Could a slow refractor hit a sweet spot for planetary and solar, that a fast reflector misses?

[Giles_B]

While the skies are cloudy I'm back to my fantasy telescope collection I'm afraid.

I understand that for really good planetary views I'd need a slow catadiotropic scope of some sort. However, I also have a burgeoning interest in solar, I wondered if there a sweet spot where a slow refractor will get me both clearer images of the planets than I have with my 10" dob, and be a good candidate for upgrading my solar viewing?

To give some background, I have a quite lovely f4.8 10" dob, and this gives me great deep sky views - even better if I can get it to some dark skies (Bortle 3-3.5), which I managed last week when on holiday. Quite mind blowing, and I should really be very content. However, it's not often that a long drive to a dark sky is feasible - there are only so many clear nights when I don't have family fun or need to be fresh the next day. DSOs from my Bortle 5-6 garden are much more accessible and good, especially on a good moonless night, but there are inherent limits to what can be seen under a these conditions. I'd also say that, while lunar views are amazing, planetary views with the dob are slightly disappointing. Planets are small even at high magnifications - and don't take the magnification well. I'm happy to take this if this is how things are, but I have been wondering if a slower telescope for planetary might fill the planet gap.

I've also been enjoying a dabble with white light solar. While recently my repurposed, aperture filtered, 130mm newt does the job, my understanding is that a refractor with a Herschel wedge would improve the contrast for white light. Futhermore, as a future project, Solar in Ha is tempting, but the entry level Ha scopes (PSTs and 50mm Lunts) seem to have very narrow angles of view and this is an evident frustration of users... If I had a larger frac for white light then this would give me a clear upgrade path that might avoid these issues, assuming I one day was wealthy enough (and remained interested enough) to get a Quark.

Window shopping for second hand refractors (I hasten to emphasise this is fantasy shopping for now), I can see there are only a few on UKABS that fit the bill i.e. are around the F11+ mark with 4"+ of aperture - for example there is an Istar 127mm f/12 R30 and a 6" F12 D&G  - These are also expensive scopes (for me) even second-hand, and I imagine that any refractor with these specs would be. Is it worth saving for something like these? Would they outperform my 10" dob on planetary? Would they be good candidates for Solar?

Help me out here - I'm a bit worried I'm just getting telescope fever and not thinking this through correctly 🤩

 

[dweller25]

Planetary views in your 10” Newtonian should be really good if the scope is well cooled and collimated.

You would need a fairly large refractor to out perform it.

For solar you don’t need a lot of aperture for good performance, perhaps the Skywatcher 100ED or 120ED may be worth considering.

[neil phillips]

Agree with David look at my sig scopes. The best on planets is the 245mm F6.3 Newtonian

My 5" F9.5 Achro frac cant touch it. Good as it is. guess which is which ? 

How good is your mirror ? what about collimation ?

Edited August 2, 2022 by neil phillips

[Giles_B]

@neil phillips Are those pictures under normal, rather than exceptional, conditions?

I'm really not getting that sort of banding on Saturn. As I don't get out that often, I assumed it was the conditions.

Feel pretty confident the 10" scope's in good collimation. Dark site DSO views are crisp, bright and generally excellent.

[globular]

I suggest you make a simple off-axis aperture mask for your 10".  You should get something like 4" fully unobstructed.

I think you'll find the views will be improved on planets on nights with average seeing.  And you just take off the mask on the odd night with perfect conditions (or when not viewing planets, of course).

Costs nothing... must be worth a try at least?

Edited August 2, 2022 by globular

[neil phillips]

52 minutes ago, Giles_B said:

@neil phillips Are those pictures under normal, rather than exceptional, conditions?

I'm really not getting that sort of banding on Saturn. As I don't get out that often, I assumed it was the conditions.

Feel pretty confident the 10" scope's in good collimation. Dark site DSO views are crisp, bright and generally excellent.

Its a little unfair the top image was good seeing the bottom not so good. But still, you can tell which is the leader. And no you will not see what I have produced here visually not even with a 16" scope. The banding has been enhanced post processing. As has the colour. The point wasnt to say look what you should be seeing. ( because you wont) the point was to show you a good 5" F9.5 Achromatic refractor. Can not compete against the larger reflector. Not in detail or colour. You mentioned a couple of larger achros . The point was they wont do what you want. Longer focal length mirrors help a bit. Mine is F 6.3 Mirror quality can help quite a bit. Especially if prior quality is bad, like a turned down edge or something. As can good seeing. And collimation. If your mirror quality is fine. Then you just might be expecting more than you can get. Without a observatory class instrument. A smaller ED scope wont change this position btw. Might be tighter with better colour. But it will still be limited no matter how perfect it is. 

Edited August 2, 2022 by neil phillips

[Giles_B]

Okay, thanks - I got a bit worried for a moment! Yes, I suppose there is still some pent up expectation as I've not long moved to a 10" reflector, and even more recently used it on planets for the first time.

I think you have answered my question really, which was whether I could get a frac that would "do it all" - looks like it's a firm "no" and I should just concentrate on what is needed for solar - which is probably quite a bit cheaper (in the first instance) anyway.

By the way @globular the tip on stopping down is really appreciated - I'll try it next time I'm out (this weekend looks likely to be clear 😀)

[ED Splitter]

Is this a case of the best telescope for you is the one you will use the most? 
The 100ED is my only scope. I did consider reflectors as an alternative but a number of factors pushed me to a refractor. Those that may be of relevance to you were, the ease to set up, quick cool down time which together mean more frequent shorter observations if that’s what your lifestyle allows. Also dark sky location observing would be easier. 
A refractor would be a good complimentary scope. Why not wait for a used refractor and try it, if it doesn’t work out you can sell it on for the same money. 
I second stopping down the scope, especially for Jupiter. Good eye pieces also help ring out the last of available detail. 

[globular]

1 hour ago, Giles_B said:

By the way @globular the tip on stopping down is really appreciated - I'll try it next time I'm out (this weekend looks likely to be clear 😀)

Please report back how you get on... be interesting to hear.

[Peter Drew]

An interest in both night time and day time astronomy is best served with two telescopes.  You already have a potentially excellent 10" for the former and just need a refractor around 4" aperture for solar.  Most telescopes are struggling to perform at their best with the planets currently being so low.     🙂 

[Alkaid]

For planets, the image scale of the 10" F5 dob vs a 4" F11 refractor is about the same.   1270mm focal length as opposed to 1100mm focal length.  Not much in it, so comparing the same eyepiece between the two scopes would result in a planetary image of about the same size.  You should however get more detail with the dob due to it's distinct aperture difference.  The refractor will show less detail overall, but it shows what it can with more contrast, so it appears sharper in many cases, even if it's not actually delivering as much visual information.  

Due to living in a city, my targets are only the Moon & Planets.  From my experience, you need 4 main things for planets:   Large aperture for detail.   Long focal length for image scale.   A very stable atmosphere (rare in the UK).  A cooled telescope - make sure that the scope is cooled to outside ambient temp.    Any one of these four not conforming can cause the view to be 'off'.   Add to that the quality of the optics (mirror quality and collimation of mirrors, lenses, eyepieces, diagonals with fracs) and you soon learn that planetary observation has quite a lot of elements to it, whereby if any one of these is poor, so will be your viewing.

But don't be put off.  Once in a while it all comes together and the views can be very good.

[Macavity]

When I was calculating the "best" f-number for a Solar Frankenscope, I seem to remember
there was a GOOD reason for Solar scopes using f/7 (or slower?) objectives. Basically, the
Sun is "extended" (half a degree in diameter) etc. But, you do the Maths? (I forget now!). 😅

[Giles_B]

On 02/08/2022 at 17:22, globular said:

Please report back how you get on... be interesting to hear.

Okay - here's the full report. Actually I didn't prepare and use the off-axis mask until Sunday, but it is interesting to compare the two days

I was out on both Saturday 2-4am and Sunday 2-4am (on Sunday I'd planned to observe sooner when Saturn was higher but I overslept *sigh*)

Saturday, Saturn was somewhat low. I could get good magnification down to about 6mm, but lower than that there was too much wobble. Saturn was pretty unvarigated white disk and ring, perhaps a hint of banding but very faint. Once again I found it impossible to make out the Cassini division, which has been one of my main gripes with my observations. I tried a neutral density filter, which made things darker, but didn't help much. Then, reminded of something I thought I had read about, I tried a UV/IR blocking filter. This was a revelation, cleaning up the wobble, allowing me to advance the zoom eyepiece a stop down to 5mm (but no further) and for my first time giving me a clear difference in the banding of the inner and outer ring - although I couldn't make out a clear black line between the two. Anyway I was happy that I had seen the Cassini division at long last 😃 - I eventually stopped observing when I lost sight of Saturn behind a tree.

I turned to Jupiter - much higher in the sky than Saturn - and was amazed I could take the magnification down to a full 3mm and still see a well defined disk with clear banding and - to my delight - the shadow of Europa in transit. In all I spent about 30 minutes looking at Jupiter, and 90 on Saturn, before calling it a night.

All in all very successful, so I was full of enthusiasm to try again with a stopped down scope. I fashioned a mask with a 3.5" aperture - maybe a little small in retrospect - and once again got Saturn in my sights. The seeing was, if anything, a bit poorer than the night before, and the UV/IR filter produced a less dramatic effect, although again I could make out faint banding and differentiate the inner and outer sections of the ring. Trying the off-axis filter - I was presented with a slightly dimmer (although nevertheless quite visible) image, and no real discernable effect, except perhaps slightly fainter details. Switching to Jupiter, the effect was more pronounced dimmness and fainter details.

On 03/08/2022 at 13:15, Alkaid said:

For planets, the image scale of the 10" F5 dob vs a 4" F11 refractor is about the same.   1270mm focal length as opposed to 1100mm focal length.  Not much in it, so comparing the same eyepiece between the two scopes would result in a planetary image of about the same size.  You should however get more detail with the dob due to it's distinct aperture difference.  The refractor will show less detail overall, but it shows what it can with more contrast, so it appears sharper in many cases, even if it's not actually delivering as much visual information.  

Due to living in a city, my targets are only the Moon & Planets.  From my experience, you need 4 main things for planets:   Large aperture for detail.   Long focal length for image scale.   A very stable atmosphere (rare in the UK).  A cooled telescope - make sure that the scope is cooled to outside ambient temp.    Any one of these four not conforming can cause the view to be 'off'.   Add to that the quality of the optics (mirror quality and collimation of mirrors, lenses, eyepieces, diagonals with fracs) and you soon learn that planetary observation has quite a lot of elements to it, whereby if any one of these is poor, so will be your viewing.

But don't be put off.  Once in a while it all comes together and the views can be very good.

So my feeling is that Steve was right here - essentially I got the same viewing experience, probably slightly worse, because I'd stopped down to a 3.5" not a 4". And yes, for one night of the two the view was really good, and I went to bed tired but excited having at last seen some detail in the rings and viewed some of that wonderful transit!

Anyway, Thank-you all for the help and advice (and sorry to sneak an observation report into Scopes and whole setups!). As far as new scopes go, @rl is very, very kindly gifting me an unwanted 90mm f/8.8 Meade refractor, so I am very excited about getting a 'first frac' and trying this with some white light solar 😁