Measuring focal length, new lenses and telescope ID?

[Loopnova]

Hello, I'm new here and to telescopes in general, though I've been doing some heavy research. I'm looking for guidance with a reflector telescope we picked up in a second-hand shop some years ago now (photo attached). The only identifiable marking on it is a sticker that says "Orion Optics". No model number or anything. The measurements are as follows:

-The tube is 6.75" (170mm) in diameter, 27.5" (697mm) length.
-The primary mirror diameter matches the opening aperture, just shy of 6" (148mm)
-The distance between the primary mirror and the centre of the eyepiece slot/secondary mirror is 21.75" (550mm)
-We have two unbranded Plossl lenses (25mm and 10mm)

Now, because there is no model number, I can't look up what the focal length might be. I'm assuming it is the raw distance between primary/secondary mirror, so 550mm? I think this puts it in the f/5 ratio, which is good for wide-field viewing as I understand it. We get lovely views of the moon with both lenses, but have had less satisfying views of venus, jupiter and saturn. If the focal length is as I assume 550mm, then the magnifications we are getting with 25mm and 10mm lenses would be 22x and 55x respectively, right? I've read that to achieve good views of the planets you need upwards of 150-250x magnification (if your telescope has that capacity). If the capacity is roughly 50x the diameter, then we should be able to achieve 300x magnification (atmospherics allowing) with this scope. I've been looking at getting a Barlow to use with my current lenses (the 10mm should net us 130x with this) and buying another lens closer to 5 or 6mm to use with it also. Is this wise? Does anyone have any recommendations?

Summary of my questions:
-Are my assumptions about the focal length of this scope correct?
-Will a Barlow lens and a smaller focal length lens help achieve better views of the planets with this scope?
-Would replacing the unbranded Plossl lenses yield clearer results? (Are all lenses created equal?)
-The scope has never been collimated by myself, so I imagine it needs a tune up, what's the best way to go about this without dropping £££? I have good practical ability so no worries there.
-I can see lots of dust on both mirrors, is it advisabe to clean them?

One last thing. Last night I managed to locate Andromeda galaxy, but it was only faintly visible with binoculars (if I looked around it rather than at it), less visible in the viewfinder and totally invisible through the telescope itself. Is the only way to see deep sky objects like this to have longer focal length and wider diameter scopes to collect more light? I know increasing magnification decreases brightness of the image so more powerful lenses won't help for deep sky objects. It was fairly low on the horizon, in a well-lit town, with a nearly full moon in the sky, so yes, visibility conditions were less than ideal! I'd like to start planning to build a telescope specifically for deep sky objects, so if anyone has any suggestions please let me know.

I know I had more questions but I've run out of steam and don't want to scare anyone off from answering this beast. Thanks in advance for any replies!

Edited March 7, 2020 by Loopnova

[Cornelius Varley]

The focal length will be more than 550mm, the distance from the primary to secondary. The point of focus will fall outside the tube, probably about 200mm from the secondary, which will make the telescope 750mm in focal length which is a common focal length for 150mm telescopes.

[John]

I agree with Peter (above) - most likely an F/5 with a 750mm focal length.

I think that scope is one of the older Orion Optics ones that uses a single vane secondary support like this ?:

Orion used to have a series called the Europa which yours could well be one of.

It is worth blowing the dust off the mirrors using a manual blower but don't touch the mirror surfaces.

The collimation may need attention if it's not been adjusted for some time.

M31, the Andromeda Galaxy should certainly be visible through the scope but you will need a low power eyepiece and even that wont show the whole thing - it's a big object !

 

 

 

[Loopnova]

2 hours ago, Cornelius Varley said:

The focal length will be more than 550mm, the distance from the primary to secondary. The point of focus will fall outside the tube, probably about 200mm from the secondary, which will make the telescope 750mm in focal length which is a common focal length for 150mm telescopes.

Ah that's good to know! Is there a way to find the point of focus to get a proper measurement? I've tried shining a light into the telescope and holding a piece of paper outiside the eyepiece but can't seem to get a focussed beam.

1 hour ago, John said:

I agree with Peter (above) - most likely an F/5 with a 750mm focal length.

I think that scope is one of the older Orion Optics ones that uses a single vane secondary support like this ?:

Orion used to have a series called the Europa which yours could well be one of.

It is worth blowing the dust off the mirrors using a manual blower but don't touch the mirror surfaces.

The collimation may need attention if it's not been adjusted for some time.

M31, the Andromeda Galaxy should certainly be visible through the scope but you will need a low power eyepiece and even that wont show the whole thing - it's a big object !

 

 

 

I think it could well be part of the Europa series, it does have a single vane secondary support instead of a "spider".

I'm looking at possibly picking up a cheshire, is a laser collimating eyepiece worth the extra expense?

I'm also looking at barlows now. I can get a Celestron Omni for £25 instead of £60, but wondering if it would be better to invest in a higher quality barlow for any future high quality eyepieces/telescopes I might get. Looking at a Meade #126 x2 Barlow Lens 1.25" or a Revelation Astro 2.5x Barlow Lens 1.25". Does anyone have advice in regards to these choices? Would I really be better off buying say, a 6mm eyepiece for higher magnification instead of a barlow?

Edited March 7, 2020 by Loopnova

[Ricochet]

1 hour ago, Loopnova said:

I'm looking at possibly picking up a cheshire, is a laser collimating eyepiece worth the extra expense?

Only if it is a Howie Glatter laser, and even then I would suggest that as an addition to a standard sight tube/cheshire. Any other laser can be used for the primary if you use the barlowed laser technique.

1 hour ago, Loopnova said:

I'm also looking at barlows now. I can get a Celestron Omni for £25 instead of £60, but wondering if it would be better to invest in a higher quality barlow for any future high quality eyepieces/telescopes I might get. Looking at a Meade #126 x2 Barlow Lens 1.25" or a Revelation Astro 2.5x Barlow Lens 1.25". Does anyone have advice in regards to these choices? Would I really be better off buying say, a 6mm eyepiece for higher magnification instead of a barlow?

I'm not sure that any of the barlows you mention are "higher quality" than any of the others. I don't know who is claiming the Omni is usually £60, the £25 is normal.

With regards to eyepieces, how much you are looking to spend per eyepiece and/or in total and what you want to observe with the telescope (DSOs, planets, everything!) would be a good starting point for people to give suggestions. The 50x aperture rule for magnification is also too optimistic, not applicable to all types of object and basically a marketing tool to sell telescopes.

[Loopnova]

1 hour ago, Ricochet said:

I'm not sure that any of the barlows you mention are "higher quality" than any of the others. I don't know who is claiming the Omni is usually £60, the £25 is normal.

I suppose I'm looking for some guidance with what constitutes a high quality or lower quality barlow, and what brands tend to fall in each category. It seems I'm looking at a particular price range! Amazon and Tringastro are showing a sneaky £59 £24.95  "sale" price.

1 hour ago, Ricochet said:

With regards to eyepieces, how much you are looking to spend per eyepiece and/or in total and what you want to observe with the telescope (DSOs, planets, everything!) would be a good starting point for people to give suggestions. The 50x aperture rule for magnification is also too optimistic, not applicable to all types of object and basically a marketing tool to sell telescopes.

I don't know really, in the range of £30-50, which on further inspection limits me to more entry level equipment I think. I'd like to know more about choosing the right eyepiece for an object. Is it a good rule of thumb to say higher magnification is better for viewing planets? I'd like to make the image of the planets larger, I know they won't necessarily become more detailed with magnification. Is it possible with this telescope to view deep sky objects well? Or is it really down to increasing focal length/mirror diameter?

Edited March 7, 2020 by Loopnova

[John]

A 150mm F/5 newtonian can show the planets well. Magnifications of 130x - 230x will prove the most effective for this purpose. 

Many (hundreds) deep sky objects are within the grasp of such a scope as well. Generally lower to medium magnifications are used to observe deep sky objects, eg: 25x - 80x

Decent quality eyepieces can be bought for between £30 and £50 new, less on the used market. A good example of such an eyepiece are the BST Starguider range as sold here by First Light Optics:

https://www.firstlightoptics.com/bst-starguider-eyepieces.html

It is useful to have at least 3 eyepieces to give low, medium and high power magnifications. Many folks end up with quite a few more though !

You might find that an illuminated reticule / red dot type finder is a good addition to the scope. These are zero magnification but appear to project a target against the sky showing just where the scope is pointed. This is a good one and it can sit alongside the optical finder you currently have:

https://www.firstlightoptics.com/finders/rigel-quikfinder-compact-reflex-sight.html

Whatever finder you are using, it is vital that it is adjusted so that whatever the finder is centered on is also exactly what the main scope is centered on. This can be done in daylight using a distant target (ie: tree, chimney etc) and a low power eyepiece in the scope.

A good star chart is also essential to help you work out where your target objects are in the sky. There is also the free software Stellarium which shows you what is where and when:

https://stellarium.org/

 

[Ricochet]

13 hours ago, Loopnova said:

I'd like to know more about choosing the right eyepiece for an object. Is it a good rule of thumb to say higher magnification is better for viewing planets?

As far as "rules of thumb" go I am of the opinion that selecting eyepieces by exit pupil size is the best starting point. Exit pupil = Eyepiece focal length / telescope focal ratio so for your f5 scope we can select eyepieces using Exit Pupil * 5. I am also going to assume that due to your location and light pollution the maximum exit pupil you want is 5mm.

• Technical limit for separation of double stars, aesthetically unpleasing:  Exit pupil > 0.5mm => EP > 2.5mm
• Optimum Planetary Observation (no APO or binoviewers, ignoring atmospheric conditions): Exit pupil 0.8 - 1.0mm => EP 4 - 5mm
• Open and globular star clusters, aesthetically pleasing separation of double stars: Exit pupil 1 - 5mm => EP 5 - 25mm
• Unfiltered nebulae, galaxies: Exit pupil 2 - 5mm => EP 10 - 25mm
• Diffuse Nebulae, UHC filter: Exit pupil 2.5 - 5mm => EP 12.5 - 25mm
• Diffuse Nebulae, OIII filter: Exit pupil 3 - 5mm => EP 15 - 25mm

The last two are probably the most contentious and I think it really does depend on the transmission of both the eyepiece and the filter as to how high you can push the magnification and how much the image is dimmed. At the price range you're talking about they might be a bit optimistic. Also notice that for all the DSOs there is a range up to your maximum exit pupil. This is because many cover quite a large expanse and you may need the extra true field of view that a longer focal length eyepiece will provide.

For DSO eyepieces I like 1.4x steps in focal length as this results in a doubling or halving of image brightness, which is what your eye needs for there to be a noticeable difference. For example with my f6 scope I use 5, 7, 10, 14, 28. I could also fit a 20mm between the 14 and 28 but so far I've not felt the need to fill that gap.

For planetary observations you will find the atmosphere at the time of observation plays a big part. You will either want a closely grouped selection of eyepieces or a zoom + barlow combination which will allow you to dial in the optimum setting.

At the £50 price point I would suggest you look at the 5, 8 and 12mm BST Starguiders and the 10 and 18mm Baader BCOs. These TMB clones are supposed to be pretty good planetary eyepieces but I've not used any myself to confirm that. At the low end I would recommend you look out for a second hand 24mm Explore Scientific 68°/Maxvision/Meade SWA to maximise your field of view in a 1.25" format.

There are also the following second hand eyepieces with active adverts you may be interested in:

Opticstar zoom

10 + 18mm Baader BCOs

16mm Maxvision

I have no affiliation or previous dealings with any of the sellers listed.