Lunar / Planetary scope - Classical Cassegrain or 4" ED

[Davesellars]

I think I may be in the market for a large(ish) focal ratio scope specific for lunar observation as I'm getting more interested in the Moon and would also like to participate in this program at the Department of Physics at Aberystwyth University when possible:

https://users.aber.ac.uk/atc/LS_present/Birmingham.htm

I have the 12" dob which I use for DSOs and with spot on collimation has produced very good results when the seeing is very good for lunar observation.  Certainly, for some observations I would (have to) use this.  However, for the majority of the time I would prefer using a a scope that I get out relatively quickly when the oppotunity arises and sit on the EQ5 mount (manual) which I orginially bought when I had my 8" SCT (which was very stable)

Also, when the planets become more available later this year this scope would be used more for this as the dob rather has more restricted location (I can take my EQ5 or AZ4 perhaps with another scope and view from the park next to us - something I definitely wouldnt lug the dob out to.

I've been looking at these two:

a.  6" Stellalyra Classical Cassegrain F/12  - £460

b.  4" Altair Astro F/11 ED - £670

Collimation doesn't particularly phase me but I gather it would have to extremely precise in the CC...   I'm a little more concerned about the effect of contrast from the central obstruction particularly for planetary observation but if this brings it "down" to more like a 4.5" to 5" APO refractor performance then I'd be sufficiently satisfied.   The other concern for me would be if the figuring of the mirror is sufficiently good to perform as well or better than the 4" refractor.

A major pro for the 6" CC would it would provide 183x with a 10mm Delos and 131x with the 14mm Delos - both very comfortable eyepieces.  I also have a 5mm and 7mm Pentax XW which would have to be used to reach approximately the same power in the 4" refractor or for when the seeing is truly excellent with the CC.  My doubt is if a mirror in the CC would be able to go much higher in the CC without going mushy though.

Difficult decision...  Will the 6" CC give more than the 4" refractor?  I'd see the refractor as lasting longer as mirrors degrade and are a pain to clean...  lots of other pros and cons!  But then the CC is pretty cheap...

Any advice or from other members that perhaps have both as a comparison.  I've read through the extensive long thread started by FLO when the Stellalyra CCs  were made available...

 

 

[Nik271]

A 4 inch F11 refractor will not be very easy to use: that's a 1.1 meter long tube plus a dew shield on top. I just got a 4 inch F7 refractor for widefield and being used to Maks am finiding it a bit trickly when pointing to the zenith. Also wind will mess up your high magnification view. A 6 inch CC seems a better compact 'grab and go' option. Mirors with modern coats degrade slowly, I think 10 years minimum of good life in them. 

PS worth checking out some of the 5/6 inch Maks out there too!

[johninderby]

I use the 6” CC as my grab’n’go scope and also have a 5” APO and on the moon they are pretty equal however the CC can be used on a smaller mount than the 5” APO so better for grab’n’go and way, way cheaper. 🙂

The mirrors in the CC are quartz with a dielectric coating so pretty tough and last indefinitely without degrading. Holds collimation about as well as a mak. Never had to touch the collimation on mine and still spot on. I find a 7mm eyepiece works nicely although have used a 4.7mm when seeing is very good. 

The CC will have better resolution than the 4” frac and contrast is surprisingly good as well. Find the CC design has the edge over a mak or SCT in on axis sharpness and contrast.

I have upgraded the focuser to a Baader Diamond Steeltrack. The stock focuser is usable but no Steeltrack. 🙂

 

 

Edited February 13, 2022 by johninderby

[dweller25]

I found the 6” Celestron SCT, the 6” Skywatcher Mak and the 6” Classical Cassegrain to be no better than my 4” refractor in terms of visual Lunar and planetary views and I could sense the central obstruction when viewing 🤪.

The refractor cooled MUCH faster and gave steadier views.

But for best stability on an EQ5 a 4” F/7 ED may be a better match……

https://www.firstlightoptics.com/starfield-telescopes/starfield-102mm-f7-ed-doublet-refractor.html

And there’s always the good old 6” F/8 Dob option….

https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-150p-dobsonian.html

Edited February 13, 2022 by dweller25

[Davesellars]

Thanks guys.

F/7 is not really sufficiently long for a 4" to get enough magnification at times without a very small eyepiece focal length which then is uncomfortable and reduces the exit pupil too much for my liking.  I use a 4mm (cheap TMB planetary II clone) with the 80mm ED which is just about OK but that's giving 150x which is not really sufficient.  I certainly wouldn't want to go start using a small focal length eyepiece than that.

I didn't have any stability issues with my 120ED on the EQ5 (probably similar weight and length to the 4" f/11) although noted it would be uncomfortable on the mount because of the length when the Moon is high up so that's a good call.   OK, probably erring towards the 6" CC but in no hurry to decide at the moment!

[johninderby]

Yes just take your time. No need to hurry. Every scope has it’s good and bad points but you will find the best compromise for your needs.

BTW find the increased aperture of the CC gives better resolution than a 4” frac on the moon and as to central obstruction just a total non issue to me.

 

[Mr Spock]

It doesn't get better than this:

For high power I use a 2" ED Barlow in front of a 2" diagonal. It needs a 35mm extension between the scope and the Barlow to come to focus. Your 14mm would give x188, and the 10mm x263. Both very useable with this scope.

With both size, and quality, far better than the f11 scope you mentioned. A 6" CC would be good, but, there's a special feel to a good apo.

Perfect for solar too 

[mikeDnight]

2 hours ago, Davesellars said:

Thanks guys.

F/7 is not really sufficiently long for a 4" to get enough magnification at times without a very small eyepiece focal length which then is uncomfortable and reduces the exit pupil too much for my liking.  I use a 4mm (cheap TMB planetary II clone) with the 80mm ED which is just about OK but that's giving 150x which is not really sufficient.  I certainly wouldn't want to go start using a small focal length eyepiece than that.

I didn't have any stability issues with my 120ED on the EQ5 (probably similar weight and length to the 4" f/11) although noted it would be uncomfortable on the mount because of the length when the Moon is high up so that's a good call.   OK, probably erring towards the 6" CC but in no hurry to decide at the moment!

I get around the short focal length eyepiece dilemma by using long focal length eyepieces in a cheap binoviewer. With a 2X short barlow attached to the nose of the binoviewer the amplification increases to approx 4X, so a pair of 18mm eyepieces in my F8 refractor gives ~X178 which is perfect for Jupiter or the Moon. And because you use both eyes, the views are so sharp and detailed you'll see fine detail much more easily. Also, you don't need expensive eyepieces, as good plossl's or Orthoscopic's will perform as well if not better than the finest eyepiece in mono mode.

The short focal length eyepieces below cost approximately  £1000, where as the binoviewer, plossl's and 2X barlow combined cost approx £250. I use the binoviewer for 99% of my lunar and planetary observing, and 99% of the time the binoviewer image out strips everything else by a large margin.

[Davesellars]

Many thanks Mike.  I'd not considered before going down the binoviewer route....  I can see how that would increase the comfort quite a bit.

I don't think I can budget a £900 refractor for this though especially with other extras so may have to re-think. I do like that idea with the binoviewers though!

I've read that the contrast is basically calculated by deducting the central obstruction diameter from the aperture - since the CC6 has a 58mm obsruction this would essentially give it less contrast than the 102mm refractor.  While resolution would be better in the 6" CC... the low contrast details for Jupiter and finer lunar features may well not be as good in the CC 6" as the 4" refractor...

[Ricochet]

3 hours ago, Davesellars said:

F/7 is not really sufficiently long for a 4" to get enough magnification at times without a very small eyepiece focal length which then is uncomfortable and reduces the exit pupil too much for my liking

Do you mean exit pupil? Exit pupil is related to magnification by the formula 

Magnification = Aperture / Exit pupil 

This means that for any given magnification both f7 and f11 100mm telescopes will have identical exit pupils. If you want to increase the exit pupil at a certain magnification it is the aperture rather than the focal length that you must increase. 

For example if you choose to set a magnification of 200X, all 100mm telescopes will have an exit pupil of 0.5mm, all 150mm telescopes will have an exit pupil of 0.75mm and all 200mm scopes will have an exit pupil of 1mm. 

If instead you mean eye relief, then this can be countered either by using an eyepiece range with consistent long eye relief or using a longer focal length eyepiece and barlow, although the 4mm planetary you mention fits into the former category. 

As Mike says, for planetary observing binoviewers are the way to go, providing you get a decent pair and can merge the images. 

[johninderby]

Remember when calculating the obstruction of a scope you need to take the area of the obstruction and not the diameter of the obstruction which gives very misleading results. 

The effect of a central obstruction is probably the most misunderstood subject in astronomy. It has far less effect than is commonly believed. The CC has great contrast and probably better than a 4” frac. There have been tests where users were unable to detect the effects of central obstruction.

[Davesellars]

Thanks Ricochet.  Yes it's exit pupil is the issue as I start to get some floaters with my 80ED and the 4mm. which gives an exit pupil of 0.53.  While it's not overly bad. I certainly wouldn't want to go any lower and find using the 5mm Pentax much more comfortable for prolonged periods (exit pupil 0.67).

John: after some research I'm sure you're right that this calculation for loss of contrast (not talking about loss of resolution) is overstated and to me would seem an over-simplification anyway not taking other factors into account.

[johninderby]

CO is just one factor among many others that affect performance of a telescope. 

[johninderby]

Found this statement in a article on telescope performance by Thierry Legault.

“Because in the image quality field, obstruction is the tip of the iceberg. The photographical and electronic amateur production shows us that almost all working instruments are not limited by the obstruction but by other causes like misalignment, thermal equilibrium defect or bad focusing, often at a level where obstruction becomes a negligible phenomenon. The qualities of an instrument are not limited to its obstruction, there are more important factors in practice: easy thermal equilibrium, image stability, reliability of the focuser, reliability of the optics supports, etc.”

Edited February 14, 2022 by johninderby

[vladmanm]

John, did you upgrade to the RT or SC version of the Baader focuser?

[johninderby]

49 minutes ago, vladmanm said:

John, did you upgrade to the RT or SC version of the Baader focuser?

It’s the RT version. Just need the one extension normally. If imaging you might need another one.

Edited February 14, 2022 by johninderby

[vladmanm]

13 minutes ago, johninderby said:

It’s the RT version. Just need the one extension normally. If imaging ypu might need another one.

Thanks, I think I'm getting one too. It's gonna help improve my collimation (no tilt like on the stock focuser) and the long travel means less fiddling with the extension rings. The tube might bump into the ota when inwards, but it has to be extended to reach focus anyway. I'm guessing it takes an M90 adapter to fit.

[johninderby]

Yes you need this adaptor.

https://www.firstlightoptics.com/baader-diamond-steeltrack-focusers/baader-steeltrack-diamond-rt-adapter-m90x1-for-refractors-gso-rc-scopes.html

[Davesellars]

11 minutes ago, vladmanm said:

Thanks, I think I'm getting one too. It's gonna help improve my collimation (no tilt like on the stock focuser)

Wait..  what??? the focuser is not straight?

[johninderby]

15 minutes ago, Davesellars said:

Wait..  what??? the focuser is not straight?

The stock focuser is a basic crayford and not as precise as the Steeltrack. The drawtube isn’t as well supported. 

The stock focuser isn’t terrible but suffers compared to a Steeltrack.

Edited February 14, 2022 by johninderby

[dweller25]

10 hours ago, Davesellars said:

I've read that the contrast is basically calculated by deducting the central obstruction diameter from the aperture - since the CC6 has a 58mm obsruction this would essentially give it less contrast than the 102mm refractor.  While resolution would be better in the 6" CC... the low contrast details for Jupiter and finer lunar features may well not be as good in the CC 6" as the 4" refractor...

That is exactly what I found 👍

[Deadlake]

11 hours ago, mikeDnight said:

The short focal length eyepieces below cost approximately  £1000, where as the binoviewer, plossl's and 2X barlow combined cost approx £250. I use the binoviewer for 99% of my lunar and planetary observing, and 99% of the time the binoviewer image out strips everything else by a large margin.

I have the same set of Vixen HR's as Mike and since I've got the BV's for looking at the moon I don't really have the need for the HR's.

Having two eyes certainly increase resolution. I also find BV's good for DSO's.

Note: Also make sure focuser has enough back focus to enable focusing when using BV's.

Edited February 14, 2022 by Deadlake

[nfotis]

Personally, I am a fan of Maksutov scopes when speaking about lunar/planetary observation and imaging.

My vote would be for a Skymax 150, if the 180 is out of your budget. The nice thing about the Maks is that their long focal ratio leads to high contrast and refractor-like views.

N.F.

 

[Davesellars]

I've not tried a Mak however I really didn't like the mirror shift in my 8" SCT when I had it which i believe is also present in Maks as well.

[nfotis]

You don't *have* to use the internal focuser all the time, you could use it for going at the region of focus, then use an external focuser for precision focus and let the mirror steady?

N.F.