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Rocket_the_Raccoon

Celestron Astromaster 70AZ - Review and Improvements

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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:

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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. 

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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
------------------------------

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.

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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.

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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.

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Edited by Rocket_the_Raccoon
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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. :icon_biggrin:

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