Aperture Mask for 16" f4 dobsonian

[Moonshane]

I finally got round to painting my hardboard aperture mask matt black today, but not before I cut a further two holes. I now have three holes of 170mm, 130mm and 100mm. These provide further options when the seeing is not so good and effectively provide 'four scopes' in one:

16" f4 (actually f4.5 with the Paracorr as focal length is increased to 1840mm)

6.7" f11

5.1" f14.2

4" f18.4

I have made some flaps held in place with velcro to cover the holes not utilised and it will be interesting to see what effect this has and whether there's a real difference between the different focal ratios and apertures (other than brightness /light gathering of course).

The scope's mirror is figured at 1/8PV so I am quite hopeful that these unobstructed views will be every bit as impressive as the equivalent refractors.

I'll add some pics when the paint has dried and it's in the scope. It's amazing how small the 100mm looks.

[swamp thing]

Look forward to reading your conclusions.

This is something I've always had mixed results with myself.

I would think the larger hole may help but the smaller ones would only result in too much resolution being lost.

Regards Steve

[Moonshane]

Hi mate

Hopefully I'll get a chance later tonight if the clouds play ball. I was inspired to do this by some comments of others about great levels of detail with e.g. 100mm and 120mm refractors and wondered what it would be like with a slower version. Also, I think it was Olly said something about a larger aperture scope working above the seeing (or something similar) and again I wondered if, on those nights where seeing is pants, it would benefit the views to reduce aperture / increase focal ratio. As you say, will be interesting to see the results and if nothing else, I have reduced the weight of the mask to aid balancing!

There's no doubt from my experiences so far that the 170mm f11 hole makes a massive difference to visible detail and contrast (although to be fair, the detail is generally there at full aperture, you just seem to have to work harder for it (apart from those 'special' nights).

[Moonshane]

hi all

here's some pics of the mask in situ and with the various apertures (170mm, 130mm and 100mm).

Not really had a chance to use it yet - hope to compare it to a 100mm ED refractor tonight.

[nightfisher]

The man goes to all the trouble of building the astro version of an iraqi super gun, then shrinks it :D

seriously, at 100mm f18 this should be a right planet moon killer, look forward to how it performs

[angusb1]

Looking forward to hearing how it performs Shane

[Moonshane]

The man goes to all the trouble of building the astro version of an iraqi super gun, then shrinks it :D

seriously, at 100mm f18 this should be a right planet moon killer, look forward to how it performs

no, seriously, this is a space SAVER. it means I only need one scope! (honest)

[Gina]

If you say so

[swamp thing]

no, seriously, this is a space SAVER. it means I only need one scope! (honest)

Yeah, you may only need one, but how many have you actually got?

[Moonshane]

OK OK

I have three and they are all out at the moment. hoping to have a shootout between my various guises and a Celestron 100ED. Never looked through an ED scope so will be interesting to see how they compare. Targets will be full(ish) moon, Mars, Saturn and a few choice doubles.

[Stu]

So are we going to tempt you over to the 'dark side' of refractors then Shane? You know it makes sense

Stu

[Moonshane]

no way Jose! I have tried several and as nice as the images often are, I really don't get on with them. My logic is (I hope) that I'll look through the frac and decide that I am not missing anything with the kit I have. I can see the attraction of fracs but they are not for me.

[Stu]

Didn't think so

Mind you, with your scopes, not sure I would bother either. Maybe I'll show you the Double Cluster through my apo with the 31mm nag at PSP to see if that works.

Very interested to hear the results of the aperture masks tonight, it's such a simple idea but a fantastic way of getting unobstructed optics.

Stu

[Stu]

How did it go then Shane? Was a great night down here so I hope the seeing was good for you too

Stu

[Moonshane]

hi Stu, yes I was just about to post - had a lie-in and just been out taking the dog for a walk - trying to train him to 'come back' - not the easiest thing to do!

Well, last night Telescope40 and I (John) had a bit of a head to head - main targets were the solar system objects available, Venus, Mars, Saturn and Luna. With the moon so full it was really a waste of time to try and observe much else but we did also have a brief look at some other objects later in the night to mix things a little.

This is not a totally objective/scientific report, but I have summarised my (our) findings to broadly reflect what we could see. We discussed the images throughout the night and generally agreed with each other so hopefully I have accurately summarised John's viewpoint too - I am sure he'll advise if not.

Consider when reading this update, the following:

• I am totally biased towards newts, really don't get on with chromatic aberration and like to observe in comfort and without any yoga.
  
• I have three newts and want to keep it that way and tonight was for me at least about ruling out once and for all the chances of getting a refractor, albeit that I had never looked through a good quality refractor before.
  
• John's main aim was to establish whether or not a slow newtonian of 6-8" would be a good bed fellow for his 100mm refractor and 10" SCT.
  
• We tried to use the same quality of eyepieces during the evening, mainly Televue, Baader Hyperions (in John's f9 frac) and BGOs.
  
• We used a Baader Neodymium filter on all targets other than Saturn. This filter really enhances contrast on Mars and Moon and reduces a lot of glare on Venus.
  
• We concentrated on Mars as this was the most challenging object available to us.
  

Equipment

The scopes used were as follows:

• 16" f4 manual dobsonian with 1/8PV optics - used at full aperture and also masked to 170mm, 130mm and 100mm. This scope has a focal length of 1840mm with the Paracorr inserted.
  
• 6" f11 manual dobsonian with 1/6PV optics on a driven equatorial platform.
  
• 6" f5 newtonian with 1/8PV optics on a manual alt-az/pedestal mount.
  
• 100mm f9 Celestron ED refractor on a manual Sky-Tee 2 mount and tripod.
  

Targets

Venus

This is always a target which provides little in the way of detail but really tests optics as it is so bright. The position of the planet in the west makes observing tricky with anything that cannot be moved easily. Therefore we tested only the 6" f5 and the Refractor on this object.

The view was broadly the same in both scopes although there was possibly a little more false colour in the refractor. Not at all troublesome though and potentially could have been due to the refractor not being fully acclimatised at this stage. A little later, we managed to get Venus and the Pleiades in the same field in the 26mm Nagler; a lovely sight but not as good as it would have been to see it the night before when the planet was amongst the cluster apparently!

Low power was used in both scopes at about 50x.

Mars

This was the planet that I knew would be the biggest test of the scopes and was the one that I was most looking forward to observing.

Using the full aperture of the 16" give a bright and very occasionally sharp view but personally, I prefer a slightly dimmer, steadier and more contrasty view and this is where the aperture mask really shows its worth. We started with the 100mm aperture and this provide a rather nice image which was broadly identical to the refractor as might be expected. With increases in aperture to 130mm and then 170mm the scope came into its own providing a slightly bright and still very detailed view of the planet.

The northern polar cap was very obvious throughout and the detail seen was pretty much as shown in the images in this thread http://stargazerslounge.com/imaging-planetary/184187-mars-5th-april.html. Obviously the fine detail was not seen visually but the main features of Syrtis Major and Utopia were readily detected as well as the southern pole towards the end of the night - As you can tell, I have not yet got my head around the geography of Mars. In summary, the best views of the night were by far and away through the 16" masked to 170mm. Toward the end of the night we were able to use much higher power with this scope that the others, managing 300x quite readily.

The 6" f11 provided extremely good views which whilst not as detailed as the 16", were certainly displaying all the main features seen in the larger scope. This was helped a little by the tracking I think. The maximum magnifications were not as high as the 16" but still good at around 240x.

We (I) forgot to use the 6" f5 on Mars but previous observing has confirmed that on this target, the views are achieved at less power and with less sharpness than the other two options available to me. It provides a respectable view though with the main features still present with patience.

The refractor was also achieving very respectable magnifications with the Nagler zoom set at 4mm (225x). I have to say that although I preferred the view through the newts, the view through the refractor when on axis was virtually the same as the 6" f11. Off axis, I did find there was a little false colour but not much.

Moon

The moon was very full and not an ideal target but we had a go.

The results here were more evenly matched with the refractor providing really nice sharp views of the surface which certainly matched the 6" scopes. There was not a lot to choose between them.

The 16" masked to 170mm had a little more resolution but this was not really useful as there was very little contrast on the terminator.

One thing I did confirm with John was how little I like observing the moon with my 13mm Ethos. It's nice to fit the whole moon into the field of view at 125x but there is so much chromatic aberration (created by the eyepiece) that it's really quite uncomfortable - almost like looking at Sirius! It just shows that Ethoi are not necessarily the best eyepieces for all objects. I am sure others will disagree but the image of the moon was better through the 15mm TV Plossl and 12.5mm BGO.

Saturn

Saturn is a strange beast and although quite low in the sky was relatively unaffected by aperture with all of the scopes demonstrating approximately the same view. Again the refractor when off axis did have a little false colour to my eyes but nothing horrible and when on axis was very nice indeed.

The Cassini Division was certainly visible with all scopes and we managed to find initially just two moons (Titan and Rhea) but eventually with the full aperture of the 16" managed to detect a total of five (the other two plus Dione, Tethys and Enceladus); not a bad haul for such a bright moonlit night and with Saturn so low.

A few Double Stars

During the night we also had a look at a few double stars to compare the views. These included Alcor/Mizar, Algieba, Polaris , the double double and Izar. The objects were all very similar in appearance with the refractor, as would be expected, having the slight edge of star roundness/tightness. There really was not a lot in it though when using the aperture mask or the 6" f11.

Conclusions

For me the main conclusion is that I really have no need for a refractor. I can achieve the same results and better with the more flexible animal that is my 16" dob. Obviously the larger scope really excels at finding and seeing faint targets at dark sites (and at home in good conditions) but it's good to know that even at home where light pollution can be terrible, I can just add the mask and have the equivalent of a good quality refractor for unobstructed observing of the brighter solar system objects and double stars and on an extremely stable platform.

The results were very pleasing and as far as I am concerned I was a lot more comfortable using the dobs than the refractor on the tripod/Sky-Tee2. I suppose it's what you are used to using though.

I for one will not be looking for a refractor anytime soon. John on the other hand seems to have formed the view that as he already has a great quality refractor, and it more or less matches the smaller slow newtonian then the really does not need to go for the additional scope, especially as he has the 10" SCT for aperture. What he did realise is that he really likes the flexibility of the 6-3mm Nagler zoom in the refractor which gives him magnifications of 150-300x and therefore would make a better addition to his collection than a further OTA (sorry John if this sessions creates a TV habit for you! ).

Personally, I have never looked through a Hyperion and I was really very impressed. OK the scope is a slow f9 but the view through these eyepieces was really excellent and I had no major issues with eye placement which is an oft quoted 'problem'. They have a large eye lens and this creates a very immersive feel and if you have a slow scope I'd have no issues recommending them.

This was a really interesting comparison session and it's good to finally get a look through a good quality frac. As good as it was, it made my mind up once and for all - it's Newts all the way for me.

[angusb1]

Good writeup Shane. The comparison with the 100mm apo is really helpful as a large newt or a 100mm apo is a choice plenty of us have considered.

[Moonshane]

cheers Angus - it helped me decide and hopefully will others too. I think that we all wonder what if..... and this sort of sharing of gear helps us to establish what we like best with no cost implications. It's always nice to observe with others too!

[swamp thing]

Great write up Shane. Sounds like a very familiar night (I have an observing buddy with a 4" ED). Lovely little scopes.

Regards Steve

[Moonshane]

cheers Steve

they are great scopes but not for me.

[Stu]

Very interesting and detailed report Shane, thanks for posting it. Would you say that there ended up being any benefit to the smaller aperture masks or was the 170mm better? Perhaps in worse seeing conditions the smaller ones may show an advantage?

Any idea why the neodymium works so well on Mars and other targets but not on Saturn? I independently came to the same conclusion myself last night.

As I posted earlier, with your choice of scopes, I'm not surprised that you concluded you didn't need another

Stu

[David Smith]

Thanks Shane, Interesting read, especially after this thread -

http://stargazerslounge.com/discussions-scopes-whole-setups/183938-new-scope-planetary-observing.html

Might try this with my 250mm see what the results are like.

[Moonshane]

hi Stu

I think as you say the smaller apertures might be better when the seeing is very poor and possibly with some double stars but in general the 170mm wiped the floor with the 100 and 130mm holes.

Anecdotally, I think that given the Neodymium filter seems to put a bluer caste to the view, and perhaps this just suits the reflected light better for Mars/Jupiter/Moon but not Saturn. I really don't know though.

[Moonshane]

Thanks Shane, Interesting read, especially after this thread -

http://stargazerslounge.com/discussions-scopes-whole-setups/183938-new-scope-planetary-observing.html

Might try this with my 250mm see what the results are like.

I was just about to post on that thread as it happens! Definitely worth a try

[Telescope40]

Hi Shane. Just had a good read of your write up and agree totally with all your conclusions.

The 170 aperture mask was definately the overall winner of "Best views" award.

Highlights for myself were the view of the Pleiades and Venus in the same FOV. The views of Mars using the 170 mask were stupendous and I am happy to confirm the details achieved in the images Shane has noted, were all seen.

Saturn, even though low and not far from the full Moon was also more than I expected. First time I had seen our ringed friend for a while and to pick up the 5 moons, after a bit of patience, and convincing each other we were not seeing things, just shows how well the f4 was working, masked off as described.

As for the scopes and the refractor/reflector shoot out, I did conclude the differences between my C100 and Shane's two 6 in reflectors were very subtle and I think a purchase of a third scope is not the way to go. They dead heated TBH in practically all respects. The frac may be slightly ahead on the sharpness of doubles here and there.

Televue's 6-3mm Nagler zoom, that I got to use in the refractor, may well be on the shopping list. Credit card be warned.

All in all, it was an excellent night even with Mr Moon being rather overpowering.

I have to recommend the mods Shane has actioned on his Scud launcher and can vouch for the excellent views it gives in whichever mode of operation he sets it up.

My thanks to Shane for the hospitality, coffees and conversation. We will be arranging another meet up soon.

Regards John

[Adz]

A brilliant write up and comparing session on a topic I think a lot of people are pleased to hear more about. Thanks for taking the time to share it all with us John and Shane.

I have a 4" ed frac and it definitely punches above the dob on planetary and lunar with a lovely non washed out image. Its main advantage for me is its much easier to travel with and grab and go etc.

I have found the nagler 3-6mm zoom works brilliantly with it as you guys mentioned. Both on planetary and especially lunar, being able to dial in to a specific detail. CA seems to only be apparent when I push it to 3mm on a bright moon. Also the image dims a little.