refractor and reflector view query

[spaceboy]

Hi All

Need a bit of help figuring this one out I think I already know the answer but wanted to make sure I didn't mislead myself.

8" 1000mm Reflector 31mm 72' EP Vs 4 3/4" 1000mm Refractor 31mm 72' EP

Which is going to give the wider FOV ? I ask as calculators are saying they are both going to give the same FOV yet looking through the EP they don't

I have an idea in mind but what's others views on this? Please not I haven't done a side by side but I know from the view of a certain object that this is the case as one over fills the view while the other fits nicely.

SPACEBOY

[John]

If the scopes and eyepieces match the quoted specs then they should give the same true field of view (2.23 degrees). Perhaps one of the setups is vignetting a bit

[MjrTom]

The FOV should be the same as the focal length is the same

[spaceboy]

That's crazy I must be going mad Would poor collimation have an effect ? Being a former slave to laser collimator's I thought I would give a Cheshire a try and my reflector was a little way out. The views after although no actual difference in quality did seem to give more magnification. The object is out of site for a while now so I can't re-check to see if it will give the same FOV now. It was only by chance getting a new EP that I checked on a calculator and noticed I should be getting the same FOV in each.

[Ad Astra]

Hi Spaceboy,

You didn't mention which scope had the narrower FOV. My guess is that it is the reflector - in which case I think you may have a vignetting problem (undersized secondary or rather bad collimation).

If you find the refractor has a smaller FOV, I would almost bet that the reflector may not be exactly the advertized focal length. If the mirror is a bit shorter than advertized, you would have lower mag and wider FOV.

Just a thought,

Dan

[spaceboy]

Hi Dan

It's the reflector. TBH I don't understand vignetting as I have never seen a before and after to make the comparison. I take it you mean darkening of the edges ?? Trouble is M45 is now out of the way so I can't re-check to see if the correctly collimated reflector is now equal in it's view to the refractor. The collimation was bad and I had suspected this hence why I purchased a cheshire. I admit I wasn't 100% sure if it may be the different design of the refractor over the reflector and I wanted to see if anyone else had noticed any difference. I think it is more likely the collimation was to blame but I've got a wait now to find out

[Ad Astra]

Hi Spaceboy,

FYI, photographic vignetting is darkening at the corners due to a lens not fully illuminating the sensor or film frame.

In a reflector, you can have problems when the secondary is too small to capture the entire cone of focused light from the primary mirror, thus losing light and brightness, (but improving contrast). With a over-large secondary, you cut off too much of the light before it gets to the primary (reducing contrast). But neither of these look like the photographic effect where one part of the field is darker than other parts. That usually happens with collimation errors, where the light cone coming into the focuser can be off-axis or otherwise tilted from the optical axis of the EP.

I'm willing to bet if you check collimation, you will improve things a lot. Over size / undersize secondaries are rare in commercial scopes today, and unless someone has swapped yours out, I don't think this is the issue.

I'd still be willing to bet that if it truly is a different FOV, you have one of the two scopes slightly different from the 1000 mm advertized. This is rather difficult to test without removing the optics from their tubes (not the best idea if it isn't absolutely necessary), and as long as collimation is good - I wouldn't worry much about a simple difference in FOV like that.

I hope that helps a bit,

Dan

[spaceboy]

Thanks Dan

What you say makes sense. My secondary was to far down the tube (close to the primary) so I'm assuming in effect this would along the focal path make the secondary either appear smaller/larger ??? It's crazy because the images of Saturn I was getting were amazing and I could make out loads of detail

[Ad Astra]

Thanks Dan

What you say makes sense. My secondary was to far down the tube (close to the primary) so I'm assuming in effect this would along the focal path make the secondary either appear smaller/larger ??? It's crazy because the images of Saturn I was getting were amazing and I could make out loads of detail

Having the secondary far down the tube usually means that the manufacturer extended the tube beyond the length where it actually needed to be as a dew/light shield. If the detail on Saturn and Luna is good, and you can look at an area with lots of stars along the milky way and see that the stars are round and 'pin-point' all the way to the edge of the field, and the collimation was spot on, I wouldn't worry that much. Sounds like your scope is fine.

With regard to the differences between the two scopes, I think that telescopes are rather like women in that regard.... it doesn't do your mental health any good to compare yours to someone else's. And if you should be fortunate enough to have two of them, comparing them to each other could be disastrous!

Dan

[spaceboy]

Having the secondary far down the tube usually means that the manufacturer extended the tube beyond the length where it actually needed to be as a dew/light shield.

Dan

More like I should pay more attention re-fitting the spider after flocking

Thank you for all your help Dan