Testing a small Maksutov

[mkk707]

Tonight I tested a small Maksutov-Cassegrain catadioptric telesope on the waxing Moon. I normally use an ED-apo, so the Mak required some getting used to. It has an aperture of only 90 mm (3.5") but a focal lenth of 1250 mm, so the image is much darker than what I am accustomed to.

I attached my Canon 1000D DSLR with a T2-adapter to the 1.25" eyepiece opening. The Moon almost fills the camera's CCD vertically so one has to allow for a fair amount of re-pointing. Typically for this kind of 'scope., color fringes are not an issue. The sharpness is, in my opinion, not very satisfactory, but then, today's seeing wasn't that great. Due to the small aperture and lomg focal distance, the exposure time is fairly long, with 1/15 s in the attached image. Much more than what I am used to with my refractor, of course. That might also account for some lack of definition.

Still, given the attractive price of the telescope (around 150 Euros) and its low weight and dimensions, it's not a bad choice if you are looking for something that is easy to carry and can safely be mounted on any reasonably sturdy camera tripod. One just has to be aware of its limitations.

http://bit.ly/JQFIAN

[RickJ]

This looks like it is simply out of focus. I don't see any motion type blur. Try making a Bahtinov mask for the scope and focus on a bright star using the mask. Then move back to the moon to take the image after taking the mask off Google the term if it is new to you. You'll find instructions on how to make and use one or vendors selling them. Home made work just as well as the neatness and dimensions aren't critical at all. Sounds like you don't have a drive on the scope? If not, when in good focus 1/15th second would cause a one second of arc blur due to motion of earth and moon. That is likely just below your resolution with a 90mm scope. Certainly you'd not want to go any longer.

Noise shouldn't be much of an issue so push the ISO a bit and see if you can use a 1/30 or even 1/60 second in movie mode and then use Registax to find the best frames out of hundreds in the stream to stack and process those. The faster the exposure the better you freeze the seeing. Even nights of poor seeing will have fleeting instants of good seeing and a video stream can capture those whereas using simple individual exposures it takes luck to snap a shot at one of those times. The more you stack the less the noise. Low noise allows more aggressive processing to be used without creating artifacts out of the noise. The more good frames you can capture the better.

Rick

[mkk707]

Rick,

Thanks a lot for all those helpful comments and pointers, they are very much appreciated. I had never heard of a Bahtinov mask before, but following your comment, I read it up on wikipedia.

Your recommendation to go to higher ISO numbers makes perfect sense, of course. I am just so used to setting ISO 100 when I am photographing the Moon through my refractor and then needing exposure durations of between 1/125 and 1/320, depending on the Moon phase, that it probably just didn't occur to me to increase the ISO setting to get a faster exposure duration when using the Mak. That is part of the familiarization process. One might also call it "learning the hard way".

You are probably absolutely correct in stating that the problem here is the focus. That again, though, has to do with familiarization. I find this cheap little Maksutov rather difficult to focus. I guess I am spoilt by the two-speed Crayford focuser that came on my refractor. The focuser on the Maksutov is a somewhat unwieldy stub that sticks out at a location where, intuitively, I don't expect it to be. So when I've finished fumbling for it, then in the absence of a reduction gear I am constantly changing the focus beyond the point where it would be sharp. I hope I will get used to that, with time.

Sure, I could have shot a series and stacked them, but this time I was trying to see what that little scope can do and I wanted to compare the "unenhanced" images to those I have obtained with my refractor. However, I certainly will try to do it the way you recommended the next time I get a chance.

Thanks again, and Clear Skies

Michael from Germany

[RickJ]

Due to the different image scales comparisons of single short exposures aren't very helpful. The low image scale covers a multitude of issues that stick out with higher image scale. Unless you get super lucky the raw larger scale shot, even with a perfect scope, will not look as good.

The optical quality of cheap Maks can very greatly. I've seen some really great ones and some really lousy ones coming from the same optical manufacturer. Moving mirror focusers in these are often poor as you've discovered. If the optics are worth it you might add a good focuser for fine focus. I have a friend who uses his that way for H alpha solar work. Optics were good but focus impossible to achieve so he added a Crayford focuser to get the focus he couldn't achieve otherwise with the very crude moving mirror system.

For serious planetary imaging I consider an 8" scope the minimum with 12" to 14" the most cost effective. Going larger doesn't help all that much and adds greatly to the cost. A member of our club home built a super 8" imaging Newtonian for less than $50 US dollars including equatorial mount and sector arm clock drive. He's even published work with it in peer reviewed scientific journals! Proof, yet again, it isn't so much the equipment as the person behind the equipment that is most important.

Rick

[mkk707]

Rick,

Thanks again for taking the time and helping out with useful advice. I don't think this particular small, cheap Mak is worth the investment of a Crayford focuser, but when I get a bigger one, I will certainly pay more attention to that aspect.

You are very right that raw larger scale images will not look better than the small scale ones taken with a scope with a shorter focal distance. I wasn't really after the looks, though ... what I wanted to do is to compare the images carefully and see whether I can spot more detail in them than I had been able to get with other scopes. Such as the small sub-craters in crater Albategnius.

In the meantime, I have stacked some of the images of the series I shot on April 28. The result is here: http://bit.ly/In9tfc
It is slightly better than the one shot I had posted before, but not much ... the problem is of course that also the stacking was based on individual frames rather than stills from a movie.

BTW, I do have a motorized mount, but I didn't use it this time ... for one, I thought that a 25 cm long Mini-Mak looks kind of silly on an EQ5. :-)

I think I'll just have to try again, and again, when I get the chance. It is true, as you say, that it's the person, not the equipment, that counts. In my case, this particular person needs more experience, and the only way to get it is to go out and take more pictures.

Greetings and CS
Michael from Germany

[grapes]

Good stuff, Rick! Good luck with the scopes, Michael.

[shanevic7]

Rick,

Thanks again for taking the time and helping out with useful advice. I don't think this particular small, cheap Mak is worth the investment of a Crayford focuser, but when I get a bigger one, I will certainly pay more attention to that aspect.

You are very right that raw larger scale images will not look better than the small scale ones taken with a scope with a shorter focal distance. I wasn't really after the looks, though ... what I wanted to do is to compare the images carefully and see whether I can spot more detail in them than I had been able to get with other scopes. Such as the small sub-craters in crater Albategnius.

In the meantime, I have stacked some of the images of the series I shot on April 28. The result is here: http://bit.ly/In9tfc
It is slightly better than the one shot I had posted before, but not much ... the problem is of course that also the stacking was based on individual frames rather than stills from a movie.

BTW, I do have a motorized mount, but I didn't use it this time ... for one, I thought that a 25 cm long Mini-Mak looks kind of silly on an EQ5. :-)

I think I'll just have to try again, and again, when I get the chance. It is true, as you say, that it's the person, not the equipment, that counts. In my case, this particular person needs more experience, and the only way to get it is to go out and take more pictures.

Greetings and CS
Michael from Germany

Wow this is an amazing picture. The moon looks like it is closer than ever. Thank you for posting it!



_____________

http://natoma08.com