Terms of use

🔴 Never point a telescope at the Sun without a front-aperture solar filter

This is the one genuine way to be permanently injured by amateur astronomy, and it happens faster than you can react. An eight-inch telescope concentrates roughly a thousand times the light and heat your unaided eye receives. Pointed at the Sun without the right filter, it can destroy the retina in under a second — painlessly, because the retina has no pain receptors, so there is no reflex to stop you and the damage may not be obvious until hours later.

The only safe filter is a purpose-made front-aperture solar filter: one that covers the full opening of the tube, rejecting the light and heat before it enters the optics, made for the purpose and certified to ISO 12312-2. Fit it over the front, check it for pinholes or scratches every time, and secure it so it cannot be knocked or blown off while you are at the eyepiece.

Never use a screw-in “sun filter” that threads into the bottom of an eyepiece. Some cheap telescopes still ship with one. It sits at the point where the full aperture has already been concentrated into a few millimetres of focused light, it gets extraordinarily hot, and it can crack without warning while you are looking through it. If your telescope came with one, throw it away rather than leaving it in the box for someone else to find. Eclipse glasses are rated for the naked eye and are not a telescope filter either — nothing that goes near your eye instead of over the aperture is safe.

Two more things people forget. Cap or remove the finderscope — it is a small unfiltered telescope of its own and will burn an eye or a face just as effectively. And never leave an uncapped telescope unattended in daylight, especially where children can reach it: an unattended scope that drifts onto the Sun can scorch what is beneath the eyepiece and will not wait for you to come back.

Solar observing done properly is safe, common and worth doing. It is done with equipment bought for the job, and this site is not a substitute for the instructions that come with it.

Our figures are planning aids, not performance guarantees

The calculators and tables on Aperture are computed from published specifications and standard optical formulae — magnification from focal lengths, exit pupil, true field, Dawes’ limit, limiting magnitude, imaging sample scale. They tell you what a given combination of numbers implies. They are honest arithmetic, and arithmetic is not an observing report.

What you actually see on a given night depends on things no formula on this site can know: atmospheric seeing and transparency, which routinely cap useful magnification far below the theoretical maximum; collimation, since a misaligned mirror throws away aperture you paid for; thermal equilibrium, because a tube brought out warm shows tube currents until it has cooled; light pollution at your site; and sample variation — two units of the same model can differ in optical quality, and the specification sheet does not record which one you received. Mount stability, eyepiece quality and your own dark adaptation matter as much as the numbers.

Treat every figure here as the ceiling that physics permits, then expect a real night to fall short of it.

Purchase decisions are yours

We publish research and computed comparisons to help you choose. We do not sell telescopes, we do not handle your order, and we have no part in the transaction beyond sending you to Amazon. The decision to buy, and the consequences of it, are yours. Check the current listing, the return window and the seller before you commit, and satisfy yourself that a telescope suits your site, your storage and how much weight you are willing to carry outside. To the fullest extent permitted by law we are not liable for a purchase you make after reading this site.

Accuracy, prices and updates

We work hard on accuracy and we show our workings, but this is a website and errors happen. Prices, star ratings, review counts, availability and specifications change constantly, and the figures here are a dated snapshot rather than a live feed. Amazon listings also drift — a listing can quietly become a different model or a different aperture. Treat any price on this site as historical and check the current one before buying. If you find something wrong, tell us; corrections are welcome and we would rather fix a page than defend it.

No warranty

The site is provided “as is”, without warranties of any kind, express or implied. We do not warrant that the information is complete, current or fit for a particular purpose, and to the fullest extent permitted by law we are not liable for loss or damage arising from your use of it — including damage to equipment, to optics, or to a night you spent setting up for something the numbers suggested you would see. You are responsible for how you use your own telescope.

Third-party links

We link to Amazon, and to other sites — including Cloudy Nights, a forum that sells nothing and pays us nothing, where the hands-on comparisons we cannot make already exist. We do not control those sites and are not responsible for their content, their products or their privacy practices. Commercial links are disclosed on our affiliate disclosure page.

Content and reuse

The writing, calculators and original analysis on this site are ours. Quote us, cite us and link to us freely — that is what the internet is for. Republishing pages wholesale is not on. Product names, brands and trademarks belong to their respective owners and are used here descriptively; no affiliation or endorsement is implied by either party. Product images are served from Amazon and remain Amazon’s or the manufacturer’s.

Governing terms

These terms are governed by the laws of [STATE — TO BE CONFIRMED BY THE SITE OWNER], United States, without regard to conflict-of-laws rules. If any part is unenforceable, the rest continues to apply. We may update these terms, in which case the date at the top of the page changes. Questions go to contact.