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Telescope buying advice May 26, 2012

Posted by aquillam in Astronomy, MichiganAstro.
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One of the most common questions I get is “what telescope should I buy. ” Prices have come down significantly in recent years, so it’s time for an update. First though, I’ll reiterate one more time my old advice: consider binoculars. They’re easy to use, portable, affordable, and useful for lots of things besides astronomy. If you already have considered them, and you’ve decided that you really do want a telescope, read on.

Decide on a budget before you start. You can’t really spend less than $100 for a telescope plus the accessories, but there really is no upper limit, so decide how much you are willing to spend first. Don’t forget that there are likely to be accessories so be sure to include those in your budget. If your budget is under $200, you’ll probably need a few non-standard accessories, like a table. I’ve listed a few examples at the end.

The best thing to do is go someplace where you can look at the ‘scope before you buy it. What I’ve written here assumes you have that option. If not, you’ll have to rely on pictures and descriptions in catalogs or online. Check the return policy before ordering! Cloudynights.com has a lot of equipment reviews, but I recommend that you have some idea what you want to know about before going there or its likely to be pretty overwhelming.

The most important thing is to make sure it is something you will use.  If it is too heavy for you to set up by yourself, you’ll either need a helper, or a permanent mount with a building to protect the ‘scope. A 16″ Dobsonian is not going to fit in a compact car, or in a minivan with 3 kids. German Equatorial mounts have several pieces that have to be assembled if you try to travel with it, including a counterweight that can be heavy and awkward. If you want to do photography, you need a mount that can track. Computerized mounts can be nice, but they require some extra set up time, so they aren’t appropriate if you want to be able to grab the ‘scope for a quick look before bed.

One of the easiest ways to tell if the telescope is worth spending money on is to look at the mount. Most telescopes come with a tripod. If it is wobbly, or unstable, don’t get it. If it’s too light for the ‘scope, don’t get it (a good mount does not require a sandbag!) If it is stiff or hard to adjust, skip it. Unlike terrestrial observing, things in the sky move, so you want the mount to move too. If the mount will track, make sure the locks are snug enough that the telescope won’t move if bumped, but they do still allow the fine motion control to work smoothly. I have run across several Meade ‘scopes made for department and discount stores with the normal, excellent optics of Meade, but the tripods were so bad the ‘scopes were basically unusable.

The next thing to decide on is what type of ‘scope to get. In general, a wider telescope makes a brighter, clearer image. However, if you’re observing in light polluted skies, the ‘scope will also collect more of the background light and wash out deep sky objects.
– Refractors have only lenses so they are the traditional spyglass type. They never need adjusting, and most people find them very intuitive to use. However, wide refractors are also generally very long and heavy.
– Reflectors use mirrors, and sometimes also have lenses, usually called corrective optics.  Most of them require occasional minor adjustments.  They can be less intuitive to use, especially the Newtonian style which has the eyepiece coming out of the side. However, most people adapt fairly quickly, and reflectors are generally lighter and less expensive for the same width.
– As a general rule of thumb, refractors are better for observing planets, the Moon, and other solar system optics, and reflectors are better for deep sky objects such as nebulae.

In addition to making sure the telescope is something you will use and that the mount is good, you should also check out the telescope optics. The image should be clear, with no distortions near the edges. The image should not have extra colors, and objects should not have auras or be partly out of focus. The focuser should move smoothly and easily, but hold when you let go.

Good names include Celestron, Meade, Orion, Williams Optics, and Edmund Scientific. In general, anything made by these companies will be good ‘scopes as long as the mount is good. You may also find a good ‘scope made by another company, but watch out for anything under $500 from another company.

Some examples (prices are estimates based on manufacturer’s websites in May 2012)

A Celestron FirstScope, a book with charts like Nightwatch by Dickenson, and a small folding camp table make a nice starter set for under $100.

An Astroscan from Edmund Scientific plus the carrying case so you can travel with it will run you about $300. You’ll probably want a small folding camp table too, but the Astroscan comes with star charts to get you started.

If you want to get the classic refractor type, a 90 mm (3.5″) with a good mount should cost at least $300. Be skeptical of anything less than that.

A small, compact telescope called a spotting ‘scope is also very useful for terrestrial observations, like bird watching. They also make great zoom lenses for SLR cameras. A beautiful, compact APO refractor from Williams Optics with a tripod, diagonal, zoom eyepiece, and camera adapter will probably run you around $2500, but you’ll be able to take amazing photos and fit everything into a carry-on. These are also good deep-sky ‘scopes, despite being relatively small refractors. Orion and Celestron make much less expensive spotting ‘scopes. A Maksutov-Cassegrain spotting ‘scope from Orion with a camera-type tripod should run you about $500

A Newtonian telescope on an alt-az mount (short for altitude-azimuth) like a Dobsonian is very quick and easy to set up, so it makes an excellent grab-n-go ‘scope. These also tend to be no-frills ‘scopes, so you can get an 8″ scope for about $350 from Orion. Add the computer controlled base and it will cost you $600.

An 8″ Schmidt-Cassegrain is a classic amateur telescope, and is still reasonably easy for one person to set up and take down. Celestron began manufacturing these in the late 60s and you can still find them used with a mount and a couple eyepieces for around $800. If you want a new one, Celestron and Meade both make nice Schmidt-Cass ‘scopes for around $1000 for just the optical tube, $1500 for the tube and a German equatorial mount. Make it a computerized, motorized fork or arm mount, and it’ll run you about $2000. Add things like cooling fans and stabilizers to make it a really great astro-imaging system and you can quickly reach $4000 before you’ve invested in the camera!

c8 repair – removing the corrector plate February 29, 2012

Posted by aquillam in Astronomy, telescope maintanance.
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This post is one in a series about C8 repair and maintenance.  You should see the introduction and disclaimers before trying anything here.

A Cassegrain telescope has a large mirror at the bottom with a hole for the eyepiece and a small secondary at the top of the tube. They can have expensive, hyperbolic mirrors, or they can have some sort of corrective lens. If the corrective lens is on the front, it is a Schmidt-Cassegrain. That is what the old orange-tube Celestron 8 telescopes are, and you can tell because it says it is a Schmidt telescope right on the plate with the serial number.

That corrector plate is NOT just a plane piece of glass. It is an integral part of the optics of your telescope, and was ground to match your primary. If you damage the corrector plate, it has to be replaced by Celestron, and it’s going to cost you as much as a new telescope. So, first,  a really really really important warning.

Really. Very. Important.

You must be extremely careful doing anything with the optics.

In addition to the obvious problem if you break the optics, there is also the problem of keeping them aligned. If you mis-align your optics, your telescope will never be the same. You may still be able to see things, but your $800 classic telescope might be no better than a $40 department store special.

There are very few reasons to open the ‘scope, so make sure your problem warrants it. A little dirt inside is fine, and won’t do much of anything to the view. Even a few pits won’t be noticeable. Dew inside can be dried with a hair dryer pointed in the eyepiece opening, and a few light water spots also won’t affect the view. Of course if the ‘scope has seen 30 years of heavy use from people who only vaguely know what they’re doing, or if you get caught in the rain, it might be worth opening things up. Also, on the older C8s the only way to get to the focuser is to take off the corrector and remove the primary. I won’t be covering that here anytime soon (I have 7 hard-used, 30+ year old C8s, and ONE of them needed some help with the focuser about 10 years ago.)

If you decide that you really do want to open up your scope, you will need a small allen wrench and a pencil. Tweezers are good if you don’t have slightly long nails. Also, whatever equipment you need for whatever maintenance you want to do.

Before you begin, make sure the dec lock will lock easily and tightly (see adjusting the dec lock.)

Mark the secondary and outer tube

The very first thing to do is mark the telescope in several places. Note I placed marks on the secondary cell, the ring that holds the corrector in place, and the outside edge (the ring orientation doesn’t matter much, but being over-cautious is valuable when dealing with the optics.)

Rmove the screws from the retaining ring

Remove the screws from the retaining ring. It helps to tilt the 'scope so the screws can't roll away onto the corrector plate.

Remove the screws that hold the retaining ring in place. Be careful not to drop the screws as they could scratch the corrector plate. You may want to adjust the tilt of the telescope while you do this so the screws are less likely to fall onto the corrector plate.

Remove the ring. I do this by hooking my thumb nails on either side and pulling outward while lifting. If you don’t have long nails, I would suggest one of those flat plastic toothpick/flossers to lift the ring until you can get hold of it without touching the part of the corrector plate that you want light to go through. Make sure whatever you use is though enough to lift the ring but won’t scratch.

Corrector plate serial number

Mark the position of the serial number on the corrector plate on the outside support ring. Also note the cardboard spacer

With the ring off, you can see the serial number on the corrector plate. It probably won’t be the same as the ‘scope’s serial number. Note the position of the serial number. Mark the outer support ring so you can realign this latter. I placed 2 marks so I can’t get confused about which part of the serial number I marked.

There are some boxboard or cork spacers holding the corrector plate in the correct position. Try to remove these first. A jeweler’s screwdriver or fine tweezers may be helpful, but again be careful, but again, be careful not to scratch anything.

The ‘scope will be very back-end heavy once the corrector is removed, so the closer to vertical it is, the better. However, I usually find that it’s a little easier to see what I’m doing if I have the ‘scope tipped just a little bit. Make sure the dec lock in engaged and firm.

Lift the corrector

Lift the corrector using the secondary. Be very careful since this is the place you are most likely to do damage to it. If it sticks, it may come free suddenly, so be prepared for that.

Hold the secondary cell and lift gently. If it won’t move, try to shift the corrector plate a little to one side. DO NOT FORCE ANYTHING! This is the part where you are most likely to damage the corrector plate accidentally.

If your ‘scope has been exposed to moisture (left in the rain, used in very dewy conditions, etc.), the corrector plate may be stuck to the interior mounting ring. You’ll have to weigh  the risks of damaging the ‘scope with the benefits of getting the corrector plate off. Patience is key. You might try something like one of the plastic toothpicks around the edge. Use something that has a little give (NOT a jeweler’s screwdriver) so you don’t chip or crack the corrector. It may help to warm it a little, like with a hair dryer. Additionally, be careful not to damage the supporting ring underneath, which has a soft cover on the rim. If the corrector plate is really stuck, be warned that when it does come free, it may come free suddenly. Don’t get so immersed in trying to force it free that you don’t have a grip when it does come free. Again, patience.

If you are opening it to clean it, be careful what you use. Check your manual if you still have it. Mine recommends a solution of one drop mild dish soap in a gallon of distilled water. I use a cleaner from Orion, but I usually find I have to follow that up by “huffing” on the optics to remove the final bit of film. Some commercial optics cleaners are available, which may also contain rubbing alcohol. This helps the cleaning solution dry faster and with fewer streaks or spots, but isn’t safe for all optical coatings.

A note about canned air:  if you get the nozzle too close to your optics, if you shake the can, or the can is not perfectly upright, you might not be pleased with the results. Condensation and propellant can come out in a glob and damage the optical coatings. You can find bulb dusters and fine brushes at camera shops 9assuming you can find a camera shop.) High quality lens paper does a very nice job too. When cleaning the inside, be even more careful than when cleaning the outside. Use a very light touch (almost not touching), and expect to go through at least half a bag of cotton balls.

When reassembling, you basically do everything in reverse.

Marks to aligh serial  number with

Set the plate into place with the serial number aligned between these marks.

Identify the location of your marks so you can replace the corrector plate.

You may want to clean off the support ring the corrector plate sits on, especially if the corrector was stuck. Be sure not clean off your marks! Clean it with the same solution as the optics and give it time to dry before putting the corrector plate back in. Do NOT put anything on it, no matter how hard it was to get the corrector out. Oils and solvents can damage the soft covering or generate a film that will cover the inside of your corrector plate.

replacing the spacers

Replace the spacers. Tweezers may help.

Set the corrector in with the serial number aligned with your marks.

Slip the spacers back in around the corrector plate. There should be 3 – 4, depending on how many are needed to keep your corrector in place. If the cork spacers are damaged, you can replace them with boxboard (like a cereal box) or folded paper. Use the cork as a template for the replacement pieces, and make sure they are shorter than the corrector plate so they don’t interfere with the outer ring. It doesn’t matter where they go, as long as they are roughly evenly spaced. Tweezers may be helpful for getting them into place. You may also need to slide the corrector plate to the side to open the gap. Push sideways on the secondary cell to move the corrector.

Replace the ring

Carefully set the ring in place...

Once the corrector and spacers are settled in place, set the ring on.

Align the ring

...rotate the ring to align it with the holes in the support ring below, and push it into place.

Carefully rotate it so the holes align with the holes in the support ring below, then push it down into place.

Replace the screws, tightening firmly but not too hard.

Chances are good you’ll have to collumate your telescope after that. But that’s a post for another day. One that follows a clear night without students.

Much gratitude to Bob, who taught me  everything I know about repairing and maintaining telescopes. Miss you man.

Summer Observing for Urban Observatories. May 17, 2011

Posted by aquillam in Astronomy, Detroit Observatory, MichiganAstro.
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I normally create a star chart and list of highlights for each observing session during the summer at UofM when I’m not going to be there. This year, I thought I would get them done ahead of time and do the ones I will be at too.

Having spent a couple hours putting this together, I thought it might be worth sharing. Especially since I spent some time sorting through things I know we can find and things that might be a challenge for our urban observatory. Hopefully other urban observers will also find this useful.

Charts were created using Starry Night College and are formatted for printing. The pages are black with red text so they won’t ruin your night vision if you look at them on a screen.

Summer star charts and Observing Highlights.

One note for anyone in the area looking at this, please note that most of these are not public events.  Public Open houses at Angell Hall are posted at http://www.astro.lsa.umich.edu/sas/openhouse.html and Viewing Nights at the Detroit Observatory are at http://www.astro.lsa.umich.edu/outreach/detroit.php.

c8 repair – opening the base part 3: the gears March 3, 2010

Posted by aquillam in Astronomy, MichiganAstro, telescope maintanance.
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This post is one in a series about C8 repair and maintenance.  You should see the introduction and disclaimers before trying anything here. This is part 3 (obviously.) You need part 1: opening the base before you can do the things on this page. That’s also where you’ll find the links to the other things you can do with the base open.

The Top Part of the Base


Removing the ring that holds the gears in place

The gears will stay with the top part of the base when you take it apart.  This is also the part where the RA lock and fine adjustment are, but that’s another post. To get to the guts, remove the ring that holds the gears to the top. As I mentioned in part 1, I usually put the screws in masking tape and tape them to the ring so everything stays together.

With the ring off, the gears slip off the axis.  There are two gears separated by the RA dial and a spring spacer. Because of the spring, you really should not take the gears apart. There isn’t a good reason to take them apart, and the spring can be really hard to put back in place. It is important that the gears be greased so they don’t wear too much or bind.  It is easiest to grease the large gears, and they will transfer grease to the smaller gears on the motors and RA fine adjustment knob. Be careful not to get grease along the top and bottom of the gears – a greasy gear can stop the RA lock from working. The surface on the inside of the gears, next to the axis should also be greased so it can slip past the ring that holds the gears in place. The edge of the bottom axis  also needs to be greased (part of it is next to the RA lock so be careful where the grease goes.)

The gears shifted slightly aside to show the edge that needs to be greased

Lifting the gears off the axis. They are shifted slightly aside and show the edge that needs to be greased (dark with old grease)

If the ‘scope has never been opened, and it is stiff, it may be time to clean out that 30-year-old factory grease and apply some new stuff.  It needs to be a good, long-lasting grease that won’t get too sticky in cold temperatures. I use whatever the department machinist has on hand. However I can tell you that the lightweight white (probably lithium) grease he gave me about 3 years ago was not a good choice.  It is thin enough that it worked its way onto the RA lock in the two ‘scopes I used it on.

disassembling the gear assembly

Disassembling the gear assembly

In general, you shouldn’t have to take the gear assembly apart.  If you do, be warned: there is a spring that helps keep the setting circle stationary when the telescope rotates that may try to leap out at you.

Place the gear assembly on the table with the setting circle face down and remove the three screws that hold it together.

the pieces of the gear assebly

The four pieces in the gear assembly. Note the spring tucked in among the gears.

The grease may hold things together, so you may need to give the pieces a push to get them apart. Don’t take the screws out of the setting circle.  The hold the little nubs on that make it possible to move the circle around to set the RA correctly. There should be a total of 4 pieces.  The teeth and inner edge are all that needs to be greased.

replacing the spring

With the gears loosely held together with the screws, slip the spring into place.

Because of the spring, re-assembling this is slightly more than just reversing what you did to get it apart (in all probability, the spring came out on its own when you loosened the third screw!)  To get it back together, stack the two gears and the setting circle in order, and put the three screws in, but don’t tighten them all the way.  Slip the spring in between the setting circle and the top gear.  With most of the springs, it works best to put the spring on top of the top gear and fit one end in place, then feed the rest of it along until it slips into place. The spring in this ‘scope seems to be a bit shorter than usual, so it was easy to put back.

Once the spring is in place, tighten the screws down the rest of the way and you’re ready to re-assemble things.

From here, you may want to go on to: