Very New to Astronomy and a little confused ... Minimum magnification?!

[Richard W]

Hello and this is my first post, so with trepidation and a little embarrassment, could I ask a question on what would be best  eye piece  I should select for a low magnification for a wide field of view to help find objects.

Background:  In November I received my long awaited 10" Dobsonian  (f4.8) from Orion Optics UK - I had done some back ground reading and I had borrowed a 6" home made Dob from a wonderful character who is chair of a local Astronomy Association (I was hooked) but felt paying that little more for a 10" and a higher level of polish on the mirror would be a better longer term investment of enjoyment!  Also went mad and brought 5mm, 11mm and 18.2 mm Tele View Delites.

Now I'm struggling to locate objects (now they are in focus after a lot of trial and error in collimation) I have a 32mm  Plossel  that cost me £10 and when I tried it I could  see the spider and a dark centre of the secondary ... so with all this my question lead me what is the minimum magnification to getter a wider view to help me to navigate around the sky.  My finder scope is x50 (I think) but it makes it harder to identify when I search the sky.   A link from this site took me to a Tele View calculator and the Flo Optics field of view demo is great but I'm lacking  a little confidence if it should be a 27mm or a 31 mm etc .....!  Hope you can advise or am I just making things to complicated for myself?

Thank you

[NGC 1502]

Hi and welcome to SGL👍
 

I own the exact same scope as yourself.  If when using the 32mm Plossl you are seeing the spider vanes and the secondary then you are way off focus. The TeleVue Delites are truly excellent eyepieces but as with most 1.25” fitting eyepieces from TV they come to focus with the focuser racked further out than most makes. That’s not a fault, just a characteristic.

Try again, and remember to refocus as necessary. As you only paid £10 for the 32mm Plossl it won’t be TeleVue (unless the seller was barmy😁). The TeleVue 32mm Plossl is parfocal with the Delites. Parfocal means coming to focus at the same focuser position with possibly a minor tweak.  If you could afford it a 27mm TeleVue Panoptic would be an excellent lowest power eyepiece to complement the Delites. The 27mm Pan will give 44x and a 1.5 degree true field, that’s about 3 moon diameters on the sky.

There are of course many other possibilities for a lowest power eyepiece. But as you’ve already purchased top quality Delites then I’m thinking a similar quality would be great for lowest power.

BTW, the 2” fitting 27mm Panoptic focuses further out than the Delites, again not a fault.

Your finderscope won’t be 50x. The 50 refers to the diameter of the finder objective- 50mm.

Please come back for further enquiries, lots of folk on here to offer help.

Ed.

[Richard W]

Thank you Ed for the very informative feedback - excellent!  

Did check finder this morning and it is 8x50 so you are correct, thank you.  Yes, the TeleVue 27 Panoptic was one of the serious options I was considering.  The insert ring for 1.25", is there a technique for getting out without the difficulty of shaking/moving the whole scope!!

Thank you for the explanations also - very helpful at this stage.  Tonight looks a possibility to do some viewing!

Richard

[NGC 1502]

Hi Richard, happy to help.  If the 1.25” to 2” adapter is a bit of a tight fit into the focuser then as you’ve found it can be an issue. You may have done this already, but when removing or replacing the adapter make sure the securing screw on the focuser has been slackened enough.  

If that wasn’t the problem you could try the following if you’re ok with basic DIY.  Please be cautious if you try this- does the focuser have a compression ring inside where the adapter fits?  You can tell by looking when the adapter has been removed. If there is a compression ring take a careful look at it when the securing screw has been fully slackened. Has it fully opened up into the machined groove inside the focuser?  If it hasn’t then that could possibly be the cause of the problem. Before proceeding further make sure the tube assembly is horizontal to prevent the possibility of anything falling into the tube and even worse contacting either mirror. PLEASE be careful and if in any doubt at all don’t do the following- Using a small thin screwdriver it’s usually a simple job to remove the compression ring from the focuser. Using your fingers carefully open up the compression ring so it’s slightly larger in diameter. Replace, try the adapter again, that may have sorted it.

If that didn’t sort it then you could simply use a 1.25” eyepiece for your lowest power, like the 32mm Plossl you already have, then the adapter stays in the focuser with all of your eyepieces. Or maybe lovely 24mm Televue Panoptic, that’s also 1.25” eyepiece the same as the Delites and is parfocal with them too. The 24mm Panoptic will give 50x and from memory gives about a 1.3 degree true field.

As always there’s so many eyepiece options, the above is just some of them.

Ed.

 

 

[Stu]

@Richard W you’ve already had great advice from Ed, so I just wanted to add the concept of exit pupil being used to define the lowest focal length eyepiece to use in your scope.

As you probably know, the generally accepted maximum dilated pupil size is 7mm, so if the exit pupil of your scope/eyepiece combination is bigger than this then you are effectively wasting light, and in extremes can see the shadow of the secondary appearing in the view. Depending on age, your max may be less than this, so often 5 or 6mm is a good place to be, especially if you have a level of light pollution in your skies.

So, exit pupil is given by the focal length of eyepiece divided by the focal ratio of your scope. Reversing this, you can multiply the focal ratio by your desired exit pupil and get the eyepiece focal length which will give this results.

eg 4.8 x 7mm = 33.6mm focal length eyepiece

or

4.8 x 6mm = 28.8mm focal length

The eyepieces being discussed fall into this range so all is good, I just wanted to explain this way of looking at it.

The 27mm Panoptic would give 5.6mm which is fine. A 31mm Nagler is significantly more expensive but would give x38 and a 2.2 degree field of view with a 6.5mm exit pupil. Depending on your skies and eyes, this may give a more washed out result though, so the 27mm is possibly the better option.

One more thing to add…. If finding objects is the biggest issue you are trying to solve, then adding a Telrad to your scope would likely be a good option. This is a unity gain finder which just projects four red circles against the sky making it easy to get your scope pointing in approximately the right direction before switching to your finder to hone the accuracy. Also, a RACI (Right Angle, Correct Image) finder can also make life easier if you don’t already have on. The right angle makes it easier to view through, and the image is the correct way up, so a little easier to work with.

 

[Richard W]

Hi Stu and Ed

Again fantastic information and I was reading about Exit pupil size as I'm 63 and looking at 6mm for myself is best guess.  So 27 mm Panoptic looks the best solution.  

I'll check my focuser as regard your suggestions Ed. .... just checked and it is a brass collar which moves around as the lock screw is turned inwards.  The insert is coming out a little easier now so thank you.

Telrad, I've heard good things, so my question is how is it attached if you still use the finder scope and that attachment to the body of the scope?  My finder has a 45 degree eye piece which was an option I thought would make life easier - which it dose and the image is the correct way round.

Another reason I was taken by TeleView was the astigmatism correction and I would look into this next using my prescription but funny question now: which eye?? 

Thank you for your time in responding to my post, really great information and brilliant.

Richard

[Stu]

1 hour ago, Richard W said:

Telrad, I've heard good things, so my question is how is it attached if you still use the finder scope and that attachment to the body of the scope?

The Telrad is supplied with a base and sticky pads. The base is a shallow V shape and is very easy to attach with just a little care, ensuring that it is aligned with the OTA. The Telrad then attaches to the base and can either be left there or removed each time. The Telrad itself has adjustment knobs to accurately align it with the view in your eyepiece.

I would agree that the 27mm Panoptic would likely be a good match for your scope and eyes.

Regarding which eye, this is likely to be something that you arrive at just by observing. We all normally have a dominant eye, which is the one you naturally favour. I found some info on this here:

https://www.allaboutvision.com/en-gb/resources/dominant-eye-test/
 

So, establish your dominant eye and the one you prefer to observe with and then choose the dioptrix to match that.

[Richard W]

Hi Stu

Well it's the left and I'm right handed.

Thanks for information on Telrad that helps immensely to explain in pictures also.

Yet again thank you and now it's post Christmas budget review and a little shopping!

Excellent

Richard

 

[Spile]

I can also recommend a Telrad especially with RACI.https://astro.catshill.com/finding-and-finder-scopes/

[Second Time Around]

Dark-adapted pupil size can vary hugely from person to person to say the least.   I'd therefore suggest you measure your own at your observing site.  It isn't difficult and there are several methods that a search will show you.  This is what I did.

I got dark-adapted for a few minutes to allow my pupils to dilate (there's no need for longer as full dark adaption is a chemical process).  I then got my wife to take a flash picture whilst I held a ruler just above my eye.  The flash is so fast that your pupil won't react.   Make sure though you haven't got red eye reduction or pre-flash on.

As Stu said, your pupil size will allow you to work out the maximum focal length eyepiece you can use without  effectively using a smaller aperture scope.

Unfortunately my maximum pupil size is only 4.5mm at my Bortle 4/mag 20.87 site.

I too have a 10 inch f/4.8 Dob.  I use an Explore Scientific coma corrector with mine that increases the focal length by a factor of 1.06.

So the maximum focal length eyepiece I can use without effectively using a smaller aperture scope is 4.5 x 4.8 x 1.06 = 22.9mm.  As there are few if any 23mm eyepieces, plus my pupil size is likely to get even smaller with age, I use a 22mm.

I'm further constrained as I have astigmatism and much prefer to use a Dioptrx to correct this rather than wear glasses to observe.  This does restrict the eyepieces that I can use though. 

However,  a longer focal length eyepiece is also useful for a wider field of view despite the loss of aperture - I use a 28mm/68 degree Explore Scientific, that also accepts a Dioptrx.

[Richard W]

Thank you for the post Steve and I do wonder what my pupil dilation is at 63 and a half!? 

I do not understand Coma correction but i do have an astigmatism so need to check my prescription and get the right Dioptrx.

Thank you again

Richard

[Louis D]

I don't know that the 27mm Panoptic has enough usable eye relief for comfortable use of a Dioptrx.  I've measured 14mm of usable eye relief, and that matches with my experience with it.  15mm is about the minimum recommended usable eye relief for the Dioptrx.  That eyepiece is too tight on ER to use with eyeglasses.  I actually scratched an eyeglass lens on the eye lens retaining ring years ago trying to push in enough to take in the entire field of view.

I swapped out my 27mm Panoptic for a 30mm APM Ultra Flat Field a couple of years ago.  It is flatter of field, better corrected to the edge, and has plenty of eye relief for eyeglasses.  The Dioptrx might fit since it has a 43mm top thread under the eye cup.  It has a 36mm field stop versus a 30mm FS for the Panoptic.  That's 20% wider which is very noticeably for a mere 10% growth in exit pupil thanks to lower edge distortion.  The APM is available under multiple brandings now, so it should be readily available.

Another option in this price range is the 20mm APM XWA HDC.  It would have about the same true field of view as the 30mm APM at a smaller exit pupil.  It is available under several other brandings, so search for availability and price.

[Second Time Around]

Louis, can you measure the thread under the 30mm APM?  I'd read that it was a little larger than 43mm, and that a Dioptrx wouldn't fit.

Thanks.

[Louis D]

6 hours ago, Second Time Around said:

Louis, can you measure the thread under the 30mm APM?  I'd read that it was a little larger than 43mm, and that a Dioptrx wouldn't fit.

Thanks.

If I get a chance, I'll use my digital caliper on it.  It's quite possible since the way threads are measured, the maximum male thread width could very well exceed 43mm.  Of course, if it's close and you don't need the threads, you could hone them down until it fits. 😉

[Louis D]

Maximum eye cup metal ring diameters:

12mm/17mm ES-92 = 56.5mm

30mm APM UFF = 45.0mm

22mm TV Nagler T4 = 44.0mm

17mm TV Nagler T4 = 44.0mm

12.5mm APM Hi-FW = 44.0mm

12mm TV Nagler T4 = 43.5mm

27mm TV Panoptic = 43.0mm

24mm APM UFF = 42.9mm

9mm/14mm Baader Morpheus = 42.9mm

Astro-Tech AF70 (13mm/17mm/22mm) = 42.9mm

So I was mistaken. My apologies.  The 30mm APM UFF has an M45 thread.  It is the 24mm APM UFF (and AT AF70s and Morpheus) that have M43 threads.  The 12.5mm APM Hi-FW is spec'ed with an M44.5 thread.

As such, the Dioptrx won't fit the 30mm APM UFF because 44mm is the maximum diameter it will clamp onto.