing. — Kavin Ishwaran, Aug 11 '21 at 03:47
Could you say where you are observing from - at the very least the latitude. — Dr Chuck, Aug 11 '21 at 08:05
@DrChuck I am observing from Chennai, Latitude is about 13 degrees. The major problem is only chromatic images — Kavin Ishwaran, Aug 11 '21 at 08:43

score 4 · Accepted Answer · answered Aug 11 '21 at 13:22

There are several factors that can contribute to poor quality images. In no particular order:

Tube currents – the telescope needs time to cool down or warm up to the outside temperature, otherwise air currents will form in the tube. If this happens an out of focus star will look like it’s boiling.
Rising air in line of sight – if you are observing over a busy town or city, there will likely be hot air rising from buildings which will cause poor viewing. One solution is to wait until most people have gone to bed, so that heat from buildings drops. Another is to view objects that are relatively high in the sky, say above 45°, so that you are looking through less hot and unstable air.
Seeing – the prior point relates to the lower atmosphere, but air turbulence higher up can also degrade images. There’s not much an amateur astronomer can do about this.
Low altitude of object viewed – this is similar to point 2, but if you were looking at Polaris from Chennai, there’s not much you can do. Likewise for me in London, looking at Saturn for the next few years, it just doesn’t get very high in the sky.
Collimation – this essentially the correct alignment of all the optical components. The telescope manual should explain what to look for and how to fix it. There are also plenty of information sources on the internet.
Too high magnification – although the scope manufacturer says maximum magnification of 307, that’s very optimistic. A rule of thumb is that the maximum magnification usable in a very good quality telescope, in excellent seeing conditions, is about 50x the telescope aperture in inches (so 250 in your case). Beyond that and you just magnify noise. However for the Mars, Jupiter and Saturn, which have a lot of fine, bright detail, higher magnification than this may help you see more detail, in good conditions.
Poor quality optics including eyepieces – Celestron is a respectable brand, and even for a beginners scope, the optics should be reasonable. Although the mirror is spherical, rather than parabolic, the distortions caused by that should not be too bad near the centre of the field of view
Chromatic aberration in refractors (included for completeness) – because different colours of light are bent slightly differently by lenses, different colours come to focus at different places. This can be addressed by optical designs using 2 or 3 lenses in the objective, and special types of glass, but these solutions add to the cost and weight of the telescope.

Not getting quality images in telescope

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