Minimum magnification for DSO

[Simon Dunsmore]

Hi, 

I'm a new member and have really enjoyed reading many of yourthreads, very helpful to a beginner like me!

I own a Skywatcher   150p F5 (750fl) I really want to get into observing DSO. I was considering ordering a 32mm Skywatcher Panaview 2" EP but am worried that this will only give me x23 magnification, will I be able to see much? Also any other suggestions for a good DSO lense for this scope,budget of around £60.

Looking forward to your thoughts 

Regards Simon 

[michael.h.f.wilkinson]

It is always good to think in terms of exit pupils rather than magnification. This is the width of the beam of light coming out of the EP. A simple way to calculate it is to divide the focal length of the EP by the focal ratio of the scope. In this case that would be 32mm/5 = 6.4mm. This is OK if you are in a dark location, and your pupils dilate to that diameter, but with the slightest light pollution the background will look washed out. Personally I like observing at an exit pupil of not much bigger than 5mm, or a 25mm EP in your case. Getting something like a wide-field 24-25mm may make more sense than say a 32mm Plossl. I am not sure the MaxVision 24mm 68 deg EP is still available within your budget, but it is a really nice EP.

[vlaiv]

Hi Simon,

Welcome to SGL. I think that the best DSO eyepiece will depend on where you observe from. How much light pollution is there on your observing site? If there is little light pollution go for 12mm eyepiece. If there is a lot of LP go for 15-17mm eyepiece. With DSO observing, it is all about the contrast, and contrast comes from certain magnification / exit pupil. Exit pupil is calculated by dividing focal length of eyepiece with F/ratio of your scope (in your case F/5). Many people concluded that best exit pupil for DSO observing is 2-3mm for dark site and 3-4mm for light polluted environment. I have 200mm F/6 scope and eyepieces that I most often use are 32mm to find stuff and 17mm to observe DSOs. Also check light throughput of given eyepiece - one that you intent to get. Read couple of reviews of it online and see what people think of it.

[Stu]

A lot depends upon your skies and the type of target you want to view. The term DSO covers anything from a tiny faint galaxy which benefits from high mag in a large dob, to large faint nebulae like the North America nebulae which looks best in a widefield frac at very low power, or binoculars.

Under any kind of light pollution, a large exit pupil will look washed out and you will lose contrast. A smaller exit pupil will have two effects. It will darken the sky background whilst increasing the object size, both of which help your eye to detect the contrast.

So, I basically agree with Michael, but let us know a little more about where you normally observe and what you like observing. Perhaps have a play about with so field of view calculators to see the size of objects and how they are framed in different eyepieces with your scope. Astronomy Tools is a great place to go for this, linked to from FLO's site.

[Knighty2112]

You have to realistic when wanting to view DSO's, and in the end really the most deciding factor on your ability to see them well, or even at all, is the darkness of the sky. From my light polluted backyard I can hardly see M33 except on the darkest & clearest nights for observation. However, there are still plenty of others that I can see relatively easily. Light pollution filters can help a little, but after a certain level of LP they can't cope well either. I have a 32mm Celestron Omni plossl that gives me good views, but for most DSO's I tend to use a 2" Explore Scietific 25mm EP, which gives me higher magnification for viewing, plus I still get a good exit pupil that doesn't wash the sky out. However, a dark clear sky is still required to see a lot of DSO's well in reality. 

[Ricochet]

1 hour ago, michael.h.f.wilkinson said:

I am not sure the MaxVision 24mm 68 deg EP is still available within your budget, but it is a really nice EP.

I think the Maxvision is only available in the second hand market now but the 24mm ES68 is still in the Bresser sale and is roughly in budget give or take the actual currency conversion.

[michael.h.f.wilkinson]

That ES has the same optics, I gather. Should be great, certainly for that price

[Pondus]

3 hours ago, Simon Dunsmore said:

Hi, 

I'm a new member and have really enjoyed reading many of yourthreads, very helpful to a beginner like me!

I own a Skywatcher   150p F5 (750fl) I really want to get into observing DSO. I was considering ordering a 32mm Skywatcher Panaview 2" EP but am worried that this will only give me x23 magnification, will I be able to see much? Also any other suggestions for a good DSO lense for this scope,budget of around £60.

Looking forward to your thoughts 

Regards Simon 

Not beeing sure how the panaview performs With Your F5 Scope, the eyepiece itself has generated Nice reviews (quality/price)  

I`d say go for the panaview 26mm at FLO (£69,-), instead of the 32mm.

It will give you an exit pupil of 5.2 , 29x and a very decent 2,4 degree Field of view. 

The Field of view at 2,4 is something the bigger dobs and SCT`s only can dream of.

What you can see at 29x (or any Power)  is of course depending on how dark the sky is and how good your eyes are (..and experience...).

But you should pick up plenty galaxies and most open clusters would be  quite lovely With 29x and 2.4 tfov

 

Rune

[Simon Dunsmore]

Hi Rune,Thanks, that's really helpful. How did you calculate those figures?

Really appreciate your comments 

Regards Simon 

[Simon Dunsmore]

Hi All,

Wow, thanks for all the help, you folks are on it!

I will do a little more research , but with regards to my location, I live on the sout coast in Kent. My location is pretty light polluted but with the 150 it's relatively easy to pop it in the car and head out into the sticks or to the coast and darker skies Hope that helps.

With regards to what I want to observe, I'm really not sure but galaxies we'd would be pretty high on my wish list.

I guess to start lm looking for a good all rounder to begin with and this is why I figured a low mag would be better, hadn't really thought about the light pollution or exit pupil 

Thanks again and please do keep the wisdom coming

Regards

Simon 

[Pondus]

Exit pupil  easiest way :   eyepiece fl / Scope fl =  26mm / f5  = 5,2

TFOV calc (one amongst many)  : http://www.csgnetwork.com/telefov.html

Rune

PS  I only steal from other Peoples wisdom in here.

Some of the members who has posted in this thread (absolutely not me!)  has tremendous knowledge and experience.

Read threads, you`ll learn alot, I did.

[michael.h.f.wilkinson]

There's a second-hand ES 24mm 68 deg on ABS-UK right now

http://www.astrobuysell.com/uk/propview.php?view=110042

 

[bomberbaz]

20 minutes ago, michael.h.f.wilkinson said:

There's a second-hand ES 24mm 68 deg on ABS-UK right now

http://www.astrobuysell.com/uk/propview.php?view=110042

 

Bargain

18 hours ago, Simon Dunsmore said:

Hi All,

Wow, thanks for all the help, you folks are on it!

I will do a little more research , but with regards to my location, I live on the sout coast in Kent. My location is pretty light polluted but with the 150 it's relatively easy to pop it in the car and head out into the sticks or to the coast and darker skies Hope that helps.

With regards to what I want to observe, I'm really not sure but galaxies we'd would be pretty high on my wish list.

I guess to start lm looking for a good all rounder to begin with and this is why I figured a low mag would be better, hadn't really thought about the light pollution or exit pupil 

Thanks again and please do keep the wisdom coming

Regards

Simon 

Forget the research simon, you really cannot go wrong with the ES ep above at that price

[michael.h.f.wilkinson]

Not my EP I hasten to add

[Simon Dunsmore]

1 hour ago, michael.h.f.wilkinson said:

There's a second-hand ES 24mm 68 deg on ABS-UK right now

http://www.astrobuysell.com/uk/propview.php?view=110042

 

Hi Rune,Thanks, that's really helpful. How did you calculate those figures?

Really appreciate your comments 

Regards Simon 

[Simon Dunsmore]

Hi

Thanks for this, if it's still available after pay day I will opt for this.

Regards Simon  

[Simon Dunsmore]

Hi Folks,

Back again, unfortunately I was too slow off the mark on the ES ep. So I'm back for one final dose of yourwisdom. I'm either going for the panavision 26mm or the ES 24mm  maxivision. Any comments, thoughts over which is the best? Both around £70. Thanks Simon 

[YKSE]

I agree with others, ES24mm is a no-brainer.

The diplayed item sale at bresser as linked by Ricochet is very nice priced.

[John]

I agree with YKSE, the Maxvision would be the better eyepiece. The 24mm / 68 degree Maxvision that I've had for a few months now seems very good for it's cost. It even works pretty well in my F/5.3 12" dobsonian and the stars are sharp over most of the field of view.

 

 

[mikeDnight]

The lower the power the brighter the sky background and so contrast can suffer. I prefer a medium power for nebulous DSO's, usually around the 70x mark, otherwise in a small aperture scope they can be washed out. A 10 mm XW in my 100mm F7.4 is a superb eyepiece for finding fuzzies at 74x. Star clusters are great in a 20mm XW and for large nebulae such as the North American, Pelican, or Veil, the 20mm with a UHC filter is excellent.

Don't be afraid to use high power on DSO's, many of them show up well against the darker background.

Mike

[Simon Dunsmore]

Thanks, do you think the fact that the Panavision is a 2" would give it the edge in any way?

[John]

3 minutes ago, Simon Dunsmore said:

Thanks, do you think the fact that the Panavision is a 2" would give it the edge in any way?

It shows a slightly wider field of view but the Panaview (I assume you mean that one ?) will show more astigmatism at the field edges when used in an F/5 scope, than a Maxvision does I think.

 

[Simon Dunsmore]

Hi Folks, 

After much debating I purchased the ES 24mm Ep in the Bresser sale. Took it out for a testtonight in-between clouds.

All I can say is Wow! This is my first decent ep and I cannot believe the difference! Spent a long time mesmerised by the Orion Nebula, then the cloud rolled in and spoilt the fun. 

Worth every penny of the 61, can't wait to get out again. Feel like I have a new scope!

Thanks for all the advice 

[acey]

Some responders are saying that low power (large exit pupil) means brighter sky background hence less contrast. This is incorrect. An extended target (e.g. galaxy) is magnified and dimmed to the same extent as the background, so contrast is unchanged. What changes is the apparent size of the object, and that (together with the change in background brightness) affects its visibility. A galaxy may be invisible at low power (because it's too small), invisible at high power (having been made too faint) but visible at intermediate power.

A low power eyepiece can be very useful for finding objects, before moving to a higher power to inspect them in detail. When I began collecting eyepieces one of the first I got was a TeleVue 32mm plossl. Sixteen years on, I still us it in every observing session, and while other eyepieces have come and gone, that one has served me in every scope I've owned. In my present scope it gives an exit pupil of 6.5mm. I should add that I do all my observing at a dark site - I agree that at a light-polluted one an exit pupil of 6.5mm may give less satisfactory results, because your own pupil may not dilate to that size.

So my advice (for what it's worth) would have been to save a bit more, get the TeleVue 32mm plossl, get your scope to a dark site and prepare to be blown away. But I'm sure you'll be happy with the one you've opted for.

[vlaiv]

7 hours ago, acey said:

Some responders are saying that low power (large exit pupil) means brighter sky background hence less contrast. This is incorrect. An extended target (e.g. galaxy) is magnified and dimmed to the same extent as the background, so contrast is unchanged....

You are correct that dimming is proportional (for both sky and target) with smaller exit pupil, but we have to consider the possibility that human eye/brain combo has non linear response in low light region, so we perceive or see dimming to be different if two have different brightness (so that target is theoretically detectable). I suspect that this is the reason people report that different exit pupils give best results on DSO in LP and dark site - different level of proportional dimming brings both sky and target in region where nonlinearity is the greatest and hence best visual contrast is perceived.