Celestron Astromaster 130 (Mirror - Parabolic or Spherical)

[bomberbaz]

Hello fellow members.

I have in my possession the above mentioned OTA. I am aware this is not a top end bit of kit but I am hoping to use it for imaging galaxy & basic spectroscopy. I have looked at other options but aperture/weight wise, (this is 3.6kg) this hits the spot.

I have tried my 72ED (doesn't focus well enough even though it is ED), a 127 mak seemed to throw up problem after problem with focal length, weight and balance until I gave up. 

Then quite by accident I came across this OTA very cheaply (Amazon return with a damaged box). If it does work out ok then a simple focuser upgrade could give me a very lightweight, low priced & useable OTA.

Thus far I have only done a terrestrial test visually (which seemed ok) and quick image with the 183mm and the result below seems very promising. Ignoring the dust mote and bottom right tree blur (and my awful focusing with the awful focuser), the image looks pretty tidy from lower left to upper right. Tonight the skies are iffy so might not get chance to do anything from the night sky sadly.

I have found a few posts on here and CN that are contradictory with some adamant it is spherical and similar calling it a parabolic, but at F5 I am leaning towards the parabolic camp based upon information I believe to be correct, mainly that a spherical will be a mushy mess at this focal length.

Trouble is, Celestron appear to be pretty rubbish at disclosing product information they either don't want or think it necessary to know.

So anyway, looking for any additional info that is available, preferably not anecdotal type.

Cheers

steve

Edited June 19, 2023 by bomberbaz

[ONIKKINEN]

1 hour ago, bomberbaz said:

I have found a few posts on here and CN that are contradictory with some adamant it is spherical and similar calling it a parabolic, but at F5 I am leaning towards the parabolic camp based upon information I believe to be correct, mainly that a spherical will be a mushy mess at this focal length.

My Astromaster 130 gets outresolved by a half-achromat 90mm refractor in planetary/lunar visual use so i would imagine a very spherical mirror in mine considering the not insignificant difference in aperture and that my refractor is not exactly top of the line stuff itself.

Seems like there are varying quality copies of the scope sold and sometimes you read an account where the scope is not too bad in someone else's eyes. Probably why Celestron does not advertise anything specific about the mirror if there is a lot of variation between mirrors.

[John]

The "rule of thumb" that I use is that if an instrument has a desirable and valued characteristic, the manufacturer will be quick to mention it. 

So if it is not explicitly mentioned, it is safest to assume that it is not present.

[bomberbaz]

9 hours ago, ONIKKINEN said:

My Astromaster 130 gets outresolved by a half-achromat 90mm refractor in planetary/lunar visual use so i would imagine a very spherical mirror in mine considering the not insignificant difference in aperture and that my refractor is not exactly top of the line stuff itself.

Seems like there are varying quality copies of the scope sold and sometimes you read an account where the scope is not too bad in someone else's eyes. Probably why Celestron does not advertise anything specific about the mirror if there is a lot of variation between mirrors.

 

9 hours ago, John said:

The "rule of thumb" that I use is that if an instrument has a desirable and valued characteristic, the manufacturer will be quick to mention it. 

So if it is not explicitly mentioned, it is safest to assume that it is not present.

OK not quite ready to give up on this & as you responded, I hoped you could give me a further opinion. Had a play with a FOV app and here's what I find. The camera, a 183 will magnify at about the same as a 15mm plossl, or about x40.

Would the lesser quality mirror (assuming it is spherical) underperform the parabolic cousin at this magnification?

[Nik271]

I used to own Astromaster 130 and while it was fine at low magnifications it never gave a sharp image at x80 or above. The star test was rubbish as well, at the time I didnt have the experience to measure precisely but everything points to the mirror being at fault. Since you have the telescope do a star test with polaris, spherical aberration is easy to spot: the diffraction patterns will be very different both sides of focus.

Bear in mind that this will only be obvious at high magnification, at F5 you need to use a 5mm EP or lower. At low magnification the spherical aberration does not matter so much and will give you good widefield views. But you want to image galaxies which I take it to mean that you need high resolution from your scope and this one will not give it.

[ONIKKINEN]

3 hours ago, bomberbaz said:

 

OK not quite ready to give up on this & as you responded, I hoped you could give me a further opinion. Had a play with a FOV app and here's what I find. The camera, a 183 will magnify at about the same as a 15mm plossl, or about x40.

Would the lesser quality mirror (assuming it is spherical) underperform the parabolic cousin at this magnification?

Magnification for imaging purposes is not useful at all, and actually more questions arise when you think about it a bit more. Lets say you view the image on a smartphone at arms length, what is the magnification now? What about a 50'' TV, surely the magnification is larger now that the image is larger? Or if you crop the image to only half its size, is the magnification now doubled? As you can see for imaging purposes where the size of the image can and will change in the end a "magnification at capture" thing is misleading and mostly useless.

But we can use a better metering stick for something similar: The resolution of your scope/camera combo which in this case is 0.76'' per pixel. Unfortunately the 183 has tiny pixels so you are already at a disadvantage here, but you can bin the image later of course. Just as a coincidence i image at a resolution of 0.76'' per pixel, with a premium 8'' newtonian and a very good coma corrector (that costs twice the astromaster 130 alone). Furthermore unfortunately 95% of the times i am significantly over sampled, and the remaining 5% i am just mostly over sampled so i would say the likelihood of your astromaster/183 combo being a good match for galaxy imaging is 0. Probably looking at binning to x4 or further to get a not-soft looking image at 100% viewing. (which i think is a decent metric of how an image performs, if the image is soft at 100% there are too many pixels to show the information. Its all subjective though so many will disagree on what is appropriate)

There are more problems with the scope however. The focuser is one of them, being entirely plastic and 1.25'' in size. Meaning there are no coma correctors that you can easily fit to it. And even if you could fit one in, will it work? After all a coma corrector is designed to fix coma that comes from a parabolic mirror, but if the mirror is spherical then it would not fix the aberrations that come with it. Without a corrector you are limited to using the central 5mm (at best) of the sensor to get a somewhat sharp image.

No reason to give up though since you already have the scope. Try it out under the stars, maybe yours has a better mirror and is not entirely soft? Then you could think about fitting a more suitable focuser and maybe a coma corrector too and maybe it does turn out to be a decent scope.

However if you dont already have a 2 inch focuser you could slot into it you would need to buy one, which will probably cost more than the scope. And at this point you could just skip the headache and get a 130PDS.

[bomberbaz]

cheers @Nik271 & @ONIKKINEN, I do have this ota on sale or return basis, so (weather permitting|) I have time to have a little play with it to ascertain if I can get anything worthwhile from it. 

For now just going to see if the spectroscopy results are any good, then take the rest from there.

cheers

steve

[bomberbaz]

UPDATE:

After thinking this through I have initiated a return, main reason the focusing issue.

The focuser unit provided is triangular in shape at its base & sits into a plastic housing with a recess that is also triangular in shape. Any new focuser which are usually square in shape at the base would have to deal with this somehow. Removing the recessed housing didn't seem to be an option as it appeared the entire upper plastic part also held the secondary spider.

So it seems that Celestron have done their best to ensure modifications of any type are very difficult. I decided not to dig any deeper as I didn't want to lose my return warranty and so I gave the idea up as a bad job.

Steve

[AstroMuni]

On 18/06/2023 at 21:05, bomberbaz said:

I do have this ota on sale or return basis, so (weather permitting|) I have time to have a little play with it to ascertain if I can get anything worthwhile from it. 

I have used this OTA extensively and you can see a few images taken with this in my signature link. As to whether it has a spherical or parabolic mirror - I have no clue. But images of DSOs come out pretty decent I must say. Focuser is tricky to use manually as there is no fine focus and it can be wobbly as well.

EDIT: I even managed to attach the ASI533 pro to it (no coma corrector) and imaged these. So good luck and look forward to seeing your images 🙂

 

Edited June 20, 2023 by AstroMuni