Planetary Scope Advice

[Guest]

Hello,

I am going to the UK Astro Show in November and will be looking around at some of the Scopes on the stands there for a Planetary/Lunar Scope for visual and imaging purposes.

I have seen lots of discussions on the Skywatcher Skymax 180 Pro and tend to be leaning towards that but I am looking for advice/opinions on other similar scopes before taking the Plunge.

For reference it will be permanently set up in an Obserbvatory, so the cool down rate on the 180 Pro would not cause too much of a problem and the camera I would be using for imaging is the ZWO ASI 224MC USB 3.0 Colour Camera.

Thanks

Neil

[vlaiv]

Have a look here:

There is 10" version as well if you want a bit more oomph since it will be observatory mounted.

[johninderby]

Had the Skymax 180 but recently replaced it with a TS 8” Classical Cassegrarin as a planetary scope. Initial impressions are extremely promising and a big improvement over the Skymax. Similar weight to the Skymax and no problems with dew at all. Bit more expensive though.

 

[johninderby]

Ha ha. Beat me to it. 😁😁

[chiltonstar]

IMO it is worth reading a lot of member's views with each type of scope before you make a decision! If you intend to do a lot of imaging from a fixed observatory, then looking at the websites of the planetary imaging experts, they tend to use the largest SCT they can get, eg a C9.25, and C11 or a C14.

Personally, after four years with a 180 Mak, cooling down and dewing issues have not been a problem for me, particularly after thermally stabilising the scope. Mine will perform to its physical limit ( Airy disc 0.7 arcsec) and show eg surface detail on Ganymede. For me, any improvement would simply be from adding more inches to the aperture - a case of DAF (the dreaded aperture fever).

Chris

[Guest]

Many thanks to you all, funny enough I was looking at the TS Optics after posting this, but its also good to see the Skymax get a good review. I do like the sound of the TS Optics Classic Cassegrain though which I see already comes with a crayford focuser and sounds like it will not require dew protection like tge skymax. I will deffiniteley also look at the large SCT's as well, but they generally seem to be just out of my price bracket. Will have a good look at the show.

Many thanks

Neil

Edited October 27, 2019 by ALIEN NEIL
Typo

[John]

If you can find one, the Russian mak-newtonians are superb planetary scopes. They rival top quality apochromatic refractors of a similar aperture. They are quite heavy and their tubes are newtonian length but their tiny central obstruction and excellent baffling really produce very contrasty and sharp images of the planets.

 

[wouterdhoye]

Hi Neil,

Before making some advice I'd first like to know a few things. One thing you answered already. The telescope will be permanently set up. So here are my next questions.

1) What's your budget? (It's too easy to spend other people's money 😉)

2) What mount will you put the new telescope on? And do you have this mount or is it incorporated in the budget of question 1?

3) What will be the prime use? Visual or photographic. as I believe the needs for visual althoug similar are not exactly the same)

Wouter.

[Guest]

Hi Wouter,

Thanks for responding.

I would like to keep the budget to below £1000 or there abouts, dont mind spending a bit more if it is worth the extra.

The mount is a pier mounted SkyWatcher AZ-EQ6 GT, which I have already got.

The primary use will be imaging, but also some visual, probably a 75/25 split.

Neil

[wouterdhoye]

Hi Neil.

I believe you can't go really wrong with the 180 Mak. However, a C9.25 is not that far out of your budget. And especially for imaging the extra aperture will make a difference. For visual use the difference will depend mostly on how often the seeing is good enough the allow the C9.25 to perform well. I don't know your budgetary constraints, though If I would have to make the decision I'd save up a little for the extra aperture. The C9.25 is a known value for lunar/planetary imaging.

The 8" classic cassegrains are an option however their only real advantage is the open tube design allowing faster cooldown. Note however that a classic cassegrain is trickier to collimate than a SCT. Central obstruction is similar to a regular SCT (33%) so in theory similar performance to a regular SCT is to be expected. The only real advantage they have to me, is that they have a crayford focuser as standard instead of a primary mirror focuser on the SCT's.

For optimal results and convenience I would also suggest you allocate some budget to a decent electronic focuser. Focusing is really critical for high res imaging. So if you can focus without introducing any vibration that will make it a lot easier.

Wouter.

[johninderby]

I found it easier to colimate my classical casegrain than an SCT and I’ve owned three SCTs. About as difficult as collimating a newt. The lack of dewing is a huge advantage over a mak or SCT. Remember it is a specialist planetary scope and delivers better lunar / planetary views than my C8 ever did. and of course zero image shift due to the fixed primary.
My Skymax180 was a very good lunar / planetary scope but the classical cassegrain is better. 
 

Now if only the weather would cooperate.🤬

Edited October 30, 2019 by johninderby

[Guest]

It sounds like I have got some serious mulling over to do, thanks for your advice Wouter, it is much appreciated.

I am glad to hear that collimating the classical cassegrain is no more difficult than a newt, as I have a 8" newt which I manage to collimate, I check the collimation about once every six months and to date it hasn't changed, but thats probably because it's sat in an observatory either in a case or on the mount, so dosent get moved around much. Do you find the collimation on the Cassegrain remains stable and how often do you have to collimate it?

I agree with regards to the weather, although I did manage to get an hours worth of imaging of M33 sunday night before clouds rolled in.

Neil

[CraigT82]

2 hours ago, johninderby said:

About as difficult as collimating a newt. 

Out of interest, what is the procedure for collimating a classical cassegrain? 🤔

Are you planning on doing any imaging with the cassegrain? Would be nice to see some lunar images to compare. 

 

[johninderby]

I’ve only collimated it once but it hasn’t been used a lot with this weather. You might have to adjust the primary but should only need doing once. Adjusting the secondary is the normal method of colimating. It’s a bit different to collimating an SCT  but once you get used to it quite easy. 

[johninderby]

Mainly visual but will take some lunar shots when the weather cooperates. 😁
 

Pretty straight forward to colimate. Here’s the procedure.from the manual

The optics in your new Classical Cassegrain optical tube have been aligned at the factory. However, rough handling during transit may warrant periodic re-adjustments. We have precisely center marked the secondary mirror with a small adhesive ring to aid in collimation. This ring does not affect the view through the telescope, so it should NOT be removed. You will need the quick-collimation cap or a Cheshire eyepiece to check and adjust collimation. We recommend doing the collimation during daytime. It can be done indoors or outside.
1. Remove any extension rings and attach the focuser directly to the optical tube.
2. For indoor collimation, set up your telescope in a well-lit room with the telescope oriented horizontally, and point it at a light colored wall. For outdoor collimation, point the telescope toward the sky but keep the telescope as close to horizontal as possible.
3. Insert the collimation cap or Cheshire eyepiece into the focuser via the included 1.25" adapter. If using a Cheshire eyepiece, make sure that the 45° reflecting surface of the Cheshire is aimed at a bright source of light, like a ceiling light or a clear sky outdoors.
4. Look through the collimation cap/Cheshire eyepiece. If using a Cheshire eyepiece you should see a small black dot and a dark ring within a larger bright circle. The dot is the hole of your Cheshire eyepiece. The dark ring is the center mark on the secondary mirror. And the bright circle is the reflective 45-degree surface of the Cheshire. The larger black circle outside that is the secondary mirror holder. (Figure 8A

If your scope is in good collimation, the black dot will be dead center in the dark ring, which will in turn be centered in the bright circle. If that’s the case, no further adjustments to the secondary mirror will be necessary. The optical axis is denoted by a thin white circle on the outer edge. You can disregard that for the time being; it will be covered in the following section.
If the view looks something like Figure 8B — with the dot of the collimation eyepiece NOT centered in the secondary center ring — you will need to adjust the three collimation setscrews at the front of the secondary mirror holder (Figure 9). This will adjust the tilt of the secondary, changing the relative position of the secondary center ring when peering through the collimation eyepiece.

Optical Axis (Primary Mirror) Adjustment
The optical axis is denoted by a thin outline of light (white) around the perimeter of the view through the collimation cap or Cheshire eyepiece (Figure 8A). If this outline is not a perfect circle of uniform thickness, as in 8C, that’s an indication that the optical axis (primary mirror) needs adjustment. This adjustment will require 3mm and 2.5mm hex keys.

Then it goes into the detailed explanation but the above will give you the idea.
 

Edited October 30, 2019 by johninderby

[Guest]

Thats a better description than I got with my SkyWatcher Explorer 200p, as I ended up using the Astro Baby instructions which were easy to follow.

Makes a change to get a manual with easy to follow instructions.

[CraigT82]

Yes, very good instructions for a change! 

[johninderby]

Should mention these are the instructions that come with the Orion version of the scope. 

And more of the instructions.🙂

Secondary Mirror Adjustment
NOTE: Only adjust the three screws around the perimeter of the holder -- do not adjust the center screw! (Figure 9). Adjusting the center screw can cause the secondary mir- ror to fall off and will not be covered under warranty.
A 4mm hex key is required to perform collimation on the sec- ondary mirror (Figure 9).

When adjusting one of these screws you will need to make counter-adjustments to the other two. Therefore, if you are loosening one screw you will need to tight- en the other two. At the end of the process you want all three collimation screws to be reasonably tight so the secondary mir- ror won’t shift while the scope is in use.
Always start by loosening one screw. Adjust the screws only very slightly -- by no more than 1/10 turn at a time, and one screw at a time before checking the view through the Cheshire again to see how things changed. Only tiny adjustments should be required to achieve collimation. This will also aid in the pre- vention of accidently putting the telescope grossly out of colli- mation. With each tiny tweak of a screw, make a mental note of which way and how far the center dot moved, as that will inform which screw to turn next and by how much. Experiment with different combinations of loosening/tightening the three screws one by one until the collimation eyepiece’s black dot is centered in the dark ring of the secondary mirror. The correct alignment of the secondary mirror is critical in determining if the optical axis requires alignment. Be sure you have properly aligned the secondary mirror before proceeding to the next step.

Optical Axis (Primary Mirror) Adjustment
The optical axis is denoted by a thin outline of light (white) around the perimeter of the view through the collimation cap or Cheshire eyepiece (Figure 8A). If this outline is not a perfect circle of uniform thickness, as in 8C, that’s an indication that the optical axis (primary mirror) needs adjustment. This adjustment will require 3mm and 2.5mm hex keys.
Note that there are three pairs of screws on the rear cell of the optical tube where the focuser attaches (Figure 10). Each pair consists of a small black screw and a larger chrome screw. The small black screws are merely locking screws, which you should loosen before adjusting the mirror’s tilt. The larger chrome screws are spring-loaded collimation screws that actu- ally adjust the tilt of the primary mirror. Turn these collimation screws only a fraction of a turn at a time. Turn one and check the view through the Cheshire to see if it improved the white optical axis ring. Keep tweaking the collimation screws, each time checking the optical axis ring, until it is concentric and uni- form in width. Then tighten the three lock setscrews to fix the mirror in that position.
After adjusting the optical axis, re-check the collimation of the secondary mirror and make any necessary adjustments, then recheck the optical axis collimation. Optical axis collimation will not need to be performed very often, if ever.

Star Testing
A star test can be performed to confirm the collimation accuracy of the telescope. The adjustment procedure on the telescope is the same as described above; testing, however, will be done in the night sky using a real star and you will not use thE collimation cap or Cheshire eyepiece. Choose a star close to the zenith (straight overhead) rather than at the horizon to minimize atmospheric distortions. Using Polaris as your target star can be helpful as minimal drift adjust- ments will be required.
It is recommended that you not use a star diagonal while per- forming this procedure. Rather, place an eyepiece directly into the 1.25" adapter in the focuser. It should be an eyepiece that provides moderate to high magnification. You may need to add all three of the extension rings in front of the focuser to be able to reach focus. Center the star in the field of view. Slowly de-fo- cus the image with the focusing knob until you can see a series of concentric diffraction rings form around the dark disk in the center. That dark disk is the shadow of the secondary mirror. In a well collimated telescope, the diffraction rings should appear round and concentric, with the dark disk exactly in the center (Figure 11). If the dark central disk is off center, the scope is out of collimation. Adjust the collimation of the secondary mirror and, only if necessary, the primary mirror while monitoring the defocused star until the dark central disk is exactly centered in the diffraction rings.

NOTE: It is important when checking or adjusting the collimation using a star, that the star be positioned in the center of the eyepiece’s field of view. If it isn’t, the optics will always appear out of collimation, even though they may be perfectly aligned! It is critical to keep the star cen- tered, so over time you may need to make slight corrections to the telescope’s position.

Edited October 30, 2019 by johninderby

[joe aguiar]

you do know thwe cooldown is gonna take a long time?

about 2006 or so I got my hands on one of the last Meades 7"f/15 UHTC mak cass OTA this was the verion without the extra weight to balance it like the fork mounted ones had.

which is the the same size as the SW mak. Mine had a built in powered fan that ran off 10AA battery pack and 1 intake air vent.

Views were pretty good from 1 to 2 hrs cooldown time BUT it was even better after the 2 to 3 hr cooldown time.

so if you get that one it doesn't have any air in take vents or fan so it will take even longer to cool

joejaguar

[joe aguiar]

altho some of the views I got for instance of Saturn were pretty much the best I have ever seen.

joejaguar