Advice about upgrades

[Capstone55]

I've just got back to observing by buying a Skywatcher 250p f/4.7 Dobsonian for visual astronomy only. It comes with the usual SW 25mm and 10mm EPs and I also have a Telrad finder, Cheshire EP and neutral density filter for the moon. I have Stellarium and all the books and charts I will ever need.

I'm very pleased with it so far but am now considering upgrades. What should I get next? Replace the EPs with higher quality, or initially get a higher power EP or should I get a Barlow? And which filters would be best?

Any advice gratefully received.

 

[PaulM]

Right angled viewfinder (RAIC) is a must

Based on my observations through my 200, 300, 350p dobs I would get a wide angled high focal length eyepiece to take in the larger objects up there

[Pixies]

2 hours ago, PaulM said:

Right angled viewfinder (RAIC) is a must

Based on my observations through my 200, 300, 350p dobs I would get a wide angled high focal length eyepiece to take in the larger objects up there

Yes and yes.

Perhaps not 'a must' for everyone. However, if you are not proficient at yoga, it does help when finding objects near the zenith!

Getting a 2" wide field EP was an eye opener (excuse the pun). Get the lowest powered EP you can, that gives you an exit pupil of 5-7mm depending on your age.

[Capstone55]

Thanks for these interesting responses. Not what I'd considered. One question though. I'm 64 and have doubts whether my eyes can handle the exit pupil of anything longer than a 25mm EP (ie about 5mm). Presumably an exit pupil larger than my pupil diameter will look dimmer? If so, is it especially noticeable and would the wider field of view still be advantageous?

[Pixies]

With a reflector, with an exit pupil > your dilated pupil, then the shadow of the secondary starts to make up a much greater proportion of the image entering your eye, so it starts to dim. With a refractor, it's just a case of wasted light.

So yes - I'd say 25mm would be the longest focal length you would use in an eyepiece.

In which case, go for a 25mm EP with the widest apparent FOV you find comfortable.

[Capstone55]

1 hour ago, Pixies said:

With a reflector, with an exit pupil > your dilated pupil, then the shadow of the secondary starts to make up a much greater proportion of the image entering your eye, so it starts to dim. With a refractor, it's just a case of wasted light.

So yes - I'd say 25mm would be the longest focal length you would use in an eyepiece.

In which case, go for a 25mm EP with the widest apparent FOV you find comfortable.

Thanks very much

[Capstone55]

Are there any makes of 25mm wide-angle EPs that anybody would recommend for a f/4.7 telescope? I understand that the low f-number creates an optical challenge so I guess that means a more expensive EP. I'm not really restricted by budget but neither am I an EP obsessive and can tolerate some degree of fall-off in quality around the edges of the field. Probably goes without saying that this is for visual astronomy only. 

Thanks

[Second Time Around]

I'd very much recommend that you actually measure your pupil size as it varies hugely from person to person, even of the same age.  It's not at all difficult to do.

Just stay in darkness for a couple of minutes at your observing site for your pupils to dilate; you don't need to get fully dark-adapted as that's a chemical process that occurs after your pupils have fully dilated.  Then just put a ruler above your eyes and take a flash photo - the flash is so fast your pupils won't react.  But do make sure that you have red-eye and pre-flash turned off if your camera has these settings.

Why is pupil size important?  Well, if the exit pupil of your scope is greater than your pupil size you'll be wasting some of the aperture of your scope (although the field of view is likely to be bigger).  So it's a good idea to have an eyepiece that matches exit pupil and pupil size.

You also need to take into account that the magnification and therefore the exit pupil will change if you use a coma corrector.  For instance, a Paracorr reduces the exit pupil by 15%, an Explore Scientific coma corrector by 6%.

To give an example I measured my dark-adapted pupil at 4.5 mm.  I have a 10 inch f/4.8 Dob fitted with an Explore Scientific coma corrector.  So the calculation is 4.5 x 4.8 x 1.06 = 22.9 mm eyepiece.  As there are few 23 mm eyepieces available, plus my pupil size will almost certainly get even smaller with age I rounded this down to 22mm.  Mine has a 65 deg FOV.

However, I do also use a 28mm/68 deg eyepiece.  This gives me a 33% wider field by diameter, 77% by area.  As Pixies said, there's a limit how far you can go with a reflector though before you see the shadow of the secondary mirror.  There's no such limit with a refractor.

Edited August 26, 2020 by Second Time Around