Is it worth upgrading my 8" SCT to C11 for planetary imaging?

[algady]

Hi guys,

I really need some advice, I'm getting more and more interesting in planetary imaging now and I've got ASI120mm camera and Baader LRGB filter set and I'll get Xagyl 5 Position USB Filter Wheel.

So, do you think that 3" difference are worth getting the c11 ota or not?

Thanks in advance

[Bizibilder]

It should make quite a difference - more light for a brighter image and greater resolution.  The downside is that you seem to have an HEQ5 which is not big enough to take the C11 (the mount would be overloaded) - you will need to upgrade to an EQ6 at least.

[algady]

Thanks  Bizibilder,

For the mount I'll try it as I think it would be ok as planetary imaging don't required that much of stability.

In the following links people successfully use this combination.

http://www.rkblog.rk.edu.pl/w/p/stability-heavy-scopes-popular-mounts/

http://www.footootjes.nl/Astrophotography_Planets_Stars/Astrophotography_Planets_Stars.html

[riklaunim]

HEQ5 can handle C11 - with an extended counterweight shaft... and no wind blowing in the OTA I used C8 and C11 and the gain of the aperture was quite nice, noticeable.

[knobby]

Don't people recommend the 9.25 for planetary, a bit lighter for the mount too?

[algady]

Don't people recommend the 9.25 for planetary, a bit lighter for the mount too?

I thought 1.25" difference is not worth the money!

[knobby]

I thought 1.25" difference is not worth the money!

Fair point :-)

[ollypenrice]

I thought 1.25" difference is not worth the money!

Damian Peach (who might be considered moderately competent...) has lots of praise for the 9.25. Check out his website.

Olly

[riklaunim]

More aperture the better. C9,25" isn't very special and will be inferior to bigger telescopes. It just has some slight difference in both mirror f/ratios.

[algady]

Thank you all,  I think that C11 would be better for me, I forgot to mention that I'll use it in excellent seeing conditions there in my country.  

[lensman57]

Don't people recommend the 9.25 for planetary, a bit lighter for the mount too?

It is also reputed to have the best strehl  ratio of the optics including some expensive Russian maksutovs, but who knows.??

A.G

[riklaunim]

It is also reputed to have the best strehl  ratio of the optics including some expensive Russian maksutovs, but who knows.??

A.G

That's just legends. Every SCT has good optics, "diffraction limited". It's good to read for example this: http://www.cloudynights.com/item.php?item_id=1847

What Celestron did this time out was go to a slightly slower primary (f/2.3 instead of f/2), which allowed a smaller, less radically curved secondary to be used. That results in the scope�s somewhat flatter field and slightly longer tube. Good, not magic.

[lensman57]

That's just legends. Every SCT has good optics, "diffraction limited". It's good to read for example this: http://www.cloudynights.com/item.php?item_id=1847

My comment was based on the report of the actual tests carried out by Astrotreff.de  on a number scopes and not an article.http://www.astrotreff.de/topic.asp?ARCHIVE=true&TOPIC_ID=98314&whichpage=1

The term diffraction limited is arbitralily set at a Strehl ratio of 0.8, any optical system with a Strehl ratio of 0.8 and above can be  conventionally called "Diffraction limited". http://www.telescope-optics.net/Strehl.htm

I have an Apo triplet with a test certicate declaring a Strehl ratio of 0.964. This Apo does not even qualify as a "High Performance" optical system even by the manufacturer themselves, for that it would need a Strehl Ratio of at least 0.975. I can't imagine what a strehl Ratio of 0.8 would mean in a real world but the loss of contrast and definition would be significant to say the least.The numbers in the tests have to be viewed with the caveat that a small aperture typically has a higher Strehl ratio than a large scope but it is still a good indication of the quality. 9" and 11" are not that far apart in terms of the percentage aperture.

Regards,

A.G