Considering bigger SCT for planets

[IDM]

After a couple of years using my StellaLyra 8 inch classical cassegrain and getting reasonable images of the planets. Then seeing some peoples astonishingly good images I am starting to get aperture envy. So I was wondering whether it is worth me getting a larger SCT either 9 1/4 or 11 inch, is this going to be worthwhile? I do know that cool down and dew are a bigger issue with SCT's but think I can allow for cooling.

I also see the Celestron SCT's come in XLT and HD variants. The HD's being considerably more expensive. Given the primary use is for planets and that the focal length of and SCT is huge (unless using Hyperstar) then I cannot imagine when I would use it for astrophotography. Though if there is a good reason for paying the extra for the HD I would consider it. I think I am entering into paralysis by analysis so would be grateful for any guidance or thoughts.

Thanks,

Ian 

[Andy ES]

Ian,

I’m using a CPC925 XLT, which even in  my inexperienced hands has produced some half decent planetary images.

Visually I find it stunning and very easy to use.

The only downside is the weight. I’m  pretty strong and a reasonably  fit 57 year old and I personally wouldn’t want to go to 11” on a CPC mount. (At least not without someone to help put it on the tripod)

Probably ok on a different mount when it is just an OTA

Can’t really comment on the extra benefits of HD but reading posts not sure there is that much in it, but I have not had the chance to compare.

Good luck with your choice.

Andy

PS I think there is a CPC925 for sale on this site at the moment 

 

[IDM]

5 minutes ago, Andy ES said:

The only downside is the weight. I’m  pretty strong and a reasonably  fit 57 year old and I personally wouldn’t want to go to 11” on a CPC mount. (At least not without someone to help put it on the tripod)

Probably ok on a different mount when it is just an OTA

I should have mentioned that I am thinking of an OTA to go on my EQ6R mount. I agree that weight could be an issue but actually the 8 inch CC weighs 8kg (approx) and the 9 1/4 inch SCT is 9kg. So not much in it. Though I just checked the C11 is 12.5kg, which is getting pretty heavy to lift.

Thanks for your general thoughts on the 9 1/4. I did see the classified add, but I live in the NE England so pickup was an issue. 

[Andy ES]

10 minutes ago, IDM said:

just checked the C11 is 12.5kg, which is getting pretty heavy to lift.

I think the CPC925 is 26kg and CPC11 is 29kg😳

makes a C11 OTA seem ok to me and my back 😂

[bosun21]

You won’t really notice the difference if you go for the 9.25” over your existing 8”. The 11” on the other hand you will definitely see the difference.

[dweller25]

I believe the 8”CC has an working aperture of around 7” so moving to a C9.25 should give improvement and a C11 even more, but they seem to out of stock at the moment.

But if dew prevention comes into the equation then the 10” CC may be a good choice - but it’s 17Kgs !

Another option could be the Mewlon 210, no corrector and 1/20th wave optics, but it’s expensive.

No easy answer 🤷🏻

Edited October 6, 2023 by dweller25

[globular]

The 8"CC has an 8.4% secondary obstruction ==>  7.4" effective aperture

The 9.25" SCT has a 13.1% secondary obstruction ==> 8.0" effective aperture

The 10" CC has an 10.9% secondary obstruction ==> 8.9" effective aperture

The 11" SCT has a 11.6% secondary obstruction  ==> 9.7" effective aperture

Personally I'd make sure I was really getting everything possible out of the 8" CC before concluding I need more aperture.  It should be capable of producing stunning planetary images - not that far behind a 9.25% SCT (non edge hd). Are you really sure it's lack of aperture that's holding you back?  It could be a multitude of other things.

But if you're adamant more aperture is what you need / want then I think you need a step up bigger than a 9.25" SCT to make the extra spend worth while.

[Peter Drew]

6 minutes ago, globular said:

The 8"CC has an 8.4% secondary obstruction ==>  7.4" effective aperture

The 9.25" SCT has a 13.1% secondary obstruction ==> 8.0" effective aperture

The 10" CC has an 10.9% secondary obstruction ==> 8.9" effective aperture

The 11" SCT has a 11.6% secondary obstruction  ==> 9.7" effective aperture

Personally I'd make sure I was really getting everything possible out of the 8" CC before concluding I need more aperture.  It should be capable of producing stunning planetary images - not that far behind a 9.25% SCT (non edge hd). Are you really sure it's lack of aperture that's holding you back?  It could be a multitude of other things.

But if you're adamant more aperture is what you need / want then I think you need a step up bigger than a 9.25" SCT to make the extra spend worth while.

I think the secondary figures are a bit misleading with regards to planetary performance.  The full aperture still retains the potential resolution, the obstruction just reduces the contrast and for imaging purposes this can be mitigated by number of frames taken and processed.     🙂

[Veloman]

3 hours ago, IDM said:

I should have mentioned that I am thinking of an OTA to go on my EQ6R mount. I agree that weight could be an issue but actually the 8 inch CC weighs 8kg (approx) and the 9 1/4 inch SCT is 9kg. So not much in it. Though I just checked the C11 is 12.5kg, which is getting pretty heavy to lift.

Thanks for your general thoughts on the 9 1/4. I did see the classified add, but I live in the NE England so pickup was an issue. 

Just weighed my C9.25 and, according to my bathroom scales, it's 11kg. It does have a Baader Diamond Steeltrack focuser but I can't believe that weighs 2Kg!

I don't find the weight a problem but it can be awkward getting it safely on and off the mount.

[Cosmic Geoff]

I have found that the 'seeing' is a major factor in planetary imaging performance.  So a 9.25" will give a slight improvement, and a 11" a more marked improvement in performance, but only if the seeing is good. If the seeing is bad, the bigger scope might actually perform worse. 

The central obstruction is not really an issue. The % loss of light isn't of significance, though the obstruction does cause some loss of contrast. But you can turn the contrast up during post-processing of images. 

The EDGE HD models have extra correcting lenses to give a flatter field for deep-sky imaging, but you don't need this feature for imaging a planet. On the other hand, the EDGE HD models are said to be finished to higher standards, allegedly giving a better optical performance.  If you are contemplating using a focal reducer at any time, note that the regular f6.3 FR is not too costly but the HD focal reducer is insanely expensive.

The OP does not mention what mount he is using. This has some bearing on how manageable the scope is.  I actually use a CPC800, which is close to being the ideal planetary imaging scope (for its size).  At nearly 20Kg I find the fork/OTA assembly as heavy as I'd want to handle, deterring me from buying a CPC925 or a C11.   If you have the SCT on a different mount, then each weight (OTA, mount. tripod, counterweight) to be handled is much less. 

Note that a fat OTA can be awkward to handle and mount up unless it has added grip-handles, e.g a second dovetail bar opposite the main one.  And the bigger it is, the more awkward it will be to handle.  People have dropped these OTAs! Usually with fatal results.

So in answer to the original question, buy what you like, but be aware of potential negatives.

Also note that used SCTs don't hold their price well (think 50% of the price of a new one) so you can save a wad of money buying used.

Edited October 6, 2023 by Cosmic Geoff

[Mr Spock]

A C9.25 fits nicely on an EQ6. I wouldn't want to put anything larger on, though I did have a 10" f4.8 Newtonian on mine.

With the handle on the bottom it's very easy to lift and manoeuvre on to the mount. The 10" Newt was a nightmare to lift (bulk rather than weight) so that's why I went for the C9.25.

[paulastro]

I have some medical issues and discovered this on FLO.  I bought it, and to my amazement it really helps in moving my 10inch Starsense around.

https://www.firstlightoptics.com/telescope-bags-cases-storage/oklop_61395.html

 

[IDM]

14 hours ago, globular said:

Personally I'd make sure I was really getting everything possible out of the 8" CC before concluding I need more aperture.  It should be capable of producing stunning planetary images - not that far behind a 9.25% SCT (non edge hd). Are you really sure it's lack of aperture that's holding you back?  It could be a multitude of other things.

I think this sums things up. I have just tried to re-collimate my scope but now face at least another 7 days of solid cloud before I have any hope of testing it (so frustrating). Even if the clouds lift there is the issue of seeing, which of course may or may not be good. Certainly given the comments that aperture might not be the issue I think I need to investigate all avenues before spending over £2k to find myself in the same position.

 

[IDM]

14 hours ago, Veloman said:

Just weighed my C9.25 and, according to my bathroom scales, it's 11kg. It does have a Baader Diamond Steeltrack focuser but I can't believe that weighs 2Kg!

I got the 9kg weight from the information on FLO's website. I don't know what the correct weight is but it would be good to know how accurate the weight is when looking at vendor sites. I certainly don't want to underestimate things and either struggle to mount a scope or have my mount struggle.  

[davidc135]

21 minutes ago, IDM said:

I think this sums things up. I have just tried to re-collimate my scope but now face at least another 7 days of solid cloud before I have any hope of testing it (so frustrating). Even if the clouds lift there is the issue of seeing, which of course may or may not be good. Certainly given the comments that aperture might not be the issue I think I need to investigate all avenues before spending over £2k to find myself in the same position.

 

It would be worth doing a basic star test to check the optics, quite apart from collimation. It's probably fine but you never know.

David

[Cosmic Geoff]

I found out the hard way that even if a SCT seems to work well on double stars, the collimation can still be slightly out, with a noticeable effect on planetary imaging quality.

17 minutes ago, davidc135 said:

spending over £2k

But see the forum's For Sale section...  £1400  for a C925.