Louis D

1 hour ago, vlaiv said:

I'm not sure that Maks and SCTs have as much vignetting due to size of rear port as people think.

Beam is very slow and focal length very long, so I don't think that much of light gets blocked in percentage - and drop of about 10% is just on the edge of being perceived as difference (just noticeable difference for light is 7% if I'm not mistaken).

Here's my post on the subject:

As I say in the post, I've photographically measured about a 35% drop off in illumination toward the outer field.  Visually, I can't see it at all.

51 minutes ago, bomberbaz said:

Just to help here it was I who suggested a reducer for increasing the exit pupil for use with narrowband filters in a different thread.

The op was looking at viewing the eagle nebula and was struggling to see it with no filter and a 25mm ep at 2.5mm exit pupil.

It was suggested by one commenter that a uhc filter would help but I chipped in with reducer for the larger exit pupil or a 40mm plossl as an additional measure to improve results.

My personal experience points to a 5mm exit pupil being optimum for line filters with 4-5mm being ok for the more forgiving uhc when used on diffuse nebula. I have read numerous articles in various forums and have yet to find anything to convince me otherwise.

So if you can live with the vignetting to have a strong response from a dim object at the centre of the eyepiece then it is still something to consider, a 40mm plossl is the other contender. 

 

In that context, that should be fine.  Just as when I use a 40mm 2" eyepiece to center an object in an f/5 scope producing an 8mm exit pupil.  I don't really care about the wasted photons in that use case, I just want the object within the field of view to avoid searching about for it.

3 minutes ago, vlaiv said:

Focal reducer helps with following:

- if telescope "field stop" (or illuminated field to be precise) is larger than eyepiece field stop - it helps to "squeeze" that field produced by telescope into field shown by eyepiece

Further, assuming the OP has a C5, using a 0.63x focal reducer will show more field with a 32mm or 40mm Plossl, but with outer field vignetting as with a widest TFOV 2" eyepiece used with 2" accessories on the C5. TANSTAAFL

19 minutes ago, Bugdozer said:

OK, firstly I cannot use a 2" eyepiece, mine is a 1.25" sort. 

Secondly, I thought larger exit pupil was a good thing? In another thread I was recommended a 40mm plossl over a 32mm because even though in my scope they will give the same field of view the 40mm has a larger exit pupil. 

Thirdly, your point 2 about FOV in mm is confusing me. Eyepiece FOV always seems to be expressed in degrees. If I look through a given eyepiece I have no idea if I am looking at a FOV of 10mm, 20mm, or any other number. But I can estimate it in degrees. 

1. You could mount a 2" visual back and 2" diagonal to a C5 (assuming that's your scope).  You'll just get vignetting like I do with my 127mm Mak and 2" visual back.  The fully illuminated image circle is limited by the approximately 1 inch diameter rear baffle and rear port.
2. Exit pupils larger than your fully dilated eye pupil will waste aperture collected photons on your iris instead of sending them to your retina.  Sometimes, this is acceptable when trying to achieve maximum TFOV at any cost.
3. You're mixing up eyepiece AFOV (apparent field of view, measured in degrees and is completely independent of the scope) with TFOV (true field of view which is also measured in degrees on the sky, but is dependent on the scope it's mounted to).  However, TFOV is dictated by the field stop diameter of the eyepiece.  Between two eyepieces, the one with the larger field stop will yield the larger TFOV in any scope used in common between the two eyepieces.

Around here, we have to get below the line where drought and flooding affect the top layers of soil.  A lot of houses built on slab foundations instead of pier and beam foundations flex and crack as the soil hydrates and dehydrates over the span of months to years.  I would expect the same to happen with pier footings not set deeply enough.

You could search on ebay for a cheaper alternative.  Just make sure it supplies at least 5 Amps at 12 V.  The 12 V connector is pretty standardized by voltage with a positive tip.

Celestron lists 3.5 Amps minimum for their AVX mount with positive tip.

1 hour ago, Don Pensack said:

The term "Focal Extender" was first used commercially, I believe, by Meade or Celestron back in the '70s.

It was a tube you added to the back of your SCT to extend backwards the focal point of the SCT, and it had camera T-Threads on the rear end so you could attach an SLR camera to the piece for prime focus imaging.

I think the purpose for the device was to lengthen the focal length of the scope and increase the image scale so the Moon and planets could be imaged.

Otherwise, I could see no reason not to make it quite short.

 

Meade later introduced a line of 4-element Barlows they termed "Focal Extenders" to differentiate them from others in the market.

They were made in China by Jing Hua Optical (JOC), and still are to this day, though they sell under other labels now, like Explore Scientific.

Technically, ALL Barlows are focal extenders.  That's what they do--extend the focal length.

And, nearly every one moves the focal plane out as well, though how this impacts the focal point of an eyepiece is determined by the focal length of the Barlow and the length of the upper barrel above the lens.

 

So the term "Focal Extender" does not necessarily mean the Barlow is telecentric.  Telenegative Barlows are focal extenders too.

You must be a hoot at a party when something astro related comes up in small talk:  "No, that's not quite accurate.  You see, back when f/8 was considered a fast scope...." 😉😁

UPS and FedEx here in the States both way outperform their USPS counterparts.  I've lost track of the number of misdelivered USPS packages or packages marked as delivered when they were still in the system just to keep their on time delivery metrics high.

Since USPS packages go into locked mail slots, I can't just go and find them when they're delivered to the wrong slot.  I have to hope my neighbor doesn't check his mail and decide to just keep my package for himself (too indifferent to do otherwise).  I then need to catch my postal worker at our mailboxes the next day and force them to check the other boxes for my misdelivered package.

By comparison, this has happened only once with UPS and never with FedEx.  DHL is barely present in the US.  And with UPS misdeliveries, I just go around the neighborhood once I'm notified it's delivered but not on my doorstep and check the usual suspects' doorsteps that have the same street address but on a difference street and just grab my package for myself.  I do worry someone will think I'm a porch pirate doing that, though.

As far as cheaply priced tele-extenders, Astronomics is now marketing a 2x version under their Astro-Tech brand for $60 (plus sales tax).  I think it's getting decent reviews on CN.  Perhaps @FLO could bring it to the UK under one of their house brands?

Glad to hear you got that sorted out.

I have a 2.5mm TMB PII clone with no rattling.  For the price ($35 shipped), it's actually a pretty decent eyepiece.  It's just too much magnification most nights.

6 minutes ago, Mr Spock said:

Thu - Sat last week we had 106.2mm. You are welcome to have some of that!

Sure, send it our way.  We're short about 12 inches at this point in the year, and many more over the previous 2+ years.  Our main reservoir for drinking water is down to 35% full.  We really need a tropical storm or similar to deluge us in rain for days.

I bought a second hand 90mm TS triplet here in the States without any issues during shipping.  I'm sure FLO would do at least as well if not much better packing the scope and choosing a courier.

However, SCTs and their thin corrector plates are another issue vis-a-vis shipping:

That, and those cheap 0.5x reducers produce a highly curved field with lots of outer field aberrations.

The best use for one I've found visually is to increase the field of view of my binoviewers in scopes with limited back/in focus.  I have a 1.25" 2x Barlow element up front followed by 45mm of spacer rings followed by the 0.5x reducer.  There is some vignetting and outer field aberrations, but I can reach focus in my Dob with only 1.0x magnification instead of my normal 3x magnification of the Barlow element alone.  I've essentially created a relay lens.

Conceptually, the way it makes the exit pupil larger is by compressing the entire exiting light flux into a smaller image circle at a lower power.  This also means that widest true field eyepieces will be vignetted if used with a focal reducer.  For photography, you can't use as large of an imaging sensor when using a telecompressor (focal reducer) because of the reduced image circle.

Once you do get it, let us know how it worked out.

We're under clouds here in Texas for the foreseeable future as well, but we need the rain, so we're not complaining too loudly.

You probably have already experienced this yourself, but swapping large Barlows/Magnifiers in and out of the optical train is a pain, even on a Dob.  I tend to leave one in for an entire viewing session when conditions allow for the higher powers.  This works okay for 2x, but I'm not sure how well it would work for 4x.

I have an ES 2x tele-extender that I picked up cheap and have yet to use it critically.  I've been preferentially using my Parks GS 2x shorty Barlow with my refractors lately.  If I get time, I'll have to have a shoot out with those two and my Meade 140 APO 2x, Tele Vue 2x, and Orion 2x Deluxe (all Barlows).  The last one doesn't work well with refractors being 6 inches long.

There used to be 1.25" SCT focuser available from TS that might fit the 127 Mak, but it seems to have been discontinued:

I cut down a foam bicycle grip and built up the internal gap with racket grip tape over the original 127 Mak focuser knob.  Both together make for a wider circumference for finer focusing, and add vibration dampening during focusing.  The other thing I did was cut some vibration absorption pads from vintage 1990s 100% Sorbothane insoles for each tripod foot.  All together, I spent under $20 on ebay for everything 5 years ago.  Figure with our newfound inflation that the total cost will now be a bit higher.

2 hours ago, JeremyS said:

Are they armed, then? 😊

No, but they can make for excellent bludgeoning tools when a baseball or cricket bat isn't available. 😉

52 minutes ago, paulastro said:

 I would have thought most people would have heard about it (using a dark cloth) as its hardly a new idea - though of course, there's a difference between knowing and doing.

 

It would be one more thing to make me sweat profusely here during a sweltering Texas summer night.  It would probably be okay from fall through spring, though.

27 minutes ago, John said:

I have sometimes thought that using a large aperture scope vs a smaller one is a bit like the difference between 1.1 litre car and 2.5 litre (we had these for a while). The small engined car could certainly push along at 70 mph on the motorway but the larger one did it with a lot more ease and the engine was barely turning over (or at least gave that impression).

Right there with you.  Moving up to a 3.6 liter V6 is a nice boost as well, but going to a 6.0 liter V8 really puts a smile on your face.  I just got done running errands in my 2009 Pontiac G8 GT, and the big engine is really fun until you pull up to the gas (petrol) pump (which I also did today).

If my back was up to it, I'd take my old Tectron 15" Dob (Nova mirror) out for a spin under the stars.  If you buy these older, big custom Dobs used, they can be quite a bargain.  I doubt if I could get even $1500 for mine nowadays.  Everyone wants a scope from a current maker with a currently known premium mirror.

On 18/10/2023 at 12:42, Deadlake said:

I agree, I get bored waiting for my C11 to acclimate.

The C11 is a great scope for DSO's, however the stars will never be as sharp and pristine as an APO.

 

Unless they're in a globular cluster, then then tend to just look like fuzz in most affordable refractors.  In a large Dob with a premium mirror, they break up at high power into twinkling crushed diamonds on black velvet, at least under Texas skies.

3 hours ago, DirkSteele said:

I note the two inch visual back is optional. Wonder if it vignettes using the full 2” field stop?

Probably.  It would depend on the baffle sizes as well as the secondary size.  However, visually, you probably wouldn't notice it.

I would think cool down time would be an issue with using this scope as a G&G planetary scope when the clouds suddenly open up for a few minutes.  The description didn't mention anything about built in cooling fans and filtered ventilation ports.

I really like the concept of the Bresser's big altitude trunnions negating the need for friction devices and its tube ring saddle allowing tube rotation, forward-back balancing, and complete removal for use on an EQ mount.

20 hours ago, John said:

Plossls have eye relief that is around 70% of their focal length. 

The eye relief stated can be misleading if the eye lens top surface is heavily concave or the eye lens is inset below the eyepiece top or below an inflexible eye cup. Actual useable eye relief can be somewhat less due to these factors. 

Edit: apologies - I am repeating what I have already posted earlier in this thread 🙄

True.  I've got a pair of 26mm Sirius Plossls that are not usable with eyeglasses because of eye lens recession.  I've measure the pair to have only 11mm of usable eye relief.

Even my GSO and Sirius 32mm Plossl pair each have only 15mm of usable eye relief due to eye lens recession.  They are usable with eyeglasses.

My 20mm SVBONY 68° Ultra Wide Angle pair have 14mm of usable eye relief and are comfortable to use with eyeglasses.  So far, these are my favorite pair for BVs.

My vintage ~17mm Bausch & Lomb Wide Field 15x pair of microscope eyepieces have 19mm of usable eye relief and are a close second favorite pair to use in a BV.  Since they are designed to minimize RMD, AMD is more obvious.  However, I've never noticed this issue BV'ing.