M84 (NGC 4374) with a Smart Telescope: Common Mistakes and How to Fix Them

Planning the Night Around Altitude

Starting before the galaxy is high enough

M84 reaches its highest point at 68° from 35° N, 53° from 50° N and 42° from 35° S. At any latitude, the galaxy is lower in the sky early in the evening than it is later on. A low target sits behind more atmosphere and more suburban glow, so frames are softer, noisier and more affected by the gradient in the sky background.

Check the altitude in your planning app before you start. Wait until the galaxy is well up in the sky, ideally with a clear margin above the rooftops and tree lines around your site, before you begin a long stack. If you must image it early, expect a weaker result and plan for more frames.

Treating "best months" as a fixed time

April and May are the best months for M84, which means the galaxy is well placed in the evening sky during those months. The exact hours depend on your longitude, your time zone and the date. Do not assume that any evening in April or May will give you the same altitude. Use the app's sky view for your actual location and time, not a rule of thumb.

Expecting a Galaxy That Looks Like a Photograph

Published images of M84 are long exposures that have been processed heavily. The stacked view from a smart telescope is much more modest. The core is brighter than the outer parts, and the outer halo is faint enough that it takes many minutes of stacking to show.

Set your expectations before you start. The galaxy's apparent size is 7.4′ × 6.4′, so it fills a small part of the field, and the surface brightness is low. Within a few minutes of live stacking you should see a clear oval with a brighter centre. Do not expect spiral arms, dust lanes or bright colour, because an elliptical galaxy has none of those features.

Stopping the Stack Too Early

The most common reaction to a faint target is to stop once something appears. The first preview often shows a bright smudge, and it is tempting to accept it as the finished result. That smudge is usually only the core, and the galaxy's outer edge is still noisy.

Let the stack run until the outline of the galaxy is smooth on all sides. Check the frame count and the stacking progress in the app. Once the core is well defined, keep stacking for a further period so that the faint halo has enough data to appear. Stopping at the first recognisable shape is the most likely reason a stacked image looks grainy.

Pointing at the Wrong Patch of Virgo

Virgo contains many galaxies, and several of them are faint and similar in size. A common mistake is to select a neighbouring galaxy, or to accept a smudge that is close to M84 but not on it. This is easy to do when the target list shows only a name and a rough position.

To find M84 by star hopping, start at Denebola, the tail star of Leo. M84 lies about 9 degrees to the east of it, with a slightly higher declination. Vindemiatrix, in Virgo, lies roughly the same distance to the east, slightly south of M84. The galaxy sits roughly midway between these two stars, so you can use the pair as a check. If the galaxy you have selected sits far from that midpoint, you have the wrong target.

Once the telescope has slewed, confirm the object name in the app and compare the centred field with a planetarium chart. Expect several faint galaxies of similar brightness in the surrounding area. Only the galaxy you have selected should be centred.

Framing the Galaxy Too Small or Off Centre

Because the galaxy is only 7.4′ × 6.4′ across, a small error in centring can put it near the edge of the frame. After stacking, a galaxy at the edge of the field may appear cut off or distorted by lens and sensor effects.

Use the app's centring routine after the initial slew, and check the centred position before you begin stacking. If your device offers a crop or framing option, use it to keep the galaxy in the middle of the image rather than zooming, which adds noise and does not add detail. Re-centre if the target drifts during a long stack.

Mistaking Sky Glow for Galaxy Structure

In a suburban sky, the background brightness is uneven. A gradient, brighter towards the horizon or towards a street lamp, often runs across the frame. Because a faint galaxy is only slightly brighter than the sky, the gradient can make one side of the galaxy look brighter than the other, or can make the halo look lopsided.

Check whether the gradient is smooth and broad, which is the sky, or whether it follows the shape of the galaxy, which is the target. If your device offers a background gradient correction, apply it after stacking. If it accepts a light pollution filter, fit one before you begin. Image the galaxy when the Moon is below the horizon or far from Virgo, because moonlight adds to the background and makes the gradient stronger.

Reading Noise and Artefacts as Structure

Faint targets produce a noisy stack. Mottled blotches, faint streaks and residual star cores can look like features, and they are easy to mistake for dust lanes or a companion galaxy. Elliptical galaxies have smooth light and no dust lanes or spiral arms, so any structure with those characteristics is likely to be an artefact.

Compare the stacked image with the expected smooth oval. If the faint halo looks blotchy, add more frames. If a star's residual core sits on the galaxy, check whether the star is genuinely in the field and whether your device has a star-removal option. Do not describe a feature as real until it appears consistently across more than one stacking session.

Entering the Wrong Time or Location

A smart telescope relies on the date, time and location you enter to calculate where the sky is. If the time zone is wrong, or the location is set to a city away from your site, the telescope may point to the correct coordinates but the sky positions in the app will not match the real sky. Plate solving and tracking can fail, and the galaxy may appear in an unexpected place.

Before each session, check the date, time zone and location in the app's settings. If the target appears in the app's sky view at the wrong altitude, correct the settings and slew again. This check takes only a minute and prevents a wasted night.

Expecting Colour

Many people expect the galaxy to appear in colour because they have seen colourful images of other objects. M84 is a pale, nearly neutral galaxy, and a stacked view will show faint, uniform tones. A nearly colourless image is normal for this kind of target and does not indicate a fault in the equipment or the stack.

Quick Checklist

1. Check the galaxy's altitude for your latitude and the time you plan to observe, and wait until it is well up.
2. Confirm the date, time zone and location in the app.
3. Star hop from Denebola towards Vindemiatrix and check that the galaxy is midway between them.
4. Centre the galaxy and check its position before you start stacking.
5. Let the stack run until the outline is smooth, then continue for a further period to bring out the halo.
6. Check the gradient and artefacts before you accept any feature as real.