Celestron Astromaster 70AZ - Review and Improvements

[Rocket_the_Raccoon]

The Celestron Astromastrer 70AZ is an entry-level 70mm f/13 long refractor on an altazimuth mount which resembles an over-sized photo-tripod with a fluid-motion pan-head. Celestron also offer an EQ model which comes with an "Astromaster Styled" EQ-1 German Equatorial mount. Other "manufacturers" that offer 70mm f/13 packages are Meade, Vixen, and Skywatcher (Capricorn 70 and 70/900 AZ3, not listed on the Skywatcher website but still being sold online).

**Why I bought this? - I was searching for a light-weight, grad-and-go mount as I have been using an Orion Astroview equatorial mount with my little 70mm f/5.7 short-tube refractor, even for some quick observing. I came across this used Celestron Astromaster 70AZ from a moving sale, being sold at only $40 CAD! So, why not? Now I have an extra telescope which I intend to use it for public viewing and education.

**Should you buy one? - No! While a 70mm f/13 refractor is a great economical choice for beginners, especially those living in a heavily light-polluted city, Meade offers a much better package while the Skywatcher Capricorn is also a good choice.

 

PART 1 - THE MOUNT
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The fluid-motion of this simple altazimuth is way smoother than I expected. It is very usable for low to mid powers (< 60x). Occasional high power use also seems pretty acceptable though it starts bringing back my memories of using my 25+ years old 60mm f/11.7 on a Yoke-style altazimuth mount at 117x. However, there exists a problem: the setup is rear-heavy and thus the OTA is not balanced. This becomes more obvious when the scope is being pointed closer to the zenith. Adding a metal tube ring at the front helps a bit but not much. Due to the lack of mounting rings (the dovetail is directly attached to the OTA), there is no way to balance to tube. So how can a refractor with a glass objective with focuser, diagonal, eyepiece being all plastic be rear-heavy? Shouldn't it be front-heavy?  I would say it is caused by the metal handle of the mount, which sits "above" the altitude axis, adding weight to the rear. A simple solution is to mount the OTA backward like the photo above. This should greatly reduce the free play when the scope is moving upward. A more permanent solution is of course to use a longer dovetail with a pair of mounting rings to truly balance the OTA. (Note that the O.D. of the OTA is slightly smaller then the I.D. of the Skywatcher 76mm tube-rings.)

 

PART 2 - RED DOT FINDER
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Called StarPointer by Celestron, this permanently mounted red-dot finder from the Astromaster series is the worst finder I have ever used. It is even worse than the 5x24mm optical finders on those so-called "department store telescopes”. The main reason is that it is almost impossible to find out how far away from the finder the eye(s) should be and it is extremely difficult to align even during daytime. Another shortcoming of this finder is the brightness of its LED not being adjustable. The simplest and most effective solution is to super-glue a Synta-type dovetail finder mount on the OTA and use an “ordinary” red-dot finder or a 6x30 finderscope. However, with the only two telescope shops in town closed down in the last two years, getting an extra dovetail finder mount means paying $15 shipping for a $15 item! After some readings online, I came across someone who used the StarPointer as a finderscope mount and put a small finderscope through it. So I decided do my own version of it. Here are the steps:

Remove the StarPointer assembly by removing the two screws pointed by the red arrows. It is also a good idea to remove the battery as it is not needed any more. Then remove the two small screws circled in red. This will enable one to take out the two pieces of edged glass inside plus a metal retaining ring. 

The finderscope is a 5x20 from my 25+ years old 60mm f/11.7. I removed the single-element objective lens and replaced it with a 21mm achromat from a cheap pocket binoculars. The achromat has a shorter focal length than the original finder objective so I shortened the length of the tube to accommodate it. Using the separation between Castor and Pollux as a reference, this “new” finderscope has a field of a little more than 6 degrees. Finally, because there was too much free play when I put the finder scope through the now hollowed StarPointer, I added a small piece of self-adhesive felt to fill up the gap. This finderscope, thought small, is still up to the task of locating bright objects and works much better than any single element finder. Still, there is still one minor problem, the finder scope is mounted too close to the OTA. Not just every time I need to swing the eyepiece away from my face, a very small portion of the field is blocked because I cannot align my eye with the eye lens of the finder eyepiece.

 

PART 3 - THE DIAGONAL
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The diagonal that comes with the Astromaster 70AZ is a 90-degree Amici roof prism. This means it produces a fully corrected image but it is mainly designed for low-power terrestrial viewing. Except for some really high-end astronomical models on the market (e.g. Baader T-2), most corrected image diagonals house a under-sized, poor quality prism. This “spherical” one that came with many Celestron refractors is no exception (see photo on the left below). The only solution is to replace it with a mirror star diagonal. Since this telescope is merely a “bonus” from buying a grab-and-go mount and it was intended to be used for public viewing, I bought a $10 USD mirror diagonal (photo at the right) and a $19 USD 12.4mm Plossl (more on the eyepiece later) from an online store in the US that sells surplus from Meade.

P.S. This seemingly useless Amici prism was later used to modified a 6x30 finder into a right-angle corrected image one which I will write another post later.

 

 

PART 4 - EYEPIECES
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The scope comes with two eyepieces, a 20mm (45x) and a 10mm (90x). They are both 3-element modified Kellners. Judging from the look of them, they should be same as the Super 20mm and 10mm from Skywatcher. One might immediately wants to upgrade these seemingly low-quality plastic body eyepieces but I would suggest wait until they are fully utilized.

The 20mm is a germ covered in dirts. Skywatcher claims it has a AFOV of 58 degrees which sounds pretty exaggerated. However, if one measure the field stop, which is ~20mm, and use it to calculate to TFOV and then use the magnification to calculate the AFOV, one would find the Skywatcher spec. is in fact correct which makes me suspect the optical formula is closer to a 3-element Konig than being a modified Kellner or RKE. This also means the TFOV is almost the same as a 25mm Plossl with a 50-degree AFOV, and would certainly be wider than a 25mm “traditional” Kellner. with an AFOV of 35~45 degrees. Performance-wise, this 20mm eyepiece is in fact pretty good partly because of the highly forgiving f/13 focal ratio. Only when it is reaching the outer 10% of the field, degrade in image quality starts becoming obvious, but this is a pretty minor shortcoming for a stock eyepiece that comes with an entry-level telescope. Contrast is also good given despite being a single coated 3-element design.

The 10mm on the other hand has a bad reputation from many online reviews. However, when used on a f/13 system, it is not that bad. Sharpness and contrast are both acceptable, lacking slightly behind the Meade 12.4mm Plossl (Chinese-made Series 4000 equivilant) that I bought to replace it. The 12.4mm gives a magnification of 73x which I somehow found from experience significantly less susceptible to unstable seeing than 90x.

 

Edited July 2, 2017 by Rocket_the_Raccoon

[Peter Drew]

I get to use a number of these and similar each year, they are the main ones that visitors have problems in setting them up or using them. The majority have quite good optics but the rest of the design leaves a lot to be desired. As mentioned, the finder system is poor and the balance is upset by the weight of the telescope being over the pivot point. It's a pity that the likes of Celestron and others who have good reputations within the higher price areas offer such telescopes that seem to have the accent on form over function. Entry level is the point at which a beginner is either put off or hooked on astronomy.

[RobH2020]

On 02/07/2017 at 12:41, Peter Drew said:

I get to use a number of these and similar each year, they are the main ones that visitors have problems in setting them up or using them. The majority have quite good optics but the rest of the design leaves a lot to be desired. As mentioned, the finder system is poor and the balance is upset by the weight of the telescope being over the pivot point. It's a pity that the likes of Celestron and others who have good reputations within the higher price areas offer such telescopes that seem to have the accent on form over function. Entry level is the point at which a beginner is either put off or hooked on astronomy.

I got one of these last year... tried using it but the mount sucks! Just about usable for the moon but not really for anything else, I've found. Too much "ping back" when you've moved to where you want to view. Is that the balance?

For some reason I overcame this and bought a 6" dob in January this year, which I'm enjoying.

However, I'm wondering about keeping the 70az as a travelscope. Is there any advice you can give on this... I don't want to spend loads on a new mount/tripod, will try some things like Rocket's suggested in another thread... but want more views on whether it's worth it!

many thanks,
Rob

[EMFLOCATER]

I have the Celestron 90 AZ. However even after loosening the Altitude handle and RA knob, the mount does not spin as smooth as I would like. Tried to loosed the 10mm allen bolt at bottom but it will not budge Wanted to pry off the Dec caps to loosen the bolt to free up Dec, but have not got that far. Anyhow, do you have any advice on this? For now I been spinning the Dec and RA axis to free up the glue like grease inside which seemed to help.

Cheers

Don

[GreyDay]

Okay, i'll have a go at this one but only because i hate seeing such negative reviews. Before i start you should all keep in mind that we're not reviewing a £1000 telescope on a £2000 mount with £350 eyepieces!. It's a cheap chinese telescope aimed at beginners who are under the impression that £100 is a lot to spend on an item that may only be used once then stuck in the loft. They dont understand that what they're actually getting is a full astronomy setup for the price of a TeleVue Plossl, or a few cheap plossls or maybe a good diagonal... get the point? we're reviewing a cheap full setup that can be obtained second hand for less than 50 quid. I've had three of these, i gave two away to friends who wanted to try out astronomy and i kept one as a grab n go.

Rocket_the_Raccoon made a few good points that i have to agree with,

1) The Mount, i agree, it's very tail heavy  and not precise. When the Alt handle is tightened and you let go, the excess weight at the rear causes the telescope to sag. This can cause the object you're trying to lock onto, to disappear from the FOV. It isn't all bad though, once this has happened a couple of times you find yourself correcting for the sag and it ceases being a problem. So the mount... Useable lightweight and sturdy enough with one caveat, don't extend the legs fully! They're amazingly stable to about half extention but can get a little wobbly beyond that. I'm used to using a SkyTee II on a CG5 tripod for larger scopes but i don't find the Astromaster mount/tripod to be a bad setup for smaller scopes.

2) The red dot finder.. On two out of three Astromasters that ive had the red dot is too bright and renders the finder useless unless you turn the LED off. One way a friend overcame this was to dab the small drilled point that reflects the led with some lens blackening paint.(the sort boy racers use to make their tail light clusters look smoked) As this stuff blocks around 50% of the light it sort of fixes the problem. At this point i'll defer to Rocket's opinion.. the finder is the achilles heel on the Astromaster though not something that cant be fixed.

3)The Diagonal... This is where i have to disagree the most! The Amici prism diagonal is an absolute godsend to beginners. It's the right way up! it's the right way round! what you see through the telescope is what you see in a star chart! theres no reversed image to contend with, no questions like "why do the stars go left when i turn right?" "why doesnt it look like the pictures i've seen?" "Is this star atlas printed backwards?" The prism isn't "Undersized" it allows all the light a 1.25" field stop can take! I've tried comparing the celestron diagonal to others using a 32mm EP and didn't see any loss in light transfer. Amici's can throw up the vertical line that appears as a ghostly shimmer but then again far less obtrusive than edge distortion from a widefield ep in a fast apo without a flattener. It's a cheap diagonal and far more user friendly, i would also suggest far superior to a $10 mirror!!! The secondary on my SW200p isn't perfectly optically smooth under DPAC so what sort of mirror do you get for $10? enough said.

4)The Eyepieces... Again i agree with Rocket, They're useable and cheap to replace, To be honest the choice of cheap plossls is staggering. an extra outlay of 50 quid will get a couple of extra ep's and a cheap barlow. I do prefer the SW MA ep's that come with their cheap scopes over the celestrons.

So, Not much to add over Rocket's review except i would recommend the Astromaster to a beginner! All the things that lower it's quality are the reason they're so cheap to buy, astronomy is one of the true hobbies where "you get what you pay for" really rings true. I've bought cheap stuff and been both disappointed and surprised by the quality, same goes for expensive gear too! But i have to stand up for the Astromaster and say it's good for the price, the last one i bought i got for £30!. I've got to admit that when i first tried one of these around 10 years ago i wasn't impressed, though after a little use and coming to terms with the shortfalls i began to enjoy it. That may be the curse of cheap telescopes where a beginner may try it once and never use it again, but! look at the Astromaster compared to the nasty 70x350 and 50x350 chinese scopes that are flooding the market and it becomes a real bargain and good value for money.