Flat Field of View for visual?

morls (Stephen) · #1 29-07-2010, 08:57 AM

Hi,

I'm thinking about getting a barlow or telextender, and I'm just wondering if I need to take into consideration 'flat field of view'....

I'm very much a beginner, so it would be mainly for visual observing in my 8" F6 dob, so is there anything I should be aware of? I think I've read somewhere that, for visual, a flat field is not really ideal....

Thanks,

Stephen

Kal (Andrew) · #2 29-07-2010, 09:13 AM

A barlow will in effect increase focal length, which decreases field curvature, so you don't need to worry about this.

NorthernLight (Max) · #3 29-07-2010, 05:27 PM

HI Stephen,

flat field of view means all things in an image find focus on the same plane from centre to the edges of your field of view. It is a good thing to have and necessary for photography. People spend quite some money to get a corrected and flat field of view. It spares one the refocusing to view things closer to the edge sharp e.g..

morls (Stephen) · #4 29-07-2010, 06:06 PM

Thanks Andrew and Max. I think what I had in mind was that a flat field might accentuate any faults inherent in the setup I have....

Am I right to think that a good barlow/telextender/powermate would help 'even out' any inconsistencies in mirrors and/or eyepieces?

NorthernLight (Max) · #5 30-07-2010, 05:27 PM

well, is it really that bad with your newton? if so, my first point would be to get it collimated correctly as this influences visual appearances the most. Get a good laser collimator and follow the instructions or search the IIS forums/articles on how to - it´s a 10min task and provides a lot improvment (should be done regularly anyways with newtons).
the thing barlows is that there minus-element partially corrects field curvature but you loose brightness and field of view (FOV) as magnification for any given eyepiece doubles or more (depending on the barlow of your choice.
There is always a little imperfection even with scopes much more expensive than yours but almost all scopes produce sharp images in the centre of your FOV. Also, I might be wrong here but I guess you are using some mediocre standard occulars (or eyepieces if you want) and they too could be the reason for your lack of space-walk-feeling / or they actually prevent you from seeing the field curvature of your prime mirror at all as they only deliver 50° apparent FOV.
Can you provide some more details on the type of scope and occulars and what flaws you actually observe when using it?

morls (Stephen) · #6 30-07-2010, 06:42 PM

No probs with the scope, very happy with it....collimation isn't an issue, just trying to learn what I can about barlows....i like the idea of having a greater range of magnifications i can go to, and if there's an added benefit of improved optics, so much the better...

(to be perfectly honest, i don't have enough experience to know if my eyepieces are mediocre standard or not - i like the views i'm getting, and i'm very happy with this lot.....)

i've got a 200mm F6 dob, GSO 2" superview 42mm, couple of super plossls that came with scope (25mm and 10mm) and a meade series 5000 5.5mm. I plan on getting a 15mm GSO superview, and a GSO 2" ED barlow....but there's a thought rattling around in my head that a telextender might be a good option, but more expensive... I'm thinking of a relatively high quality barlow as I see this as being something I won't be replacing in the near future, and i hope i'm right in thinking it will not be wasted with the rest of my gear...

astro744 · #7 01-08-2010, 07:43 AM

Quote:

Originally Posted by morls

....but there's a thought rattling around in my head that a telextender might be a good option, but more expensive... I'm thinking of a relatively high quality barlow as I see this as being something I won't be replacing in the near future, and i hope i'm right in thinking it will not be wasted with the rest of my gear...

If you mean the Meade 5000 series telextender then these are 4 element Barlows and were introduced to compete with the Tele Vue Powermates.

I cannot comment on the Meades but the Powermates allow the eyepiece to see the light cone from the telescope as the designer of the eyepiece intended thereby retaining eye relief and maintaining full field without vignetting. Note not all eyepiece/Barlow combinations vignette, some do. No eyepiece/Powermate combinations vignette. To achieve this 4 elements are used in lieu of 2 elements at a negligible loss in light transmission. In essence the Powermates are invisible in the light path. I have not used the Meades so I cannot comment on their performance.

If you intend on Barlowing an eyepiece of 30mm or greater then use a Powermate otherwise the eyerelief will be too long and difficult to use. Note some sizes are avaibale in 2" and some in 1.25" only. A quality 2" 2x Barlow or 2" 2x Powermate is a good investment.

Note Powermates are also used for imaging applications particularly the 4x and 5x as they can extend the focal length of your telescope considerably to give greater image scale for objects such as planets.

Note too Powermates can be heavy and may require counter-balancing. An alternative would be to get a low cost ED Barlow to get a feel for the powers you need and then get a quality eyepiece of the required focal length to give you that power without a Barlow or Powermate.

NorthernLight (Max) · #8 01-08-2010, 10:39 AM

I have 5x powermate and am very happy with it-visual as well as for photography. Its nice to use a widefield occular on the moon and still have the magnification of a say 9mm - a bit like orbiting moon. Compared to the one-element celestron 2x barlow that came in a kit I must say that it is comparing a kit lens with an L-lens. And the powermates have an option to mount a T-ring on top (in place of the eyepiece holder) which makes them easier to use with an slr cam as the connection is tighter and the lever is reduced.
I am not affiliated to any shops or makers but whenever I read a review where televue was compared to other brands it was the televue that won. You get what you pay for.

morls (Stephen) · #9 01-08-2010, 05:18 PM

Quote:

Originally Posted by astro744

If you mean the Meade 5000 series telextender then these are 4 element Barlows and were introduced to compete with the Tele Vue Powermates.

Yeah, this is the one I was thinking of....I think could get a 2" for around $200....a lot of money, but not as much as a powermate...I'm still wondering if I can justify such an expense with the setup I have at the moment....one consideration I have is that although I don't intend to get into imaging anytime soon, I have a couple of friends who have nice cameras and would love to play around with some basic imaging....am I right to think the 2" is definitely the way to go here? Also, your point about the eye-relief is a real consideration...

Also, if I was to use it with my 2" 42mm Superview I would (presumably) get a nice view at 21mm, which would compliment my current collection very nicely....by getting a 15mm I would then have a good spread of magnifications...

Quote:

Originally Posted by astro744

An alternative would be to get a low cost ED Barlow to get a feel for the powers you need and then get a quality eyepiece of the required focal length to give you that power without a Barlow or Powermate.

that's a good option....but, I'm now becoming more sure that I would like to be able to connect a camera, so I'd like to have this as an option, even though my main (95%) interest is visual....so, I guess my question now becomes how would an ED barlow stack up against a telextender (or powermate) for using an SLR with it?

thanks astro and Max for your help......as this thread's progressed my idea of what I'm hoping to be able to do has become clearer, so I'm sorry if it's wandered a bit off the original topic.....maybe i should have called the thread "barlow or telextender/powermate?"

astro744 · #10 01-08-2010, 07:09 PM

Firstly I am not an imager so perhaps someone else can comment on the use of an ED Barlow versus a 2x Powermate or telextender to double the focal length. Note in either case it will come down to what telescope you have and what you are trying to achieve by doubling the focal length as by doing so you also double to focal ratio and slow the speed of the system by 2x, eg. 25cm f5 (1250mm f.l.) becomes 25cm f10 (2500mm f.l.). Increasing the focal length is good for planets but not deep sky and your final focal length with a 2x amplifier may still not be enough to give you the required image scale.

I would think that you would definitely need a Powermate or telextender for use with the 42mm as a standard Barlow will increase the eye relief too much. However, I have not tried this eyepiece with a Barlow so cannot comment on how bad the eye relief actually is.

morls (Stephen) · #11 01-08-2010, 07:50 PM

I'm using a 200mm dob, f.l. 1200mm, so it's an f6....

I'm interested in planets and deep sky, just beginning to find my way around really. I like the idea of having a range of magnifications, especially down from 20mm...

If slowing the system to f12 helps with planetary observation that's great....does this mean I could get away with using, for want of a better term, "less-better-corrected" eyepieces to get good results at high magnification? I'm asking this because I've read that faster scopes require, in the shorter focal lengths, more expensive eyepieces that compensate for the faults inherent in fast systems...

astro744 · #12 01-08-2010, 09:10 PM

Quote:

Originally Posted by morls

I'm using a 200mm dob, f.l. 1200mm, so it's an f6....

I'm interested in planets and deep sky, just beginning to find my way around really. I like the idea of having a range of magnifications, especially down from 20mm...

If slowing the system to f12 helps with planetary observation that's great....does this mean I could get away with using, for want of a better term, "less-better-corrected" eyepieces to get good results at high magnification? I'm asking this because I've read that faster scopes require, in the shorter focal lengths, more expensive eyepieces that compensate for the faults inherent in fast systems...

Faster 'scopes generally require highly corrected eyepieces for all focal lengths (not just short) to ensure astigmatism free views off axis. A Barlow will reduce eyepiece astigmatism and improve off-axis sharpness, (unless it's a really poorly corrected eyepiece and/or Barlow). If you want to go down the Barlow or Powemate path go the 2" to give you maximum flexibility for imaging as a 1.25" system will vignette with a typical DSLR camera.

If I may go off topic here a little and suggest an eyepiece path for you that is not too expensive but will offer you outstanding quality.

Tele Vue 25mm Plossl good low-med power to complement your 42mm wide field. You will get 48x mag. at 4.2mm exit pupil.

Tele Vue 20mm Plossl for good deep sky viewing at 60x.

Tele Vue 1.25" 2x Barlow to give 12.5mm (96x) and 10mm (120x).

Tele Vue 1.25" 3x Barlow to give 8.33mm (144x) and 6.67mm (180x).

You could later add a 15mm Plossl to give 80x, 160x and 240x and you would have a nice spread of powers and an equally nice sapread of true fields. Note you will likely view the major planets from 120x to 180x most of the time and over 200x for nights of above average seeing. Rare nights of exceptional seeing will allow you to go to say 300x but to do so you would have to add either the 11mm or 8mm (or both) Tele Vue Plossls to your collection and use them with Barlows. Note using longer focal length eyepieces with Barlows or Powermates allows for more comfortable viewing than using short focal length eyepieces as the eye lens is a lot larger and more comfortable to look through (at least for Plossls) than on shorther focal length eyepieces.

Note the above is for a 50deg apparent field of view (AFOV) set (all 1.25"). I would also add the 32mm Plossl for maximum true field for a better corrected view than say the 42mm. This would also have the advantage of allowing 1.25" filters across the board.

Some may argue that 50deg AFOV is not enough for an untracked DOB system and prefer the wider AFOV 68deg and over. However, these are much more expensive if you want to maintain abberation free views. If you want to go down this path then may I suggest a 19mm Panoptic and 2.5x Powermate giving 63x and 158x as a start and save up for other eyepieces that would suit your needs. I would recommend the Powermate over the Barlow if choosing the Panoptics to maintain eye relief and ensure no vignetting.

morls (Stephen) · #13 02-08-2010, 08:58 AM

Quote:

Originally Posted by astro744

Faster 'scopes generally require highly corrected eyepieces for all focal lengths (not just short) to ensure astigmatism free views off axis. A Barlow will reduce eyepiece astigmatism and improve off-axis sharpness, (unless it's a really poorly corrected eyepiece and/or Barlow). If you want to go down the Barlow or Powemate path go the 2" to give you maximum flexibility for imaging as a 1.25" system will vignette with a typical DSLR camera.

Thanks astro, that's just the kind of info I'm after....

Regarding your suggestions for eyepieces and barlows, that sounds like a very good combination, but I'm not ready to make that kind of investment yet....down the track I might end up with such a collection, but at the moment that's just not realistic for my budget and experience....but, it's food for thought and maybe I should have some sort of long-term plan here...

NorthernLight (Max) · #14 02-08-2010, 11:58 AM

Hi Stephen,

just a little hint for when you really connect your dslr to the dob:

set the slr to M, burst mode, iso max, fast shutter speed, remote shutter tripping and take a few shots in a row on planets/moon. Earth is moving pretty fast and the atmosphere is always a bit turbulent-hard to freeze objects without tracking in one exposure.
good luck and if you get a decent put on the beginners astrophotography site to encourage others.

morls (Stephen) · #15 02-08-2010, 09:40 PM

Thanks Max, that's really useful info...

morls (Stephen) · #16 04-08-2010, 10:55 AM

ok, I've taken the plunge, and after all the great advice here, it might seem a bit strange.....but.....

I decided to go for the 1.25" meade telextender. I had trouble finding it here in Australia, so I got it through an overseas website for AU$116 delivered, which will leave me change for an eyepiece too....

I took a few things into consideration....this is built along the same lines as a Tele View powermate, but is a lot cheaper. In one the few reviews I have been able to find, it compares favourably, and although it might not be of the same standard, I'm hoping it will still be a step up from a standard barlow.

I went 1.25" because I decided this is probably the diameter of most of the eyepieces I will use with it, so it was kind of hard to justify the extra weight and cost (AU$197 shipped) of a 2"....

Thinking of imaging, I decided anything I would be doing with a dob mount was going to be very basic, so this should probably not be a consideration, especially as I'm chiefly interested in visual, and any imaging would be experimental and fun....I will look into afocal using my 2" 42mm eyepiece, and if things develop more maybe a basic 2" barlow down the track...

So, I'm now awaiting arrival of the telextender. Reviews have been hard to find, but fingers crossed it'll be ok, and being a beginner with nothing to compare it to might be a good thing.....

Thanks for all your generous advice as I thought this through online....I appreciate the time you all took to read and respond to my posts. I'll update once the telextender arrives and I get a chance to give it a try.

Cheers

Stephen

morls (Stephen) · #17 14-08-2010, 10:37 PM

The telextender has arrived, and I love it. To my inexperienced eyes, it's beautifully made accessory, and the difference in quality between this and most of my other gear is obvious. It's made a huge difference to my appreciation of good views....
I've also bought a William Optics 15mm SWA, and this works beautifully with the telextender....

I was out this evening, and took a few shots of the moon using my iphone...I can't remember them all, as I took a bunch and was swapping things around, trying this and that, but I think maybe the second one was through the telextender and a blue 80A filter...it's a bit blurry cos all I had was an iphone up to the eyepiece...
http://www.iceinspace.com.au/forum/s...817#post624817

Overall, extremely happy with this telextender. for around $120 delivered, seems a real bargain.