True or False

Rastis · #1 03-01-2006, 09:39 PM

Please answer true or false and why to the following statement.

A 4 inch refractor will show more contrast and I personally think the images are sharper then a 8 inch DOB. The DOB's images will definitely be brighter and you will be able to get more useful power from it purely because of its larger aperture.If you are interested in planets the equatorially mounted refractor would be better because planets are seen better by using the maximum usable power that the conditions allow. The higher the power the smaller the field of view and the quicker the image will drift from view. If this becomes a problem then its much, much easier to mount a motor drive in the EQ refractor than a DOB and you can enjoy high power views without constantly needing to re-adjust the telescope.

ballaratdragons (Ken) · #2 03-01-2006, 09:51 PM

True or False to which statement Rastis? You made 7 different points!

1. A 4 inch refractor will show more contrast

2. I personally think the images are sharper then a 8 inch DOB.

3. The DOB's images will definitely be brighter and you will be able to get more useful power from it purely because of its larger aperture.

4. If you are interested in planets the equatorially mounted refractor would be better because planets are seen better by using the maximum usable power that the conditions allow.

5. The higher the power the smaller the field of view and the quicker the image will drift from view.

6. much easier to mount a motor drive in the EQ refractor than a DOB

7. you can enjoy high power views without constantly needing to re-adjust the telescope.

Can you narrow the question down a bit to what you really want to know.

asimov (John) · #3 03-01-2006, 09:58 PM

The obstruction (diagonal mirror) in a newt reflector lowers the contrast to a degree. Comparing a 4" refractor to an 8" reflector I'm not sure which would have better contrast.

Rastis · #4 03-01-2006, 10:01 PM

all please

ballaratdragons (Ken) · #5 03-01-2006, 10:12 PM

1. Don't know.

2. I can't give a true or false on what you 'personally think'.

3. True.

4. I think so.

5. True, This happens with every type of scope.

6. That's debatable. It could cause an argument between some.

7. False. You have to re-adjust all the time to follow objects whether it's a Refractor, reflector, SCT, or Maksutov.

Sorry, best I can do!

janoskiss (Steve H) · #6 03-01-2006, 10:46 PM

False.

Because if any part of a statement is false the the statement itself is false. Here is the deal (provided both hypothetical scopes are of good quality, and for simplicity let us assume that the refractor is an apochromat so false colour is not an issue):

The smaller refractor will show you less detail (4 times less in fact) than the reflector with twice the aperture. The image you see through the eyepiece is effectively blurred by diffraction effects on true angular scales that are proportional to the inverse of the aperture. The view may be more pleasing in the refractor at lower magnifications, but when conditions allow powers over about 100-150x (for your 4" vs 8" scenario), the increased detail (resolution) of the reflector's image will become apparent. In the refractor you will not be able to distinguish surface features much smaller than about 1.5 arcseconds in size, even under ideal conditions (piggybacking Hubble say). In the reflector you'll still make out details half that size. For this reason, at moderate to high magnification the larger reflector will also produce sharper images.

Dennis · #7 03-01-2006, 10:59 PM

Rastis

Organising the single paragraph of serial questions into a list of numbered questions on separate lines would make it easier for people to respond. Also, it would likely make it easier for you to compare and contrast any responses?

Unless your questions are just for curiosity, some brief words on your experience, intended use, set-up expectations, observing location, etc would also help potential respondents to “condition” their responses accordingly?

Regards

Dennis

ausastronomer (John Bambury) · #8 03-01-2006, 11:40 PM

Quote:

Originally Posted by Rastis

Please answer true or false and why to the following statement.

A 4 inch refractor will show more contrast and I personally think the images are sharper then a 8 inch DOB.

That depends on a large number of variables which you haven't specified in your question.

A bad 8" dob will easily outperform a cheap fast achromatic refractor on EVERYTHING except low power wide field views.

An 8" dob against a good 4" refractor ?

Thats marginally closer, it depends a lot on who built the dob, how good the mirror in it is, how well its baffled, how well its collimated, if its properly cooled etc etc. In addition a slow dob eg F8 or slower will perform even better as a planetary scope due to a smaller secondary mirror.

Lets assume however a good 8"/F6 dob and a good 4" flourite doublet.

The 4" APO will show nicer stars, thats about it. The 8" dob will show infinitely more detail in moments of good seeing than the refactor. The dob will also go a lot deeper. IMO "a good" 8" dob will walk all over a 4" APO in just about everything. But then thats only my opinion, but I have been there and tried it. Also tried a 4" Flourite APO against a 10" dob and the 10" blows the 4" away by a bigger margin.

CS-John B

janoskiss (Steve H) · #9 03-01-2006, 11:58 PM

Quote:

Originally Posted by ausastronomer

The 8" dob will show infinitely more detail in moments of good seeing than the refactor...

Also tried a 4" Flourite APO against a 10" dob and the 10" blows the 4" away by a bigger margin.

John, I understand what you are saying, but infinitely more?? And then the 10" goes even beyond infinity??

Rastis, detail goes like inverse aperture squared. (This is the quantitative aperture rules rule.) So 8" Dob will show you (8/4)^2 = 4 times more detail than the 4" scope. In other words, if you were to digitise it, you would need four times the number of megapixels to capture the full detail of the image.

ving (David) · #10 04-01-2006, 08:57 AM

or yet another take....

it is said that your average air cell is 6 inches. given this, if the seeing is bad then a 4 inch aperture apo will show better detail than a 8 inch dob because... well its obvious, the 8" dob will take more than one of these aircells while the 4" will only need one. or so i am told....
given average to good seeing the 8" (all things equal quality wise) should blow the 4" out of the water giving much more detail even though because of the central obstruction the stars won't be as neat...

well this is my take on it anyhow. I dont know if its 100% right but it seems to work in theory.

Rastis · #11 04-01-2006, 09:13 PM

The statement that I asked people to reply true or false to,

Quote:

A 4 inch refractor will show more contrast and I personally think the images are sharper then a 8 inch DOB. The DOB's images will definitely be brighter and you will be able to get more useful power from it purely because of its larger aperture.If you are interested in planets the equatorially mounted refractor would be better because planets are seen better by using the maximum usable power that the conditions allow. The higher the power the smaller the field of view and the quicker the image will drift from view. If this becomes a problem then its much, much easier to mount a motor drive in the EQ refractor than a DOB and you can enjoy high power views without constantly needing to re-adjust the telescope.

was made by a dealer. It was made in reply to my question as to what he recommended for a scope for mainly viewing the planets. Also the 4 incher was not a "APO". I just thought I would get a second opinion from the guns.
Have a nice day,
Rastis.

Rastis · #12 04-01-2006, 09:21 PM

Sorry just thought of this,

Quote:

In addition a slow dob eg F8 or slower will perform even better as a planetary scope due to a smaller secondary mirror.

Where can I get a f8 8 or 10 inch dob from. This sounds like a perfect scope to me. Would be a awfully long scope though. Seems to me that they are all f4 to f6

ballaratdragons (Ken) · #13 04-01-2006, 09:45 PM

The simple answer Rastis is 'Barlow an f5'.

I don't know where you would get an f8 Reflector unless you ground your own or ordered a mirror and made the rest of the scope.

Someone else may know of where to get one. I don't, sorry.

janoskiss (Steve H) · #14 04-01-2006, 09:46 PM

GS Dobs (which are like Orion XTs) you can get from Bintel, Andrews; Saxon Dobs (which are like Celestron Starhoppers) from My Astro Shop (MAS), AOE (Aquila Optical), Telescope Shed. There are others also but these are the ones that come to mind.

Most people here go for the GS Dobs (probably because we send them straight to Bintel or Andrews), but MAS's advertised prices on the Saxon Dobs are also very good; AOE is very competitive as well and their service is great. MAS also has the ED80 refractors (80mm apo) on special for a very good price at $549, should you want a refractor after all. That scope should beat the 100mm achromat that dealer of yours was pushing in every respect. Would go very well on a good AZ mount. You'd also need a star diagonal, finder and eyepieces.

But refractors do have their advantages: faster cooldown (almost none for a small one like the ED80), better grab-and-go (unless you put it on an EQ mount), easier to transport, better contrast, and more pleasing star images (because there is no diffraction spikes from the mirror support). On the downside you will see less stars, less detail at mid-to-high magnifications, and fewer deep sky objects.

Dennis · #15 05-01-2006, 06:53 AM

Quote:

Originally Posted by Rastis

Sorry just thought of this, Where can I get a f8 8 or 10 inch dob from. This sounds like a perfect scope to me. Would be a awfully long scope though. Seems to me that they are all f4 to f6

Rastis

I have a 4" f9 apo on a motor driven equatorial mount and it produces lovely views of Jupiter and Saturn, and reasonable images using a digital camera.

I also have a 9.25" f10 Schmidt Cassegrain on a motor driven equatorial mount which provides brighter, more detailed and larger views, with superior images using a digital camera.

However, when using the 9.25” SCT, if I turn power off to the motors so the mount does not track and then I try to nudge the 9.25" SCT by hand, it is a pain in the bum trying to keep the planetary image in the field of view (FOV). An f8 dobsonian would also share the same difficulties in keeping the image in the FOV as at the longer focal length (and higher magnification) the FOV becomes quite small and the planet literally races across the field of the eyepiece.

Personally, viewing a smaller but stationary Jupiter and Saturn using my 4" apo on a driven mount is much more pleasing for me, than trying to chase a larger, brighter image of the planet on a non-driven mount. However, people do practice the latter quite competently and appear to be as fulfilled as I am when using my 4" apo.

If you are keen on planetary viewing, prior to making a purchase, I would recommend that you get to look through a 4” scope tracking and an 8” dob non-tracking, so you can experience the realities of using both instruments as it is almost impossible to convey the experience via words alone.

Cheers

Dennis

ausastronomer (John Bambury) · #16 05-01-2006, 07:52 AM

Quote:

Originally Posted by Rastis

Sorry just thought of this, Where can I get a f8 8 or 10 inch dob from. This sounds like a perfect scope to me. Would be a awfully long scope though. Seems to me that they are all f4 to f6

Rastis,

Ron Royal used to run a telescope making course through Sydney observatory and they made an 8"/F8 dobsonian from a kit. Ron also used to sell the kit separately to interstate people and then offer telephone support through the building stage. I don't know that he still runs the course as the influx of chinese/taiwanese dobs may have made it difficult to compete.

Discovery telescopes in the USA make an 8"/F7 but they are expensive for what you get. Cave also used to make an 8"/F7 newtonian years ago and very rarely these OTA's are offered on the 2nd hand market. Other than that you would have to commission an ATMer to make you 1 or build it yourself.
However an 8"/F6 is a good compromise between the views it provides and portability , that is why most 8" newts are F6.

In respect to Dennis's comment about hand tracking I agree that you should try both a tracking scope and a dobsonian before you buy. Be aware that it takes a little practice to learn to hand track properly with a dob and it also helps a lot if the dob has been properly optimised for tracking. Some dobs owned by people that don't know what they are doing, or don't care, track horribly and are almost impossible to use. Those owned by people that know or care move like melted butter on toast and are childs play to track at over 500X with a very narrow FOV. Dennis's comment about manually tracking with his SCT is not relevant IMO as a SCT fitted with motor drives is not optimised to be used by hand tracking. Actually some of them are down right impossible to use with the drives switched off. On the other hand a dobsonian is designed to be used by hand tracking and has a much longer moment arm past the pivot axis and is a lot easier to hand track with, if its properly set up.

Quote:

Originally Posted by Dennis

An f8 dobsonian would also share the same difficulties in keeping the image in the FOV as at the longer focal length (and higher magnification) the FOV becomes quite small and the planet literally races across the field of the eyepiece.

AS of NSW have a 16"/F7 dob at Bowen Mountain and it tracks without issue, so I don't see any problem tracking with an 8"/f8 that has about 1/2 the focal length. Likewise I have used a 20"/f5 with a 2.54 metre focal length on numerous occasions at high power without issue. I think your making an assumption there Dennis without ever having tried to track with such a scope.

CS-John B

Dennis · #17 05-01-2006, 10:11 AM

Quote:

Originally Posted by ausastronomer

Rastis,

Dennis's comment about manually tracking with his SCT is not relevant IMO as a SCT fitted with motor drives is not optimised to be used by hand tracking. Actually some of them are down right impossible to use with the drives switched off.

Likewise I have used a 20"/f5 with a 2.54 metre focal length on numerous occasions at high power without issue. I think your making an assumption there Dennis without ever having tried to track with such a scope.

CS-John B

This is good – we are getting to the nitty gritty of some useful stuff that hopefully Rastis will find useful. Your points are valid for optimized dobs and competent users but, you appear to have dismissed some of my comments on technical issues rather than the usability issue that I was attempting to illustrate. Therefore, I offer the following comments:

The equatorial mount used in my example was a Takahashi EM200 with the Declination clamp locked - all I had to do was nudge the RA. Despite the eyepiece being at the “wrong end” and the method of handling the OTA awkward compared to a dob, this was still easy and predictable due to the quality of the movement of the Tak mount. But, when compared to the ease of tracking, it still is a pain in the bum to me, a comment which may be relevant to the original querant.

Also, I have used Peter Robinsons fine (ex) 20” f5 Obsession so I am aware of how easy a good dob can be handled, so there is no assumption here – just a data point to alert the original querant that “try before you buy” would help avoid buyer’s remorse.

Astronomers tend to favour one or the other which just comes down to personal preferences, what works for them and what trade offs they are prepared to make.

Also, I suspect that Rastis would not be purchasing a 25” Obsession

Regards

Dennis

Rastis · #18 06-01-2006, 09:51 AM

Hi chaps,
Are the Saxon dob's made by GSO? If not how do the two compare as My Optical Shop has a good price on the Saxon 8 incher.
Rastis.

janoskiss (Steve H) · #19 06-01-2006, 10:24 AM

Saxons are made by Synta, I guess. The optics should be of similar quality to the GSO (but only guessing again). The main and obvious difference is the mount. Haven't seen many of them in action, and only ever looked through one of the older f5 8" models briefly. Daniel "danielsun" has one. If you PM him, I'm sure he'd be happy to fill you in.

davidpretorius · #20 06-01-2006, 11:37 AM

rastis, my dob is now motorized, so i get very satisfactory high end magnification or imaging surface detail etc on a planet, i also get very good medium and low magnification foe viewing the planet with its moons etc. The dob is great for all types of deep space objects etc because it gathers lots of light.

What it can't do is wide field. Planet surface details and double stars are great, but how lovely is just walking outside and looking up at the milky way and going wow!!

From an imaging point of view, i would love to capture all things i see from very low magnification (if at all) milky way shots thru to 1000x lunar and planetary shots.

My solution and it is not a defeatist attitude is that i will be buying a ed80 down the track. No one scope will do everything! No eyepiece will do everything.

It is a "enjoy the journey" type hobby. I suggest starting with a dob, because it seems to be able to do most area, but you will want to plug those holes with better eyepieces, filters, low power refractors etc as you go along.

It is not one or the other, it is both, but try and start with something that will give you the biggest wow factor straight up!