Al's Astrophotography Blog

Aug 2009 – Finally, a night with stars and a chance to test out my new GM-8.

I recently bought a brand new Losmandy GM-8. I bought this mount to serve as a portable (almost grab-n-go) to take when I head up into the hills to do imaging. I found that my CGE is just too much mount for me to pack up and take remote, and my CG-5 is too finicky. I thought the GM-8 a good compromise.

On first receipt of the GM-8, I was impressed by the apparent quality of the mount, overall very nice fit and finish. However, the new equipment curse has kept the skies cloudy, and last night was the first opportunity to actually see how the mount performs.

I’ve had no previous experience with Losmandy mounts or the Gemini system so I was prepared to be befuddled and confused, but actually all went pretty well.

I set the mount up with an 80mm refractor and DSI-II camera to align and test guiding with. A polar scope came with the mount and it proved quite accurate. It must have had the mount very close to the pole because alignment was a breeze. After sync’ing to the first alignment star, every additional star landed near the center of the camera chip. I aligned to 6 stars then tried the Gemini polar correction routine which turned out to be very easy to do. The amount of correction needed was only about 8 arc-mins, which again suggests the polar scope was pretty much right on.

I tried guiding using PHD and it calibrated first try and seemed to guide pretty well. I then tried using the Gemini’s routine train PEC. Using the guide camera I captured, averaged, and smoothed, 4 PEC training runs. Then with PEC enabled, the guiding smoothed a little more. Looking at the data with PEAS showed it was guiding with the raw data (noise) at less than 3 arcsecs p-p and Avg PE less than +/- 0.2 arcsecs. For a light weight portable mount, I’m very satisfied with the out-of-the-box performance of this GM-8.

I’m a happy camper today,
al

Ever had your scope objective dew up? Frustration eh? After it happened to me I went shopping for ready made heaters.

Well… being frugal (i mean really cheap), I just couldn’t bring myself to part with the money they wanted for the heaters. So…. I looked for ways to do it myself. The solution I ended up with: an Electric Blanket!!

Actually, the heating elements from an old electric blanket. I think this stuff is really cool. Within an old electric blanket is something like 70 feet of rubber insulated, finely wound Nickel Chromium resistance wire. I cut up the blanket to get the wire out. The wire has a resistance of about 2 ohms/foot and seems to be about perfect to make low power heaters from.

I decided about 6W of heat would be a good figure to start with, figuring I’d lose 30 to 50% to the outside, so only 3 to 4 watts would get to the scope. So, 6W @ 12v = 0.5 amps, which sounds low enough. Now, since W=(E x E)/R then it would mean I’d need a resistance of R = (12 x 12)/6 = 24 ohms which meant 12 ft of wire would do it.

I also planned to try them first with two heaters connected in series so I’d use half the power from the battery, and put just 1/4 the power to the scopes. It turns out that usually the “in series” configuration is enough to keep the objectives clear. Only on the wettest nights do I have to hook them up as single heaters.

The way I made the heaters:

• Cut two strips of duct tape long enough to wrap around the scope near the objective.
• Lay the first strip on a flat surface with the sticky side up, then lay the wire back and forth lengthwise until I’d used up my 12 ft.
• Put the second strip of duct tape over the wire to trap it between the two strips of tape.
• Hook up some wire leads to the heating elements with crimps (NiCr wire doesn’t solder well) and put on RCA connector on the wire for hooking up to a power supply.
• Wrap the duct tape heater with some scrap strips of the blanket material and it’s done.

This is what it looks like in process of making one these “el cheapo” heaters:

A heater I made for Low Power Heater for use in Series configuration: 3W @ 6V (12 ohms)

I power with either my usual 12V battery or AD/DC adaptor.  I’ve built a little connector box so I can easily use either supply and also hook my heaters in serial or parallel, or just a single unit.

I hold the heaters to the scope(s) with elastic bands and velcro.  Here’s a picture of them in use on the “equipment” gallery on my main website.

See the heater bands in use here:

There are heaters on both scopes. Black elastic holding the heater on the SCT, and some white elastic holding it to the refractor.

This project was simple to do and works fine.

al

After 3 years in this hobby (I hate calling it a hobby… addiction, passion, endeavor, pursuit…) I’ve formed some opinions about what makes a mount suitable for astrophotography. Choosing a mount for your telescope is a very individual thing. It’s very much dependent on your goals, your current and future telescopes and cameras, whether it’s to be permanently mounted, or will it be used at star parties, and other factors.

So, …. I’m not going to tell you what mount to buy. I’m just going to list some of the things I think are important to consider when selecting a mount for AP:

Rigidity, Stability, Mass: It’s hard to have a good AP mount if these elements are weak. An exception can be mass, but it’s often related to the other two elements. The mass of the mount itself can be reduced if the mount is built with better precision and can transfer imposed forces from scope imbalance, wind, etc, to the tripod or pier. The bottom line is that a mount that’s good for AP should be exceptionally stable and rigid, and typically it will be heavy.

Weight Capacity: The rule of thumb is a mount’s weight capacity for AP is half the rated weight capacity. I think that’s not a bad rule, but I’d call it a guideline. When selecting a mount try to find out as much as possible about its construction. A mount’s ability to carry load relates to how rigid/stable it is, and strength of it’s drive system. However, in a properly balanced setup the force needed to move the mount should be very low, basically the force needed to overcome friction. As such, hardened or larger diameter axis shafts supported by ball or roller bearings is a good thing because of the reduced friction and increased rigidity. Wo when thinking of weight capacity, also review the constructions details of the mount, the precision of machining, the types of bearings used, what other AP’rs are supporting with the mount and their results.

Periodic Error & Drive Vibration: Getting a mount with low periodic is important, but it’s not all important. In my opinion just as important is how much jitter is induced but the higher frequency (faster moving) parts of the drive gears. This discussion can get very technical and I’ll probably discuss what I know about it in another post, but for now I’ll say it’s not just about how much PE there is, but how smooth the PE curve is. The smoother it is, the easier it is to correct, either with PEC (periodic error correction) routines, or by guiding. There is almost no published information or specifications on this subject, so the best source I know of is talking to folks who own and have measured/analyzed a mounts performance. The general question to ask is how well does this mount guide?, or can I see a graph of a recent guide log?. When looking at a guide log, look at the raw data and check the total peak-to-peak (p-p) variation measured in arcseconds. There is always variation due to seeing, but values over 4 arcsecs p-p will give you fuzzy oblong stars.

Intended use: Do you want to use your mount at star parties, short trips to remote sites, of will you normally leave it permanently mounted. My simple example is that I bought my CGE as my upgrade from my (wobbly) CG-5. I normally keep my CGE on a permanent pier but thought I’d press it into service when I went remote as well. I was wrong. For me the CGE package with it’s big tripod was just too much to pack up, transport and get setup, was just too much. Too much weight and work for me. As a result, I ended up buying a smaller and lighter mount for when I travel.

Future Plans: Plan for you future and keep your options open. As in the previous point, you may not be able foresee your next steps.

Budget: I don’t know what to say about budget. That’s your business. Just remember the most importanat part of your imaging system is the mount. No question. Get the gest you can afford.

Any of this make sense?
al

What a difference dark skies make.

Living in the great Pacific Northwest, opportunities to image under clear skies are rare.  Under dark skies, even rarer.

Lately, whenever the weather permits, I grab my gear and head up into the Olympic Mountains.  I enter up by Port Angeles and head up to Hurricane Ridge, which has an elevation of about 4000 ft.  There is a visitor center up on the ridge,  a parking lot,  and a nearly 360 deg view.

Although there is still a little glow from Seattle (roughly 70 miles away) and nearby Port Angeles, it is still very dark.  As astronomical  darkness approaches, the Milky Way starts to look like clouds passing overhead, but soon is presented with good detail and color.  It’s something I never see from my semi rural home.  The difference in images taken there vice at home is dramatic and well worth the easy 2 hour drive.  The main effect is that even with longer subs, the background sky is smooth and evenly dark which makes processing the images much easier.

One thing about imaging in the mountains is the cold.  You need to be prepared.  It can get very cold very quick.  I’ll write a long report about ways I’ve used to deal with it.

Anyway, if like me you’re stuck with light polluted & moisture filled skies, do yourself a favor and get outta town.

An example: My last Horsehead & Flame

al

This is an interesting question. Lately, with the recently relesed “AllStar” alignment routine for the Celestron GEM mounts, many would say NO. In fact, emphatically NO.

I have a different opinion, but I’ll agree that a polar alignment scope (PAS) is not necessary. Further, that a PAS does not typically provide accuracy equivalent to the software alignment provided by the hand controller routines, or a drift alignment. AND finally, that the hand controller alignment routines are “good enough” for decent long exposure photography.

However, I prefer to use a PAS. I use the PAS to get a decent (close) polar alignment when I initially set up my mount. Then, first thing (before doing a pointing alignment) I do a drift alignment. This works great because the PAS gets me very close and the drift alignment goes quick and easy since the final adjustments are small. After the drift alignment, I do a quick pointing alignment, usually just a sync to one star. This gets me close enough to find my intended target for the night, I don’t need to be able to navigate the whole sky with spot on Go-Tos. I just need to be able to find my target…. gotta fuss around with calibrating the guider and framing the target anyway.

Following this method takes a few minutes more (i think) than doing the “AllStar” routine, but it’s not much more. I prefer this method because when done, I’m SURE of the polar alignment… no guesswork.  Alignment to the NCP is the the important part for AP and I want to be sure the alignment is right where I want it. I don’t want to waste a night because my guiding wasn’t up to snuff.  I also don’t need to spend much time on doing pointing alignments which are typically not too important since I usually shoot only one or two targets per session.

Does this make sense to anyone but me?

al